Rapid-recall takeaways distilled from the FRCA Primary question bank, grouped by specialty. One pearl per high-frequency concept. Use it for last-pass revision alongside questions, not as a replacement for them.
400 pearls · 5 specialties · Generated 6 November 2026
Physiology and Anatomy
80 high-yield facts
RhD is the most immunogenic red cell antigen. 85% population RhD+. Anti-D is IgG (not naturally occurring, requires sensitisation). Crosses placenta → HDFN. Prevented by anti-D immunoglobulin prophylaxis. Rh system has 5 main antigens: D, C, c, E, e.
LAD → anterior wall + anterior septum + apex. LCx → lateral wall. RCA → inferior wall + RV (in right-dominant). SA node: RCA 60%, LCx 40%. AV node: RCA 90%. Coronary dominance defined by PDA origin: right 85%, left 10%, co-dominant 5%.
Surfactant: DPPC from type II pneumocytes. Reduces surface tension → prevents atelectasis, improves compliance. Laplace: P = 2T/r - smaller alveoli would collapse without surfactant. Deficiency: neonatal RDS (<35 weeks), ARDS in adults.
Calcium clock: spontaneous SR Ca²⁺ release (RyR2) → NCX inward current → phase 4 depolarisation. Works with membrane clock (If, ICaT, ICaL) = coupled clock model. RyR2 mutations → CPVT. Disrupted calcium clock → slower pacing. Both clocks entrain each other for reliable automaticity.
Stellate ganglion at C7-T1, anterior to first rib. Block performed at C6 (Chassaignac's tubercle) for safety. Successful block → Horner's syndrome (ptosis, miosis, anhidrosis, enophthalmos). Indications: CRPS, phantom limb pain.
Glycine = primary inhibitory NT in spinal cord/brainstem (Cl⁻ channels). GABA = primary inhibitory NT in brain. Strychnine = glycine receptor antagonist → convulsions. Tetanus toxin blocks glycine release → rigidity. Glycine is also NMDA co-agonist (separate role).
HPV: pulmonary arterioles constrict to low PAO₂ → diverts blood from poorly to well-ventilated regions → optimises V/Q. Opposite of systemic response. Onset seconds, maximum 15-20 min. Inhibited by: volatiles, vasodilators, sepsis. Preserved by: propofol, TIVA. Critical during OLV.
Etomidate = most haemodynamically stable IV induction agent. Minimal changes in HR, BP, CO, SVR. Maintains baroreflexes. Use in: haemodynamic compromise, cardiac disease, trauma. Key disadvantage: adrenocortical suppression (11β-hydroxylase inhibition, ~24-48h).
DHTRs: IgG antibodies to Rh, Kell, Duffy, Kidd antigens. Occur 3-14 days post-transfusion. Extravascular haemolysis (spleen). Kidd antibodies notorious - levels drop below detection. Features: falling Hb, jaundice, positive DAT. Much milder than ABO AHTR (intravascular).
Subclavian vein: anterior and inferior to artery, separated by anterior scalene muscle. Over first rib, under clavicle. Advantages: low infection rate. Risks: pneumothorax (2-3% landmark), non-compressible. USS reduces complications. Supraclavicular approach increasingly popular.
Radial nerve from posterior cord, C5-T1. Motor: all extensors (triceps, wrist extensors, finger extensors). Sensory: posterior arm/forearm, dorsal hand (1st web space). Injury → wrist drop. Vulnerable in spiral groove (humeral fracture, Saturday night palsy). Largest brachial plexus branch.
PEEP improves PaO₂ by: recruiting atelectatic alveoli + increasing FRC above CC + reducing shunt. Trade-off: ↓CO, ↑dead space, risk of overdistension. Optimal PEEP balances oxygenation improvement against haemodynamic cost. Consider DO₂ (not just PaO₂) for overall benefit.
CO has 240 times greater Hb affinity than O2, forming COHb and shifting the curve leftward - a double mechanism of tissue hypoxia. SpO2 is falsely normal; use co-oximetry. Treat with 100% O2 per NPIS guidance.
Blood volume: ~70 mL/kg male, ~65 mL/kg female = ~5L in 70 kg male. 60-70% in venous system (capacitance). MABL = EBV × (Hi - Ht) / Hav. Neonates ~80-90 mL/kg, pregnant women ~100 mL/kg.
TTE = first-line for assessing systolic function (EF), diastolic function (E/A, E/e'), valvular disease, and chamber dimensions. Non-invasive, bedside, repeatable. TOE for intraoperative assessment. RCoA supports perioperative echo training (FICE, BSE).
Compliance = ΔV/ΔP (easy filling). Elastance = ΔP/ΔV (stiffness, inverse of compliance). ↓Compliance (LVH, fibrosis, amyloid) → ↑filling pressures, ↑dependence on atrial kick, ↑sensitivity to preload changes. Key concept in HFpEF.
Oxidative phosphorylation = primary ATP source (~34/38 ATP per glucose). ETC: Complexes I-IV + ATP synthase on inner mitochondrial membrane. O₂ = final electron acceptor. Cyanide/CO block Complex IV → ATP depletion → lactic acidosis. Complete glucose oxidation yields ~38 ATP total.
Sacral hiatus at S4-S5, covered by sacrococcygeal membrane. Identify by palpating sacral cornua. Insert needle at 45° through membrane, flatten to 20-30°, advance 1-2 cm. Dural sac at S2 = 2-3 cm safety margin. Common in paediatric anaesthesia for sub-umbilical surgery.
Lung compliance ~200 mL/cmH₂O. Chest wall ~200 mL/cmH₂O. Total system ~100 mL/cmH₂O. ↓Compliance = stiffer (fibrosis, ARDS). ↑Compliance = floppier (emphysema). Monitor static compliance during IPPV to detect changes.
Q₁₀ ≈ 2-3 for biological reactions (rate doubles/triples per 10°C rise). Each 1°C drop: CMRO₂ ↓6-7%. At 18°C: CMRO₂ ↓~80% (deep hypothermic circulatory arrest, safe 30-40 min). Hypothermia → ↓drug metabolism, ↓enzyme activity. MH → uncontrolled ↑metabolic rate.
Normal fibrinogen 2-4 g/L. Target ≥1.5 g/L in major bleeding (≥2.0 in obstetrics). First factor to fall critically in haemorrhage. Replace with cryoprecipitate or fibrinogen concentrate. Measured by Clauss assay or FIBTEM on ROTEM. Acute phase reactant - elevated in pregnancy/inflammation.
Anatomical dead space: face mask > LMA > ETT. ETT bypasses most upper airway (↓dead space ~50%). LMA sits above vocal cords, adds own dead space (~15-20 mL). Slightly higher MV may be needed with LMA to maintain PaCO₂. Practical difference is small and easily managed.
Femoral nerve = L2-L4, passes under inguinal ligament, lateral to femoral artery (NAVY/NAVEL). Motor: quadriceps (knee extension). Sensory: anterior thigh + saphenous nerve (medial leg to malleolus). Block: USS-guided, lateral to artery, deep to fascia iliaca.
Normal pH 7.35-7.45 (mean 7.40). [H⁺] at 7.40 = 40 nmol/L. Logarithmic scale: 1 pH unit = 10× change in [H⁺]. Acidaemia <7.35, alkalaemia >7.45. Life incompatible with pH <6.8 or >7.8. Three defence mechanisms: buffers (instant), lungs (minutes-hours), kidneys (hours-days).
Polycythaemia = primary long-term adaptation to altitude. EPO (via HIF-1α) → ↑RBC → ↑Hb → ↑O₂ carrying capacity. Develops over weeks-months. Hb may reach 18-20 g/dL. Excessive Hct >55% is harmful (↑viscosity, ↑PH, thrombosis = chronic mountain sickness).
Trendelenburg: ↑venous return → ↑preload → ↑SV (Frank-Starling) → ↑CO. Trade-offs: ↑ICP, ↑IOP, ↓FRC, ↑aspiration risk. PLR is a safer alternative for fluid responsiveness assessment and temporary haemodynamic support.
Na⁺/K⁺-ATPase: 3Na⁺ out, 2K⁺ in per ATP. Electrogenic (net outward current). Inhibited by digoxin → ↑intracellular Na⁺ → ↑Ca²⁺ via NCX → ↑contractility. Stimulated by insulin, β2-agonists, aldosterone. ~40% of brain's energy expenditure. Hypothermia reduces activity.
PEEP improves oxygenation: recruits atelectasis, ↑FRC > CC, ↓shunt, redistributes lung water. Not always beneficial: excessive PEEP → overdistension, ↓CO, ↑dead space. Optimal PEEP: balance recruitment vs overdistension. Methods: compliance-guided, oesophageal pressure, EIT.
R ∝ 1/r⁴ - radius to the fourth power is the dominant factor. Halving radius → 16× resistance. Arterioles are the main resistance vessels. Poiseuille's law is approximate for blood (non-Newtonian, pulsatile, elastic vessels).
Aerobic ATP: oxidative phosphorylation (ETC + ATP synthase) produces ~30-32 ATP per glucose. Anaerobic glycolysis = only 2 ATP + lactate. ETC blocked by cyanide (complex IV), CO, NO (sepsis). Treat cyanide: hydroxocobalamin. Cytopathic hypoxia in sepsis.
ATP energy in terminal phosphoanhydride bond (~30.5 kJ/mol). Body turns over ~40-70 kg ATP/day (recycled ~750×). Produced mainly by oxidative phosphorylation. Used for contraction, transport, synthesis, signalling. ATP depletion → ion homeostasis failure → cell death.
APC + Protein S → inactivate Va + VIIIa. Protein C activated by thrombin-thrombomodulin complex. Both Protein C and S are vitamin K-dependent. Deficiency → thrombosis. Factor V Leiden = APC resistance (commonest inherited thrombophilia, ~5% Caucasians).
Atelectasis forms in dependent (posterior/supine) lung regions within minutes of induction. Affects 15-20% of lung area. Mechanism: compression (main) + absorption + surfactant loss. Worse in: obesity, elderly, prolonged anaesthesia.
GA impairs baroreceptor sensitivity (dose-dependent). All agents affect it (volatiles, propofol, opioids, benzos). Exception: ketamine preserves baroreflexes. Clinical consequence: blunted compensatory responses to hypotension → haemodynamic instability.
Foetal HPV: ↓alveolar PO₂ → pulmonary vasoconstriction → ↑PVR → blood diverted through DA and FO. At birth: ↑PAO₂ → reversal of HPV → ↓PVR → ↑pulmonary blood flow (8-10×). Adult HPV: V/Q matching mechanism. Volatiles inhibit HPV → V/Q mismatch during OLV.
Pulmonary vessels constrict to hypoxia (HPV - unique). Systemic vessels dilate to hypoxia. Opposite responses, both physiologically logical. HPV mechanism: K⁺ channel inhibition → depolarisation → Ca²⁺ entry → vasoconstriction. Critical for V/Q matching, especially during OLV.
CAP = graded summation of multiple fibre APs. Multiple peaks from different fibre velocities (Aα → C). ↑stimulus → recruits more fibres → ↑CAP amplitude. Largest fibres have lowest threshold. Temporal dispersion creates complex waveform. Individual APs remain all-or-nothing.
Heparin + AT-III → inhibits thrombin + Xa. UFH needs ≥18 saccharides for thrombin inhibition. LMWH: mainly anti-Xa. Monitor UFH with APTT. Protamine reverses UFH (1mg:100U) but only partially reverses LMWH. AT-III deficiency = heparin resistance.
CO = HR × SV (normal ~5 L/min). SV determined by preload, afterload, contractility. Four determinants of CO: heart rate, preload, afterload, contractility. CI = CO/BSA (normal 2.5-4.0 L/min/m²).
Threshold = point of regenerative Na⁺ channel opening → all-or-nothing AP. ~-55 mV in neurons. Subthreshold = graded, decaying. Strength-duration relationship for nerve stimulation. Hyperkalaemia → depolarises → ↑initial excitability then ↓(channel inactivation). Hypocalcaemia → ↑excitability.
VO₂ max: ~3000-4000 mL/min (12-16× resting). Fick: VO₂ = CO × (CaO₂ - CvO₂). CO ↑5× + extraction ↑3× = ↑15× VO₂. CPET measures functional capacity: AT <11 mL/kg/min = ↑perioperative risk. VO₂ max limited by CO in health, by O₂ extraction in lung disease.
Bezold-Jarisch = bradycardia + hypotension + coronary vasodilation, triggered by ventricular C-fibre mechanoreceptors (vigorous contraction of underfilled ventricle). Important in: spinal anaesthesia hypotension, vasovagal syncope, inferior MI.
Spinal cord: 1 anterior spinal artery (anterior 2/3) + 2 posterior spinal arteries (posterior 1/3). Artery of Adamkiewicz: largest reinforcing artery, left side 75-80%, T9-T12. Anterior spinal artery syndrome: motor loss + pain/temp loss, preserved proprioception. Anterior cord = watershed zone.
Coeliac plexus at T12-L1, anterior to aorta, around coeliac trunk. Largest autonomic plexus. Supplies upper abdominal viscera. Block: pancreatic cancer pain (neurolytic with alcohol/phenol). Complications: hypotension (splanchnic vasodilation), diarrhoea, rarely paraplegia.
Negative BE (base deficit) = metabolic acidosis. Positive BE = metabolic alkalosis. Normal BE: -2 to +2. HCO₃⁻ deficit = BE × 0.3 × weight (kg). Trauma: base deficit >6 = significant injury. SBE corrected for Hb (calculated at Hb 5 g/dL) is more clinically useful.
LFCN = L2-L3, purely sensory. Lateral thigh sensation. Passes near ASIS under inguinal ligament. Meralgia paraesthetica = LFCN entrapment (obesity, pregnancy). Not reliably blocked by femoral nerve block. Fascia iliaca block may cover it. Block near ASIS for lateral thigh procedures.
Alveolar dead space ↑ during GA: IPPV creates Zone 1 (PA > Pa in non-dependent regions), ↓CO reduces Pa. Anatomical DS ↓ (intubation). Total physiological DS ↑. VD/VT: 0.33 → 0.45-0.50. ↑MV needed for normocapnia. PaCO₂-ETCO₂ gradient widens (3-5 → 10-15 mmHg).
obturator nerve = L2-L4; motor to thigh adductors; sensory to medial thigh; articular branches to hip and knee; exits via obturator canal; block prevents adductor spasm during lateral-wall TURBT (RA-UK Plan A).
Brachial plexus: C5-T1 (Roots → Trunks → Divisions → Cords → Branches). Trunks: superior (C5-6), middle (C7), inferior (C8-T1). Cords named relative to axillary artery. Terminal branches: musculocutaneous, median, ulnar, radial, axillary. Mnemonic: Robert Taylor Drinks Cold Beer.
Adult NMJ receptor: α2βδε (pentamer, 2 ACh binding sites). Foetal/denervated: α2βγδ (γ replaces ε). Foetal receptors: longer open time, extrajunctional, ↑K⁺ response to suxamethonium. Burns >48h, denervation → foetal receptors reappear → suxamethonium contraindicated.
PCC = Factors II, VII, IX, X + Protein C, S. First-line for warfarin reversal in major bleeding. Dose by INR and weight. Always give with vitamin K (5 mg IV) for sustained effect. Advantages over FFP: small volume, no thaw, no ABO match, standardised. Risk of thrombosis.
Veins hold 60-70% of blood volume - the body's blood reservoir. Venous capacitance changes (venoconstriction/dilation) have major effects on preload. Anaesthetic agents increase venous capacitance → relative hypovolaemia → hypotension.
Haldane effect: deoxy-Hb carries more CO₂ and buffers more H⁺ than oxy-Hb. Complementary to Bohr effect. Responsible for ~50% of CO₂ exchange. Clinically: O₂ therapy in COPD → ↑oxy-Hb → ↓CO₂ carriage (Haldane) → ↑PaCO₂ (one mechanism of oxygen-induced hypercapnia).
Electrochemical gradient = concentration gradient + electrical gradient. Driving force = Vm - Eion. Na⁺ driving force at rest = -130 mV (strong inward). K⁺ driving force at rest = +20 mV (weak outward). At Eion, driving force = 0. Determines direction and magnitude of ion flow.
Group O = no antigens, both antibodies (anti-A + anti-B). Landsteiner's rule: antibodies to absent antigens. ABO antibodies = IgM → complement activation → intravascular haemolysis if incompatible. O = universal RBC donor; AB = universal plasma donor.
Duct-dependent lesions need PGE1 (alprostadil) to maintain DA patency. Systemic duct-dependent: coarctation, IAA, HLHS, critical AS. Pulmonary duct-dependent: pulmonary atresia, critical PS, tricuspid atresia. PGE1 dose: 5-20 ng/kg/min. Side effect: apnoea (12%) → have intubation ready.
Platelet adhesion = GPIb/V/IX + vWF + collagen (Bernard-Soulier if deficient). Platelet aggregation = GPIIb/IIIa + fibrinogen (Glanzmann's if deficient). GPIb for adhesion, GPIIb/IIIa for aggregation - two distinct processes.
AT = exercise intensity where lactate accumulates (anaerobic > aerobic). Detected by V-slope on CPET. Normal AT: 50-60% VO₂ max. Perioperative risk: AT >11 = lower risk, <11 = higher risk, <8 = very high risk. Guides postop care level. Prehabilitation aims to ↑AT before surgery.
RPP = HR × SBP. Clinical surrogate for MVO₂. Normal resting ~6000-8000. >12,000 = ischaemia risk. Reduce RPP by: ↓HR (β-blockers), ↓SBP (adequate anaesthesia/analgesia), avoid tachycardia and hypertension. Monitor RPP perioperatively in cardiac risk patients.
Prone position = best V/Q matching. More uniform ventilation and perfusion distribution. Heart off posterior lung. PROSEVA trial: prone >16h/day ↓mortality in moderate-severe ARDS (P/F <150). Complications: facial oedema, pressure injuries, extubation risk. Also improves secretion drainage.
causes of elevated anion gap metabolic acidosis: Methanol, Uraemia, Diabetic ketoacidosis, Propylene glycol/Paracetamol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates.
True shunt = blood bypasses ventilated alveoli → hypoxaemia NOT responsive to O₂ (distinguishing feature from V/Q mismatch). Normal physiological shunt 2-5%. PaO₂ primarily affected; PaCO₂ can be normalised by hyperventilation (linear CO₂ curve).
↓Ca²⁺ → ↑excitability (lowers threshold for Na⁺ channel activation). Trousseau and Chvostek signs. Causes: massive transfusion (citrate), post-thyroidectomy, alkalosis. ↑Ca²⁺ → ↓excitability. Monitor ionised Ca²⁺ during massive transfusion.
Beta-oxidation in mitochondrial matrix. Fatty acids enter via carnitine shuttle (CPT-I/II). Each cycle: removes 2C as acetyl-CoA + NADH + FADH₂. Palmitate → ~129 ATP. Brain cannot use fatty acids (→ uses ketones in fasting). Carnitine/CPT deficiency → myopathy.
Unpaired t-test: compares MEANS of TWO independent groups. Requires: continuous, normally distributed data, independent groups, approximately equal variances. Paired t-test: same subjects before/after. ANOVA for ≥3 groups. Mann-Whitney U = non-parametric equivalent. Chi-squared for categorical data.
Overall: CO₂ + Ca(OH)₂ → CaCO₃ + H₂O + heat. NaOH acts as catalyst (consumed and regenerated). Products: CaCO₃ (insoluble chalk), H₂O (humidification), heat (warming). 14 kcal/mol CO₂. 1 kg absorbs ~120-150 L CO₂. Water in soda lime essential for reaction to proceed (14-19% optimal).
Fresh gas decoupling: delivered TV independent of FGF. Traditional machines: FGF adds to TV during inspiration (TV = set + FGF × Ti). With decoupling: TV = set TV regardless of FGF. Methods: decoupling valve (diverts FGF to bag) or piston compensation.
Simple face mask: FiO₂ 40-60% at 5-10 L/min. Minimum flow 5 L/min (prevent CO₂ rebreathing). FiO₂ variable - depends on flow rate and breathing pattern. Venturi mask: precise, fixed FiO₂ (better for COPD). Non-rebreathing mask: highest FiO₂ from simple devices (~60-90%).
Normal distribution: ±1 SD = 68%, ±2 SD = 95%, ±3 SD = 99.7% (the 68-95-99.7 rule). Mean = median = mode in a normal distribution. Fully described by mean and SD. Basis for parametric tests and confidence intervals. SD = spread of individual values.
Desiccated soda lime → CO production (desflurane worst) + fire risk. Sevoflurane + soda lime → Compound A (nephrotoxic in rats, debated in humans). Prevent: turn off FGF after cases. Amsorb Plus = CO/Compound A-free alternative. Baralyme withdrawn (worse for CO + Compound A).
Ligamentum flavum = 80% elastin (yellow, elastic). Provides the 'loss of resistance' feel during epidural insertion. Thickest at L2-3 (~3-5 mm). May have midline gaps in cervical region. Calcifies with age, making epidural insertion harder.
Acute rule: ↑PaCO₂ by 1 kPa → ↓pH by 0.1 (and vice versa). Useful for rapid bedside ABG assessment. Chronic changes show less pH change per kPa due to renal compensation. Also: ↑PaCO₂ by 10 mmHg → ↓pH by ~0.08.
Kidney acid-base: (1) reclaims filtered HCO₃⁻ (PCT 80%, TAL 15%, CD 5%); (2) generates NEW HCO₃⁻ by excreting H⁺ as titratable acid (H₂PO₄⁻, ~30%) and NH₄⁺ (~60-70%). Alpha-intercalated cells: H⁺-ATPase + H⁺/K⁺-ATPase (apical). Each H⁺ excreted = 1 new HCO₃⁻ generated (via basolateral AE1).
Pharmacology
80 high-yield facts
Metoclopramide prokinesis: 5-HT₄ agonism → ↑ACh release → ↑gastric motility, ↑LOS tone, pyloric relaxation. Distinct from its D₂ antiemetic action. MHRA restricts to ≤10 mg TDS for ≤5 days (EPS risk). Useful pre-RSI for aspiration risk reduction.
Carbamazepine = potent CYP inducer → ↓warfarin effect (↓INR), ↓OCP efficacy, ↓NMBA effect. Also auto-induces. Onset/offset 2-3 weeks. Mnemonic: PC BRASS for inducers.
Vd = dose / C0; a Vd of around 5 L indicates a drug largely confined to the intravascular compartment, and Vd drives the loading dose calculation (loading dose = Vd x target concentration).
3-compartment model: V₁ (central - blood/organs, small), V₂ (rapid peripheral - muscle, moderate), V₃ (slow peripheral - fat, large). Rate constants: k₁₂/k₂₁ (V₁↔V₂), k₁₃/k₃₁ (V₁↔V₃), k₁₀ (elimination from V₁). Theoretical compartments (not anatomical). Drug enters and exits via V₁.
IV paracetamol peak analgesic effect ≈ 30-60 min after infusion. Give at induction for peak effect at emergence. Faster than oral (peak 1-2h) but not instant. Duration 4-6h. NNT ≈ 3.7 for postoperative analgesia.
Schnider model - fixed V1 (4.27 L), clearance adjusted for age and lean body mass, ke0 = 0.456 min⁻¹. Better for elderly patients. Marsh model - V1 scales with weight, no age adjustment.
N₂O irreversibly oxidises the B₁₂ cobalt ion, inhibiting methionine synthase, causing megaloblastic changes (>6 hrs) and demyelination (prolonged). Avoid in B₁₂ deficiency. Recreational abuse causes subacute combined degeneration.
t½ = 0.693 × Vd ÷ clearance - always reconcile units (Vd in mL, clearance in mL/min). Half-life is a derived parameter determined jointly by volume of distribution and clearance, so changes in either alter it.
Physics and Clinical Measurement
80 high-yield facts
LiDCO = lithium dilution via peripheral vein + arterial lithium sensor. No PA catheter needed. Stewart-Hamilton equation. LiDCOplus adds continuous pulse power analysis. Calibrate every 8 hours. Contraindicated with lithium therapy. Small lithium dose (0.15-0.3 mmol).
PiCCO transpulmonary thermodilution: CO + GEDV (preload) + EVLW (lung water) + CFI (function). CVC + femoral/axillary arterial thermistor catheter. GEDV: volumetric preload, less affected by ventilation than CVP. EVLW: direct measure of pulmonary oedema (normal 3-7 mL/kg).
NIRS: near-infrared light (700-1000 nm) → measures rSO₂ (cerebral tissue O₂ saturation). Normal ~60-80%. Mainly venous (~70% of signal). >20% drop from baseline or <50% = concern for ischaemia. Used in cardiac surgery, carotid surgery, beach chair position. Monitors frontal cortex only.
Coagulation = interrupted/pulsed waveform, ~6% duty cycle, high peak voltage (up to 9000 V). Tissue desiccation and protein denaturation (60-100°C). Spray/fulguration = arcing across gap. Cutting = continuous, 100% duty cycle, lower voltage. Blend modes = intermediate duty cycles.
Pacemaker + diathermy: dispersive pad far from generator, short bursts, lowest power, magnet available, bipolar preferred. Risks: inhibition, reprogramming, lead damage, VF. Not absolutely contraindicated but requires precautions. Cardiology input recommended. Continuous ECG monitoring essential.
Class III = safety extra-low voltage (SELV <25 V AC). Voltage too low for dangerous shock. Powered by safety isolating transformer. Battery devices are 'internally powered' (similar principle).
Diathermy = most common cause of theatre electrical burns. Causes: poor dispersive pad contact, direct coupling, capacitative coupling, channelling, inadvertent activation. Prevention: proper pad placement, REM system, lowest power, bipolar where possible, electrode in holder when not in use.
ETCO₂ during CPR reflects cardiac output from compressions. Sustained rise = earliest ROSC indicator. <1.3 kPa (10 mmHg) at 20 min = poor prognosis. Higher ETCO₂ = better quality CPR. Confirms ETT placement (most reliable). Recommended by all resuscitation guidelines.
Triple point = all three phases coexist (specific T and P). Water: 0.01°C, 611.73 Pa. Used to define the Kelvin scale (273.16 K). CO₂ triple point above 1 atm → sublimes at atmospheric pressure (dry ice). Phase diagram shows solid, liquid, gas regions and equilibrium lines.
TOF ratio = T4/T1. Four stimuli at 2 Hz. Safe extubation ≥0.9 (quantitative monitoring). Non-depolarising block causes fade (pre-junctional effect). Depolarising block causes equal reduction of all twitches (no fade). Tactile assessment unreliable for ratios >0.4. Always use quantitative monitoring.
Medical gases are completely dry. Minimum: ≥15 mg/L. Ideal: 33-44 mg/L at 34-37°C. Dry gas → mucosal damage, ↓ciliary function, thick secretions, heat loss, sore throat. HME adequate for short cases. Active humidification for >24-48h.
UK pipeline pressure = 4 bar (400 kPa) for all medical gases. Oxygen cylinders = 137 bar. Schrader probes at wall outlets are non-interchangeable (colour-coded). Pipeline failure → automatic cylinder backup. VIE = primary bulk oxygen supply in large hospitals.
NNH = 1/ARI. ARI = drug event rate - placebo event rate. NNH of 10 = 1 extra harm per 10 treated. Low NNH = more harmful. Compare NNT (benefit) to NNH (harm) for clinical decisions. Severity of harm matters (not just frequency). Always report CIs for NNT/NNH.
FloTrac/Vigileo = uncalibrated pulse contour analysis from standard arterial line. SVV >13% = fluid responsive (same conditions as PPV). No external calibration needed. Less accurate in: sepsis, vasodilation, rapidly changing vascular tone. Advantages: minimal invasiveness, easy setup.
Raised baseline CO₂ = rebreathing. Check: soda lime exhaustion, inadequate FGF, faulty expiratory valve, insufficient expiratory time. Normal inspired CO₂ should be zero. Cardiac arrest → sudden ETCO₂ drop to near zero. Disconnection → absent trace. Hyperventilation → low ETCO₂ but zero baseline.
Chest X-ray: ~0.02 mSv (~3 days background). CT head ~2 mSv, CT chest ~7, CT abdomen ~10 mSv. Background ~2.7 mSv/year (UK). CT abdomen ≈ 500 CXRs ≈ 4 years background. ALARA and justification mandatory. Annual occupational limit 20 mSv.
Pumping effect: back-pressure from PPV → gas compression in vaporising chamber → expansion on release carries extra vapour → transient ↑output. Modern vaporisers minimise this with long channels, check valves, pressure compensation. More pronounced with high pressures, high rates, low FGF.
Active humidifiers: heated water chamber, ~37°C, ~100% RH, ~44 mg/L. Better than HMEs for prolonged ventilation (>24-48h). Heated wire circuits prevent rainout. Placed in inspiratory limb (near ventilator). Risks: thermal injury, condensation, infection (Legionella/Pseudomonas).
Heidbrink valve = APL valve = pop-off valve = expiratory valve. Spring-loaded pressure relief. Adjustable: fully open (SV, ~1 cmH₂O) to closed (max pressure). Excess gas → scavenging. Don't leave fully closed during ventilation → barotrauma. Max limit ~60-70 cmH₂O (safety).
Compliance = ΔV/ΔP (mL/cmH₂O). Lung: 200. Chest wall: 200. Total: 100 (in series). Reciprocal of elastance. High compliance = easily distended. Low compliance = stiff. Decreased by: fibrosis, ARDS, oedema. Increased by: emphysema. Breathing circuit compliance also important (3-5 mL/cmH₂O).
Machine uses pipeline preferentially (higher pressure ~4 bar vs cylinder regulator ~3.5-3.8 bar). Cylinder = automatic backup if pipeline fails. Seamless changeover. Both sources connected simultaneously (recommended). Pre-use check: disconnect pipeline → verify cylinder backup → reconnect.
Bain circuit: co-axial Mapleson D. FGF for IPPV: 70-80 mL/kg/min (~MV). FGF for SV: 200-300 mL/kg/min (inefficient). Inner tube = fresh gas to patient. Advantages: lightweight, compact, convenient for head/neck surgery. Risk: inner tube disconnection → rebreathing.
O₂ = homonuclear diatomic → symmetric vibrations → no IR absorption. Also IR-inactive: N₂, He, Ar. O₂ measured by: paramagnetic analyser, fuel cell, Clark electrode, mass spec. CO₂ absorbs at 4.26 μm. N₂O at 4.5 μm. CO₂ and N₂O overlap → cross-sensitivity correction needed.
Mapleson B: FGF near patient, valve near patient, bag at far end (connected by tubing). Least efficient for spontaneous ventilation. Second-best for controlled. Rarely used clinically. Mainly theoretical/exam knowledge. Classification order: spontaneous A>D>C>B; controlled D>B>C>A.
FiO₂ ≤0.3-0.4 during laser use. Air as carrier gas. No N₂O. ↑FiO₂ → dramatically ↑fire risk. FiO₂ 1.0 = most dangerous. Communicate FiO₂ with surgeon before laser activation. Can increase between laser passes if SpO₂ low. Allow washout time after reducing FiO₂ before laser activation.
Aladin cassette = electromagnetic/RFID automatic agent identification. Machine configures delivery algorithm for specific agent. No manual selection needed. Electronic injection system (not passive splitting ratio). Interchangeable agent-specific cassettes. Integration with machine monitoring.
Stefan-Boltzmann: P ∝ T⁴ (absolute temperature). Body emissivity ≈ 0.98 (near-perfect blackbody for IR). Net heat loss ∝ (T_body⁴ - T_env⁴). Space blankets reflect IR radiation. Radiant warmers emit IR. Skin colour does not affect IR emission. Radiation = ~40% of heat loss in adults.
Stochastic: probability ↑ with dose, no threshold assumed, severity independent of dose. Cancer and genetic effects. Latent period years-decades. LNT model (linear no-threshold). Deterministic: threshold exists, severity ↑ with dose, appears days-weeks. Skin burns, cataracts, marrow failure.
Fleisch pneumotachograph = parallel capillary tubes maintaining laminar flow. ΔP across element ∝ flow (Poiseuille). Heated to prevent condensation. Differential pressure transducer measures ΔP. Volume = integral of flow. Lilly alternative uses wire mesh screen.
Fick: Flow = Uptake / (Ca - Cv). Applicable to any organ + any substance. CO = VO₂/(CaO₂ - CvO₂). CBF: Kety-Schmidt with N₂O. RPF: PAH clearance. Requires: measurable substance uptake, steady state, accurate AV sampling. Gold standard for CO measurement. Adolf Fick, 1870.
Significant Kruskal-Wallis → at least one group differs → need post hoc tests (Dunn's test). Same for ANOVA → post hoc (Tukey, Bonferroni). Multiple uncorrected comparisons inflate Type I error. Bonferroni: adjusted α = 0.05/number of comparisons.
Sudden ETCO₂ to zero = oesophageal intubation, disconnection, or complete obstruction. Capnography = gold standard for confirming ETT position. Sustained waveform ≥6 breaths = tracheal. Rising ETCO₂ = MH, inadequate ventilation. Shark fin = bronchospasm. Low ETCO₂ = PE, low CO.
Mann-Whitney U = non-parametric unpaired t-test (2 independent groups, ordinal/non-normal data). Wilcoxon signed-rank = non-parametric paired t-test. Kruskal-Wallis = non-parametric ANOVA. Spearman's rho = non-parametric Pearson's r. Non-parametric tests use RANKS, not raw values.
VIE: liquid O₂ at -183°C, 7-10 bar, vacuum-insulated double wall. 1 L liquid ≈ 860 L gas. Hospital VIE: 10,000-15,000 L liquid. Superheater warms gas to room temp before pipeline. O₂ critical temp = -118°C (cannot liquefy above this). Backup cylinder manifold mandatory (HTM 02-01). Tanker delivery.
Return electrode: well-vascularised area, near surgical site, good contact, large area. Pacemaker precautions needed. Bipolar: no return electrode (current between forceps tips). Diathermy = radiofrequency (300 kHz-3 MHz, not 50 Hz). Fire risk: alcohol prep, drapes, bowel gas. Check pad regularly.
Mass spec: ionises molecules → separates by mass-to-charge ratio in magnetic field → detects all gases. Measures: O₂, N₂, CO₂, N₂O, volatiles, N₂. Only method to detect N₂ clinically. Very accurate, fast. Disadvantages: expensive, bulky, vacuum required, multiplexed.
Harmonic imaging: transmit at f₀, receive at 2f₀ (second harmonic). Non-linear tissue propagation generates harmonics. Benefits: ↓near-field artefacts, ↓side lobes, ↑contrast, ↑border definition. Default on most modern machines. Especially useful for: cardiac, abdominal, obese patients.
TOE: probe in oesophagus behind LA → superior cardiac imaging. ↑Frequency possible (short distance). No chest wall/lung interference. Continuous intraoperative monitoring. Immediate surgical result assessment. Complications: dental injury (common), oesophageal perforation (rare).
Transcutaneous CO₂: heated electrode 42-44°C. Heating: arterialises capillary blood + increases CO₂ diffusion through skin. PtcCO₂ reads higher than PaCO₂ (local metabolism + temperature effect on CO₂ solubility). Change sensor site every 2-4 hrs (burn risk). Best in neonates (thin skin).
1 atm = 101.3 kPa = 760 mmHg = 1033 cmH₂O = 1.013 bar. 1 kPa ≈ 7.5 mmHg. 1 mmHg ≈ 1.36 cmH₂O. Clinical: BP in mmHg, airway pressure in cmH₂O, gas pressure in kPa/bar. Gauge pressure = above atmospheric; absolute = gauge + atmospheric.
Piston ventilator: motor-driven piston, no driving gas needed. Electrically powered. Very precise tidal volume delivery. Excellent for paediatrics. Fresh gas decoupling. Advanced modes possible. No driving gas consumption (economic). Examples: Dräger Zeus, Fabius. Cf.
Charles' law: V ∝ T (absolute, Kelvin). V/T = constant at constant P. V₁/T₁ = V₂/T₂. Must use Kelvin. Room temp to body temp: ~5.8% volume increase. BTPS: Body Temperature, ambient Pressure, Saturated with water vapour. STPD: Standard Temperature and Pressure, Dry (0°C, 101.3 kPa, dry).
Henry's law: C = kH × P (dissolved gas ∝ partial pressure at constant T). Clinical: dissolved O₂ ∝ PaO₂, decompression sickness (N₂ bubbles on ascent), hyperbaric O₂ therapy, volatile agent uptake. Solubility decreases with ↑temperature.
Attenuation ∝ frequency (~0.5 dB/cm/MHz in soft tissue). ↑Frequency → ↑resolution but ↓penetration. Attenuation = absorption (heat) + scattering + reflection. TGC compensates for depth-dependent attenuation. 10 MHz → superficial (4-5 cm). 2-5 MHz → deep (15-20 cm). Air and bone: extreme attenuation.
Daily vaporiser check: correct agent, adequate fill, seated securely, dial turns smoothly, interlock works, included in leak test. AAGBI 2012 guidelines. Anaesthetist responsibility. Full check before each list, brief check between patients. Internal inspection only during scheduled servicing.
Clark cathode reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻. Current ∝ PO₂. Polarising voltage ~-0.6 V (ensures diffusion-limited operation). Cathode: platinum. Anode: Ag/AgCl. Consumes O₂ → sample must flow. Membrane prevents contamination. Fuel cell anode: Pb → Pb²⁺ + 2e⁻ (galvanic, no external voltage).
Delayed sample: PO₂ falls (O₂ consumed), PCO₂ rises (CO₂ produced), pH falls (acidotic). Cellular metabolism continues. Analyse within 10-15 min. Ice slows metabolism ~90% (stable for 30-60 min). Expel air bubbles (air → falsely high PO₂). Leucocyte larceny in leukaemia → falsely low PO₂.
Venturi = fixed performance. Total flow = O₂ × (1 + entrainment ratio); for 40% the ratio ≈ 3:1, so 10 L/min → ~40 L/min total. Total flow must exceed peak inspiratory flow (~30 L/min) for an accurate FiO₂. Based on the Bernoulli/Venturi principle (BTS 2017 guideline for oxygen use in adults).
Bernoulli: increased velocity → decreased pressure (conservation of energy). Total energy constant along streamline: kinetic + pressure + potential = constant. Clinical: Venturi masks, jet ventilation, nebulisers, airway collapse in OSA. Applies to ideal (inviscid) fluids along streamlines.
Fast-flush optimal = 1-2 oscillations then rapid return. Overdamped = no oscillations, slow return, flat trace. Underdamped = >4 oscillations, spiky trace, systolic overshoot. Always check after setup. MAP remains most reliable measurement regardless of damping.
AGSS receiving system: positive relief (+5 cmH₂O) protects from back-pressure (blocked outlet). Negative relief (-0.5 cmH₂O) protects from excessive suction. Open system provides pressure break. NEVER connect vacuum directly to patient circuit (lethal negative pressure).
Vaporiser interlock: only one vaporiser ON at a time. Prevents: dual agent delivery → unpredictable depth, overdose. Systems: Selectatec (GE), Dräger mechanical, electronic (modern). Does NOT prevent: wrong agent filling (need agent-specific filler), malfunction.
Bourdon gauge = gauge pressure (above atmospheric). Curved metal tube straightens with pressure → pointer moves. Simple, robust, no electricity. Reads zero at atmospheric. On machine: cylinder pressure, pipeline pressure. N₂O gauge unreliable (reads 44 bar until liquid exhausted). Accuracy ±2-5%.
Simplified Bernoulli: ΔP = 4v² (mmHg). v = peak velocity (m/s). Derived from ½ρv² with blood density ≈ 1060 kg/m³. AS: peak velocity 4 m/s → gradient 64 mmHg. PASP = 4v²TR + RAP. Assumes negligible proximal velocity. Angle-dependent (Doppler). Widely used in clinical echocardiography.
Tilted vaporiser → liquid in bypass channel → dangerously HIGH output when first used. Purge for 20-30 min at high flow + max dial before clinical use. Always transport upright. Tec 6 has tilt alarm. Aladin cassettes are less affected. Unpurged tilted vaporiser = risk of lethal overdose.
SI unit of pressure = Pascal (Pa) = 1 N/m². 1 atm = 101.325 kPa = 760 mmHg ≈ 1 bar. Clinical units vary: mmHg (BP), kPa (blood gases), cmH₂O (ventilator), bar (cylinders). Gauge pressure = relative to atmospheric. Absolute = gauge + atmospheric.
Cooling saturated gas → condensation (dew point reached). Gas holds less water at lower temperature. 37°C saturated = 44 g/m³; 20°C = 17 g/m³. HMEs exploit this: expired heat/moisture deposited, then returned on inspiration. SVP increases exponentially with temperature.
Argon laser: 488/514 nm (visible blue-green). Absorbed by haemoglobin and melanin. Applications: retinal photocoagulation, port wine stains, GI bleeding. Selective photothermolysis: choose wavelength for target chromophore + pulse duration < thermal relaxation time. CO₂ = water.
HeNe laser: 632.8 nm, visible red, low power. Used as aiming beam for invisible surgical lasers (CO₂, Nd:YAG). Class 2 or 3R (not Class 4). Coaxially aligned with surgical beam. Still don't look directly into it. Gas laser (He + Ne). Essential for safe targeting of invisible therapeutic beams.
Selectatec interlock = prevents simultaneous use of two vaporisers. Mechanical interlock system on back bar. Vaporisers can be individually removed/replaced. Aladin system = electronic identification + interlock. Critical safety feature - prevents dangerous agent combinations.
Cylinder O₂ volume = cylinder pressure × water capacity (Boyle's law). Full E cylinder = 137 × 10 = 1370 L. Duration = volume / flow rate. Does NOT apply to N₂O (liquid below critical temp). O₂ critical temp = -118°C (always a gas at room temp).
Cohort study: exposure → follow over time → outcome. Can calculate RR and incidence directly. Case-control: outcome → look back → exposure. Can only calculate OR. OR ≈ RR when disease is rare. Cohort: good for common outcomes. Case-control: good for rare diseases.
Van der Waals: (P + a/V²)(V - b) = nRT. Corrects ideal gas law for: 'a' = intermolecular attractions (reduces effective P) + 'b' = molecular volume (reduces available V). Real gases deviate most at high P, low T, near critical point. Ideal gas approximation best at low P, high T.
Ritchie whistle: powered by residual O₂ pressure (pneumatic). No electricity/battery needed. Sounds for ≥7 seconds. Activates when O₂ pressure <200 kPa. Cannot be silenced. Simultaneously: N₂O cut off. Audible above theatre noise. Fail-safe design - works even in power failure.
Class 4 = all surgical lasers - eye, skin and fire hazards. Mandatory laser-controlled area, wavelength-specific goggles, LPA/LLSO, FiO₂ <0.3, laser-resistant ETT (IEC 60825-1; MHRA; AAGBI/RCoA).
Cylinder manifold: backup to VIE. Two banks of J-size cylinders (6800 L each at 137 bar). Automatic changeover between banks. Reduces to 4 bar pipeline pressure. Capacity: 2-3 days normal use. HTM 02-01 mandates backup. Alarm on bank depletion.
N₂O diffuses into closed gas spaces 34× faster than N₂ diffuses out → expansion (compliant spaces) or ↑pressure (non-compliant). Contraindicated in: pneumothorax, bowel obstruction, pneumocephalus, middle ear/retinal surgery, air embolism.
P-value = probability of data (or more extreme) IF null hypothesis is true. NOT probability that the treatment works or null is true. p < 0.05 = 'statistically significant' (by convention). Does not measure clinical importance or effect size. Confidence intervals are more informative.
ITT = analyse as RANDOMISED (regardless of compliance). Preserves randomisation, reduces bias. Conservative estimate (diluted by non-compliance). Reflects real-world effectiveness. Per-protocol = only adherent patients (estimates efficacy). ITT + PP consistent → robust.
Thermocouple = Seebeck effect = voltage at junction of 2 dissimilar metals ∝ temperature difference. Fast response, small, wide range. ~40 μV/°C (copper-constantan). Needs amplification. Reference junction required. Less common clinically than thermistors. Type T (Cu-constantan) for medical use.
AGSS: protects staff from chronic waste gas exposure. WELs: N₂O 100 ppm, isoflurane 50 ppm, sevoflurane 20 ppm. Components: collection (APL valve) → transfer (30mm tubing) → receiving (reservoir + relief valves) → disposal (active or passive). Active = powered extraction (~75 L/min).
Reynolds number >2000 → turbulent flow likely. Re = ρvd/η (density × velocity × diameter / viscosity). High density, velocity, diameter and low viscosity → turbulence. Heliox reduces Re (low density) → laminar flow → less resistance. Trachea often turbulent; bronchioles laminar.
Plenum = pressurised gas supply, high internal resistance, mounted on anaesthetic machine back bar. Draw-over = low resistance, patient's inspiratory effort, portable. Plenum vaporisers are precision, agent-specific, temperature-compensated devices.
Tec 7 (sevoflurane) max dial = 8%. Tec 5 (isoflurane) max dial = 5%. Tec 6 (desflurane) max dial = 18%. Max dial ≈ 2-3 × MAC for each agent. Safety feature preventing extreme concentrations. High settings used for gaseous induction (especially paediatric sevoflurane induction).
O₂ cylinder (size E): 137 bar full, ~680 L at atmospheric pressure. O₂ stored as compressed gas (above critical temp -118°C). Pressure gauge ∝ contents remaining. N₂O: 44 bar (liquid present - gauge unreliable). Pipeline O₂: 4 bar (400 kPa). Size E = 5 L water capacity.
Kidney produces: erythropoietin (cortical fibroblasts), active vitamin D (1α-hydroxylase in PCT), renin (JG cells), prostaglandins (vasodilation). Aldosterone is from the adrenal cortex, not the kidney. CKD → anaemia (↓EPO) + renal bone disease (↓vitamin D).
Hip joint innervation (Hilton's law) = femoral + obturator + sciatic nerves (+ branches). Complete hip analgesia requires blocking all three. Fascia iliaca covers femoral ± obturator. PENG block = newer approach targeting articular branches at anterior capsule.
HTS: osmotic ICP reduction. ↑Plasma osmolality → water drawn from brain → ↓brain volume → ↓ICP. Advantages over mannitol: no diuresis (volume expands), works with partial BBB disruption (higher reflection coefficient), anti-inflammatory, no rebound. 3% or 23.4% NaCl. 23.4%: central line only.
Epidural space: potential space, dura to ligamentum flavum/periosteum. Contains: fat, veins (Batson's plexus), arteries, connective tissue, lymphatics. NOT CSF. Full length of vertebral column. Drugs block nerve roots traversing the space. LA: segmental nerve root block.
Core temperature sites: nasopharynx, distal oesophagus, PA catheter (gold standard), tympanic, bladder. NICE: monitor temp every 30 min during GA >30 min. Axillary underestimates by 1-2°C. Core-peripheral gradient >4°C suggests vasoconstriction.
Absolute refractory period = Na⁺ channels inactivated (no stimulus works). Relative refractory period = channels recovering (supranormal stimulus works). Na⁺ channel states: resting → open → inactivated → resting. Limits firing frequency and ensures unidirectional propagation.
ALL ganglia = ACh at nicotinic Nn receptors (both sympathetic and parasympathetic). Postganglionic: parasympathetic = ACh at muscarinic; sympathetic = NA at adrenergic. Exceptions: sweat glands (sympathetic but ACh/muscarinic), adrenal medulla (modified ganglion → adrenaline/NA into blood).
Heat loss mechanisms in thermoneutral conditions: Radiation 40%, Convection 30%, Evaporation 20%, Conduction and Respiratory 10%. In theatre, exposed body cavities dramatically increase evaporative and radiant losses.
Infraclavicular block = cords (lateral, medial, posterior) around second part of axillary artery, behind pectoralis minor. Complete upper limb block with lower phrenic palsy risk than interscalene. Costoclavicular approach = cords clustered together, more superficial.
Postganglionic sympathetic = noradrenaline (NA) at adrenergic receptors. Exception: sweat glands = ACh at muscarinic. NA termination: reuptake-1 (80%, blocked by TCAs, cocaine), COMT, MAO. Adrenal medulla releases adrenaline (~80%) + NA (~20%) into blood.
Reduced FRC + increased VO2 = rapid desaturation during apnoea in pregnancy. This is why adequate preoxygenation is critical before obstetric general anaesthesia and why the 'safe apnoea time' is shortened from approximately 6 minutes to approximately 3 minutes.
Anaesthetic agents: reduce RBF/GFR 15-40% (↓CO, ↓MAP, sympathetic effects). Reversible on emergence. Sevoflurane: Compound A (rat nephrotoxicity) - no human clinical nephrotoxicity at flows ≥1 L/min. More important: maintain MAP, euvolaemia, avoid nephrotoxins.
Femoral nerve = lateral to femoral artery, deep to fascia iliaca. NAVY arrangement from lateral to medial. Hyperechoic triangular shape on US. Block provides anterior thigh + medial leg (saphenous) analgesia. Common for hip and knee surgery.
Brown fat + UCP1 (thermogenin) = neonatal non-shivering thermogenesis. General anaesthesia inhibits NST by blocking sympathetic activation, making neonates especially vulnerable to perioperative hypothermia.
CYP450 needs NADPH + O₂. Mixed-function oxidase: one O atom → substrate (hydroxylation), one O atom → H₂O. Located in hepatic smooth ER. Haem-containing enzyme (absorption at 450 nm with CO). Inducible and inhibitable.
ACE is primarily on pulmonary endothelium - AI → AII (activation) and bradykinin → inactive peptides (inactivation). Both occur in the pulmonary circulation. ACE inhibitor side effects (cough, angioedema) result from bradykinin accumulation, not angiotensin II reduction.
KDIGO AKI: Stage 1 (Cr ≥26.5 μmol/L in 48h or ≥1.5× baseline in 7d, UO <0.5 mL/kg/h 6-12h), Stage 2 (Cr 2-2.9×, UO <0.5 for ≥12h), Stage 3 (Cr ≥3× or ≥353.6 or RRT). Perioperative AKI: 5-7% major surgery, 30% cardiac. Risk: CKD, DM, CCF, nephrotoxins, hypotension.
Saphenous nerve = terminal branch of FEMORAL nerve (not sciatic). Only below-knee nerve not from sciatic. Block at adductor canal - preserves quadriceps strength (unlike femoral block). Provides medial leg/ankle/foot sensation.
Anterior spinal artery: single vessel, anterior 2/3 of cord (motor + spinothalamic). Artery of Adamkiewicz: T9-T12 (usually left), major supply to lower cord. Anterior cord syndrome: motor loss + pain/temp loss, proprioception preserved.
Suprascapular nerve (C5-C6) = ~70% shoulder joint sensation + supraspinatus/infraspinatus motor. Alternative to interscalene block for shoulder analgesia WITHOUT phrenic palsy. Passes through suprascapular notch. Combine with axillary nerve block for ~90% shoulder coverage.
Stellate ganglion = C7/neck of first rib (anterior). Block performed at C6 tubercle level. Successful block → Horner's syndrome. Risks: vertebral artery injection (seizures), RLN block, phrenic palsy, pneumothorax.
ADH primary stimulus = ↑osmolality (hypothalamic osmoreceptors, sensitive to 1-2% change). Secondary = ↓volume/BP (baroreceptors, override osmolality in severe hypovolaemia). Surgical stress → ↑ADH. SIADH = inappropriate release regardless of osmolality.
[HCO₃⁻]:[CO₂] = 20:1 at pH 7.40. Henderson-Hasselbalch: pH = 6.1 + log([HCO₃⁻]/0.03 × PaCO₂). The 20:1 ratio is maintained by respiratory (CO₂) and renal (HCO₃⁻) compensation. Both systems work to preserve this ratio.
Child-Pugh = 5 parameters: Bilirubin, Albumin, PT/INR, Ascites, Encephalopathy (mnemonic: BAP-AE). Creatinine is in MELD, not Child-Pugh. Class C mortality ~80% for abdominal surgery. MELD = bilirubin + INR + creatinine ± Na⁺.
Stress response → cortisol → lymphopenia, ↓cell-mediated immunity, Th1→Th2 shift → ↑infection risk. Proportional to surgical trauma. Hypothermia further impairs immunity. Normothermia reduces SSI by ~20%. Epidural attenuates immune suppression.
Adductor canal = sartorius (roof) + vastus medialis (lateral) + adductor longus/magnus (medial). Contains saphenous nerve, nerve to vastus medialis, femoral vessels. ACB = primarily sensory block with preserved quadriceps strength - preferred over femoral block for TKA.
Ankle block = 5 nerves: Tibial, Deep peroneal, Superficial peroneal, Saphenous, Sural. 3 deep blocks + 2 subcutaneous ring blocks. Provides complete foot anaesthesia for foot surgery when combined. All are terminal branches of sciatic (4) and femoral (saphenous, 1).
Iliohypogastric nerve (T12-L1) = suprapubic skin + lateral buttock. Ilioinguinal nerve (L1) = inguinal skin + medial thigh + scrotum/labia. Both in TAP (between internal oblique + transversus abdominis). Block for inguinal hernia analgesia. Can be blocked with TAP or specific nerve block near ASIS.
GH ↑ during stress: ↑lipolysis, ↑insulin resistance, protein-sparing. 'Anti-insulin' for glucose and fat, but anabolic for protein. Secreted in pulsatile pattern (GHRH stimulates, somatostatin inhibits). Acromegaly: difficult airway + insulin resistance.
CSE needle-through-needle: spinal needle protrudes ~12-15 mm beyond Tuohy. Combines rapid spinal onset with epidural flexibility. Common for caesarean and labour. Risk: untested epidural catheter, potential intrathecal LA migration via dural hole.
First-pass = gut wall + liver metabolism before systemic circulation → ↓oral bioavailability. Bypass routes: sublingual, transdermal, parenteral. Liver cirrhosis → ↑bioavailability of high-extraction drugs (portosystemic shunts).
CPP = MAP − ICP; target 60-70 mmHg in TBI (NICE NG232/Brain Trauma Foundation); autoregulation operates over CPP 50-150 mmHg and is often lost after injury.
Dorsal columns: fine touch, vibration, proprioception, 2-point discrimination. Ascend ipsilaterally (gracilis = lower body medial, cuneatus = upper body lateral). Cross in medulla (internal arcuate fibres → medial lemniscus). Synapse: nucleus gracilis/cuneatus → VPL thalamus → S1 cortex.
SAP block = superficial (between latissimus dorsi and serratus anterior) or deep (between serratus anterior and ribs/intercostal muscles). Blocks lateral cutaneous branches of intercostal nerves (T2-T9). Applications: rib fractures, breast surgery, VATS.
ESP block = LA between erector spinae muscle and transverse process. Transverse process = safe bony endpoint. LA spreads to reach ventral rami via costotransverse foramina. Applications: thoracic, breast, abdominal surgery, rib fractures. Safer than paravertebral block.
GA lowers vasoconstriction threshold from ~36.5°C to ~34-34.5°C (2-3°C drop). All agents contribute dose-dependently. Neuraxial: sympathetic block + altered afferent input → hypothalamus 'thinks' blocked area is warm. Active warming essential from induction.
PTH = ↑Ca²⁺ (bone resorption + renal reabsorption + ↑vitamin D activation → gut absorption). Also ↓phosphate (phosphaturic). Post-thyroidectomy hypocalcaemia: check Ca²⁺ 6-hourly, watch for QT prolongation.
PAG → NRM (serotonin) + LC (noradrenaline) → dorsal horn inhibition via dorsolateral funiculus. Endogenous opioids also involved. Opioids work partly by activating this system. TCAs/SNRIs enhance descending inhibition. PAG = rich in mu opioid receptors.
Adult narrowest airway = vocal cords (glottis). Paediatric narrowest = cricoid cartilage (complete ring, subglottic). This explains why cuffed tubes are sized for the glottis in adults. In children <8y, the cricoid limits tube size.
PGE₂ = key mucosal protector: ↑mucus, ↑bicarbonate, ↑blood flow, ↓acid. NSAIDs → ↓PGE₂ → ulceration. COX-2 selective inhibitors spare COX-1 → less GI damage. PPIs or misoprostol for gastroprotection.
SIADH = hyponatraemia + concentrated urine + euvolaemia + UNa >30 + normal thyroid/adrenal. Common postoperatively. Treatment: fluid restriction (mild), hypertonic saline (severe). Correct Na⁺ ≤10 mmol/L/24h (risk of osmotic demyelination).
Cricothyroid membrane = between thyroid cartilage (above) and cricoid cartilage (below). Site for emergency cricothyroidotomy. Incise lower half to avoid cricothyroid arteries. DAS 2015: scalpel-bougie-tube technique.
Subgluteal sciatic = between gluteus maximus (superficial) and quadratus femoris (deep). More superficial than classic posterior approach. Nerve appears as flat hyperechoic structure ~1-2 cm wide. Provides both tibial and common peroneal coverage.
Obturator nerve divides into anterior and posterior branches separated by ADDUCTOR BREVIS. Anterior branch = superficial to adductor brevis; posterior branch = deep to adductor brevis. US-guided block targets both branches around adductor brevis. Hip and knee joint innervation.
Vagus (CN X) = 75% of parasympathetic outflow. Innervates heart, lungs, GI tract to splenic flexure. Exits jugular foramen. Vagal reflexes: oculocardiac, laryngoscopy-induced bradycardia. Treated with anticholinergics.
Space of Disse = perisinusoidal space between hepatocytes and fenestrated endothelium. Allows plasma-hepatocyte exchange. Contains stellate cells. In cirrhosis: capillarisation (collagen deposition, loss of fenestrae) → ↓exchange, ↑resistance.
Angiotensin II = primary aldosterone stimulus (AT₁ receptor → Ca²⁺ → aldosterone synthase). Second stimulus: hyperkalaemia (direct). ACTH = minor role. ANP inhibits aldosterone. ACEi/ARBs → ↓aldosterone → ↑K⁺ risk.
Lateral cutaneous nerve of thigh = L2-L3, pure sensory. Passes near ASIS under inguinal ligament. Meralgia paraesthetica = entrapment syndrome (lateral thigh pain/numbness). Blocked by fascia iliaca or specific ASIS-level block.
T4 = nipple, T6 = xiphisternum, T8 = costal margin, T10 = umbilicus, T12 = suprapubic. For caesarean: need T4 block. These landmarks are tested at every spinal/epidural assessment.
Vasa recta = countercurrent exchangers (passive) - maintain medullary gradient while providing blood supply. U-shaped, parallel to loops of Henle. Low medullary blood flow (~10% of RBF) preserves gradient. High flow (osmotic diuretics) washes out gradient.
Ulnar nerve = all intrinsic hand muscles EXCEPT LOAF (median nerve). LOAF = Lumbricals 1-2, Opponens pollicis, Abductor pollicis brevis, Flexor pollicis brevis. Ulnar injury → claw hand. Complete hand block needs ulnar + median + radial.
WHO ladder Step 1 = non-opioid (paracetamol, NSAID) ± adjuvant. Step 2 = add weak opioid. Step 3 = switch to strong opioid. Modern practice: multimodal analgesia (combine classes) rather than strict stepwise escalation. Always use non-opioids as the foundation.
Countercurrent multiplication: small single effect (TAL NaCl transport → ~200 mOsm/kg horizontal gradient) multiplied into large axial gradient (300→1200 mOsm/kg) by countercurrent flow arrangement. Descending limb: water exits. Ascending limb: NaCl exits. Continuous flow amplifies gradient.
Wind-up: progressive ↑dorsal horn firing to repeated C-fibre stimulation (>0.3 Hz). Mechanism: temporal summation of slow EPSPs (substance P/NK1) → Mg²⁺ expulsion from NMDA → Ca²⁺ influx. Short-term, homosynaptic. Gateway to central sensitisation (longer-lasting, heterosynaptic).
DRG: cell bodies of all primary afferent sensory neurones (including nociceptors). Located in intervertebral foramen. Pseudo-unipolar neurones.
Sugammadex dosing: 2 mg/kg (moderate block), 4 mg/kg (deep block), 16 mg/kg (immediate reversal after RSI). For failed RSI with rocuronium → 16 mg/kg sugammadex restores breathing in ~1.5-3 minutes.
Drugs modulate voltage-gated channels by pore block or altered gating - local anaesthetics show state-dependent block (higher affinity for open/inactivated states), explaining use-dependent selectivity for actively firing nerves.
Mivacurium = only non-depolarising NMBA metabolised by plasma cholinesterase. Short duration (~15-20 min). Prolonged block in atypical cholinesterase. Also suxamethonium uses plasma cholinesterase. Not reversed by sugammadex.
Neostigmine = reversible anticholinesterase → increases ACh at both nicotinic (desired) and muscarinic (undesired) receptors - always co-administer an anticholinergic agent.
Protamine: basic protein binds acidic heparin → inactive complex. Dose: 1 mg per 100 units UFH. Give slowly IV. Adverse effects: hypotension, anaphylaxis (fish allergy, vasectomy), pulmonary hypertension, heparin rebound. Excess protamine = anticoagulant. Only ~60% LMWH reversal. No DOAC reversal.
Glycopyrrolate = quaternary ammonium muscarinic (M₂) antagonist - no CNS effects, well-matched onset with neostigmine, 200 mcg per 1 mg neostigmine.
In severe cirrhosis, hepatic clearance falls, bioavailability rises, and Vd increases - renal function typically worsens (hepatorenal syndrome), not improves. Phase II metabolism is better preserved than phase I.
Rivaroxaban → neuraxial: wait ≥22-26 hrs (AAGBI). Key DOAC intervals: rivaroxaban 22-26 hrs, apixaban 26-30 hrs, dabigatran varies with CrCl. Next DOAC dose ≥6 hrs after needle/catheter removal.
Weak bases are trapped in acidic compartments (ionised and cannot cross back); weak acids are trapped in alkaline compartments - this explains fetal drug accumulation, reduced LA efficacy in infection, and urinary alkalinisation for aspirin overdose.
Dexmedetomidine: most evidence for ↓ICU delirium. MENDS trial: dex > lorazepam. SEDCOM trial: dex > midazolam. Mechanisms: natural sleep preservation, no GABA-A risk, opioid-sparing. Benzodiazepines: independent delirium risk factor - AVOID. Haloperidol: no benefit (MIND-USA, AID-ICU).
Serotonin syndrome triad: mental status changes + autonomic instability + neuromuscular excitability (CLONUS is key). Treat: stop drug, cool, benzodiazepines, cyproheptadine. Distinguished from NMS by rapid onset and clonus.
Empirical sepsis antibiotics: broad-spectrum within 1 hour → de-escalate at 48-72h when cultures available. Cover likely pathogens based on: source, local resistance, patient risk factors. Each hour delay ↑mortality.
Intrinsic activity (α): full agonist = 1, partial agonist = between 0 and 1, antagonist = 0. It measures the fraction of maximal response a drug can produce relative to a full agonist in that tissue.
Early LAST = circumoral tingling, tinnitus, metallic taste, visual disturbance (CNS excitation from loss of inhibitory pathways). Progression: CNS excitation → seizures → depression → cardiovascular collapse. Bupivacaine is most cardiotoxic.
Palonosetron = longest-acting 5-HT₃ antagonist (t½ ~40 hrs, cover up to 5 days). Unique: receptor internalisation + allosteric inhibition. Single 250 mcg dose for CINV. 75 mcg for PONV.
Method of residuals - terminal-line back-extrapolation gives intercept B and slope β; subtracting the terminal line from early data gives the residual line with intercept A and slope α; C₀ = A + B and V₁ = Dose/(E + B).
Phenobarbital = ↑duration of GABA-A Cl⁻ channel opening (+ direct activation at high doses). Benzodiazepines = ↑frequency. Barbiturates are more dangerous in overdose due to direct activation capability.
ED95 = dose producing 95% twitch suppression. Standard intubating dose = 2× ED95. RSI dose of rocuronium = 1.0-1.2 mg/kg (3-4× ED95) for rapid onset comparable to suxamethonium.
Ticagrelor = reversible, direct P2Y₁₂ inhibitor (no CYP activation needed). Stop 3-5 days pre-surgery (vs 5-7 for clopidogrel). BD dosing. Side effects: dyspnoea (adenosine-mediated), ventricular pauses.
Therapeutic Mg for eclampsia prophylaxis = 2.0-3.5 mmol/L (2-4× normal). Loading 4g IV + maintenance 1g/h for 24h. Monitor: reflexes, RR, urine output, serum Mg levels. Toxicity progression: loss of reflexes → respiratory depression → cardiac arrest.
Carbamazepine = use-dependent Na⁺ channel blockade (binds inactivated state). Per NICE NG217, alternative (no longer first-line) for focal seizures - lamotrigine/levetiracetam preferred. WORSENS absence/myoclonic. Autoinduction, potent enzyme inducer, SJS risk (HLA-B*1502).
Extraction ratio = CL hepatic / Q hepatic. High ER (>0.7) = flow-dependent clearance; low ER (<0.3) = enzyme-dependent clearance; intermediate ER = affected by both. Oral bioavailability ≈ 1 - ER.
PADIS (2018): Pain, Agitation/sedation, Delirium, Immobility, Sleep. Key: analgo-sedation approach, light sedation (RASS 0 to -2), dexmedetomidine/propofol over benzodiazepines, CAM-ICU delirium screening, early mobilisation, sleep hygiene. Integrated bundle approach (ABCDEF). Standard of ICU care.
Neonates need higher mg/kg loading doses (large Vd from high body water) but lower maintenance doses (immature hepatic metabolism and reduced renal clearance).
Hofmann degradation is spontaneous organ-independent chemical decomposition at body pH and temperature - atracurium and cisatracurium are the key drugs, making them safe in hepatic and renal failure.
Stagnara wake-up test: lighten TIVA → patient follows commands (move hands/feet) → confirms motor pathway integrity during spinal surgery. TIVA ideal: propofol + remifentanil short CSHT → predictable wake-up (10-20 min). Reduce propofol Ce to ~1.5-2 μg/mL. Stop NMBAs. Prepare for movement/coughing.
IV administration gives the fastest onset with 100% bioavailability - arm-brain circulation time (~30-40 seconds) determines CNS drug onset, which is why IV induction agents work within one arm-brain circulation time.
MAC is strictly additive between volatile agents - 0.5 MAC of one + 0.5 MAC of another = 1.0 MAC total. This contrasts with propofol + remifentanil which is synergistic (supra-additive).
High extraction ratio = flow-dependent clearance (propofol, morphine); low extraction ratio = enzyme/binding-dependent clearance (phenytoin, warfarin).
Dabigatran: direct thrombin (IIa) inhibitor. Prodrug, low bioavailability (6-7%). t½ 12-17h. 80% renal excretion - contraindicated CrCl <30. Reversal: idarucizumab (Praxbind) 5g IV - binds dabigatran 350× > thrombin affinity. RE-VERSE AD trial. Also dialysable. Moisture-sensitive capsules.
IM absorption is primarily determined by local blood flow - exercise and warmth increase absorption; shock, hypothermia, and co-administered vasoconstrictors decrease it.
Therapeutic index = LD50/ED50 - a larger value means greater safety margin. Drugs with narrow TI (digoxin, lithium, phenytoin) require therapeutic drug monitoring.
Dibucaine number 20 = homozygous atypical cholinesterase → suxamethonium block lasting 2-4 hours. Always maintain ventilation and refer for family screening.
Context-sensitive half-time varies with infusion duration - remifentanil is context-insensitive (~3 min always), while fentanyl is highly context-sensitive, making remifentanil preferable for prolonged infusions.
It takes 5 half-lives to reach ~97% of steady-state concentration - the rate of approach depends solely on the half-life, not the infusion rate.
Codeine requires CYP2D6 activation to morphine - poor metabolisers get no analgesia, ultra-rapid metabolisers risk fatal respiratory depression (especially children). MHRA safety alert applies.
Sugammadex advantage over neostigmine = can reverse DEEP block (any depth including immediately post-dose at 16 mg/kg). Neostigmine only works at moderate block (T2 visible). Sugammadex only works for rocuronium/vecuronium, NOT atracurium.
Alfentanil vs fentanyl: smaller Vd (0.4-1 vs 4 L/kg), lower pKa (6.5 vs 8.4 → 89% vs 9% unionised), shorter t½ (90 min vs 4 h), lower extraction ratio (more susceptible to CYP3A4 inhibitors), faster onset.
Amide LAs = hepatic metabolism (CYP1A2 for lidocaine, CYP3A4 for bupivacaine). Ester LAs = plasma cholinesterase hydrolysis. Amide metabolism rate: prilocaine > lidocaine > bupivacaine. Hepatic impairment increases amide LA toxicity risk.
Non-competitive antagonism reduces Emax insurmountably - recovery requires new receptor synthesis. Phenoxybenzamine is the classic irreversible non-competitive antagonist used in phaeochromocytoma management.
Suxamethonium = depolarising block (phase I): fasciculations → paralysis. No TOF fade. Not reversed by neostigmine. Phase II block develops with prolonged/repeated exposure and behaves like non-depolarising block.
Right-to-left shunt: SLOWS gas induction (blood bypasses lungs), SPEEDS IV induction. Left-to-right shunt: minimal effect on gas induction, SLOWS IV induction. Opposite effects on inhalational vs intravenous induction.
Eleveld model = general-purpose allometric propofol model for ALL ages (neonates to elderly). Uses age, weight, height, sex, and opioid co-administration. May eventually replace Marsh/Schnider as a single universal model.
Hill coefficient determines dose-response curve steepness - nH = 1 (no cooperativity), nH > 1 (positive cooperativity, steep curve), nH < 1 (negative cooperativity, shallow curve). Higher nH means a narrower dose range between no effect and full effect.
Idarucizumab = specific dabigatran reversal (Fab fragment, 350× affinity vs thrombin). 5 g IV. Andexanet alfa = Xa inhibitor reversal. Dabigatran is dialysable (35% protein bound). Stop 24-48 hrs pre-surgery.
Clopidogrel: thienopyridine prodrug → CYP2C19 activation → irreversible P2Y₁₂ ADP receptor blockade. Effect: 7-10 days. Stop 7 days before neuraxial/surgery. CYP2C19 poor metabolisers: ↓effect (pharmacogenomics). DAPT mandatory after coronary stents. Platelet transfusion can partially overcome.
Doxycycline = tetracycline, reversible 30S binding, bacteriostatic. Long t½ (~18-22 hrs), oral. Safe in renal failure (unlike other tetracyclines). Avoid: dairy/antacids (chelation), pregnancy, children <12.
LAs bind primarily to AAG (acute phase protein). High AAG (surgery, trauma) → less free drug → less toxic. Low AAG (pregnancy, neonates, liver disease) → more free drug → higher toxicity risk. Protein binding: bupivacaine 95% > lidocaine 65% > prilocaine 55%.
Morphine oral bioavailability ≈ 30% (first-pass glucuronidation). Oral:parenteral ratio = 3:1. Bioavailability increases in liver cirrhosis (portosystemic shunting). Always use conversion ratios when switching routes.
All volatiles: ↓tidal volume + ↑respiratory rate (compensatory but inadequate) → ↑PaCO₂. Depress response to both CO₂ and O₂. All cause bronchodilation. Depress mucociliary clearance. Hypoxic drive depression persists into recovery.
Polypharmacy: ≥5 drugs common in elderly (average 5-8). Drug interaction risk increases exponentially with drug number. Continue: beta-blockers, statins, antiepileptics, steroids, levodopa. Withhold: ACEi/ARBs, diuretics, oral hypoglycaemics. Thorough preoperative medication review essential.
HIT-II = IgG anti-heparin-PF4 complex → platelet activation → thrombosis + thrombocytopenia. Day 5-14 (or <24 h with recent exposure). 4Ts score (BSH 2012); STOP heparin; switch to argatroban/fondaparinux; NO warfarin until platelets recover; NO platelet transfusion.
Sevoflurane for gas induction = non-irritant + low blood-gas coefficient (~0.65) → smooth, rapid induction. It has a relatively high MAC, no real analgesia, still depresses the cardiovascular system, and can cause emergence delirium.
All volatiles relax the uterus dose-dependently. During GA caesarean section: keep volatile ≤0.75 MAC to minimise bleeding. Therapeutic use: uterine relaxation for retained placenta, inversion, EXIT procedures.
Cyclizine = H₁ antihistamine + antimuscarinic → effective for motion sickness and vestibular nausea. 50 mg IV slowly. No EPS risk. Caution in heart failure (negative inotrope). Causes sedation and dry mouth.
Chlorhexidine: increasingly recognised perioperative allergen. NAP6: ~9% of perioperative anaphylaxis (3rd most common). Ubiquitous in healthcare (skin prep, CVC dressings, oral care, catheters). Sensitisation often unrecognised.
Gabapentin binds α₂δ subunit of VGCCs → ↓Ca²⁺ entry → ↓excitatory neurotransmitter release. NOT a GABA drug despite its name. UK Class C controlled substance since April 2019.
Midazolam has a long, context-sensitive half-time that increases dramatically with prolonged infusion - avoid for long-term ICU sedation due to unpredictable accumulation, especially with organ dysfunction.
Aminophylline = non-selective PDE inhibitor → ↑cAMP → bronchodilatation. Narrow therapeutic index (10-20 mg/L). Monitor levels. Loading dose 5 mg/kg IV over 20 min.
Clavulanic acid = suicide β-lactamase inhibitor (minimal own antibiotic activity). Protects amoxicillin from enzymatic destruction. Co-amoxiclav = widely used for surgical prophylaxis (NICE). Cannot overcome all β-lactamase types.
Opioids cause GI effects via enteric mu receptors - decreased peristalsis, increased sphincter tone, delayed gastric emptying, sphincter of Oddi spasm. Constipation does not develop tolerance. Always co-prescribe laxatives.
Angiotensin II (Giapreza): AT1 receptor → vasoconstriction. Third vasopressor axis (after catecholamines and vasopressin). For catecholamine-refractory vasodilatory shock. ATHOS-3 trial (2017): improved MAP response + ↓catecholamine doses. Dose: 20 ng/kg/min (range 1.25-80). t½ ~30s.
Thiopentone reduces CMRO₂ maximally (burst suppression) but profound cardiovascular depression may worsen cerebral perfusion. RCTs show no benefit for routine neuroprotection post-cardiac arrest. Role limited to refractory ICP elevation.
Insulin lowers K⁺ by stimulating Na⁺/K⁺-ATPase → drives K⁺ INTO cells (transcellular shift, not excretion). Emergency hyperkalaemia treatment: insulin 10 IU + glucose. Lowers K⁺ by 0.5-1.2 mmol/L in 15-30 min. Effect is temporary (4-6h).
Hypoalbuminaemia increases the free fraction of highly protein-bound drugs - clinical significance depends on the drug's hepatic extraction ratio.
TCI: computer-controlled infusion targeting drug concentration. Marsh model: weight-based, Cp targeting. Schnider model: age/weight/height/LBM, Ce targeting. Ce = effect-site (brain) concentration. keo: plasma-effect site equilibration constant.
Equipotential bonding = all conductive surfaces at same potential. No potential difference = no current flow through patient/staff. Connected to common earth bar. <10 mV allowed between surfaces. Required by BS 7671 and HTM 06-01. Regular testing essential.
Fick principle: CO = VO₂ / (CaO₂ - CvO₂). Requires oxygen consumption measurement + arterial and mixed venous blood gases. Normal: CO = 250 / (200-150) = 5 L/min. Gold standard but impractical for routine use. CvO₂ requires PA catheter sample. Reverse Fick uses CO₂.
PPV >13% = fluid responsive (in mechanically ventilated patients). PPV = (PPmax - PPmin) / PPmean × 100%. Requires: controlled ventilation, TV ≥8 mL/kg, sinus rhythm, closed chest. Superior to CVP/PAWP for fluid responsiveness. PLR test for spontaneously breathing patients.
SpO₂ 90% ≈ PaO₂ 8 kPa (60 mmHg) - critical threshold, steep curve below. On steep part: small ΔPaO₂ → large ΔSpO₂ (pulse ox sensitive to hypoxaemia). On flat part: large ΔPaO₂ → small ΔSpO₂ (pulse ox insensitive to hyperoxia). P50 = 3.5 kPa at 50% saturation.
Raman scattering: laser light → molecule interaction → frequency-shifted scattered light. Unique spectrum per gas. Can measure ALL gases including O₂ and N₂ (unlike IR). Rarely used clinically (expensive, weak signal). IR + paramagnetic = standard clinical combination.
Synchronised cardioversion = shock on R wave, avoids T wave (vulnerable period). Prevents R-on-T → VF. Used for organised rhythms (AF, SVT, stable VT). Unsynchronised = defibrillation (VF, pulseless VT). Sync mode may reset after each shock. Energy: lower than defibrillation doses.
Critical temperature: above this, gas cannot be liquefied by pressure alone. O₂ critical temp = -118°C (always gas in cylinder, pressure ∝ contents). N₂O critical temp = 36.5°C (liquid+vapour at room temp, pressure constant until all liquid gone → must weigh cylinder). CO₂ critical temp = 31°C.
Phlebostatic axis: mid-axillary line at 4th ICS (supine patient) ≈ right atrial level. Reference for zeroing all pressure transducers. 1 cm height error ≈ 0.74 mmHg (ρgh). Must re-level and re-zero after repositioning. In lateral position, use sternal mid-point.
Diathermy EMI → pacemaker interprets as cardiac activity → inhibits output (VVI mode). Dangerous in pacemaker-dependent patients (asystole). Prevention: dispersive pad far from generator, bipolar preferred, short bursts. Magnet converts to asynchronous mode (VOO/DOO).
Pulse oximetry: 660 nm (red) + 940 nm (infrared). R ratio = AC/DC at each wavelength. R=1 → SpO₂ 85%. COHb → falsely high. MetHb → trends to 85%. Only 2 wavelengths = only detects HbO₂ and Hb. Insensitive to hyperoxia. Does not measure PaO₂.
Galvanic fuel cell: self-generating (no external voltage), gold cathode + lead anode + KOH electrolyte. Current ∝ PO₂. Lead anode consumed → finite lifespan (6-12 months). Used in breathing circuits for FiO₂ monitoring. Slower response than paramagnetic. Store sealed when not in use.
Type CF = cardiac floating = direct cardiac use. Leakage limits: 10 μA normal, 50 μA single fault. Type B = 500 μA, Type BF = 100 μA, Type CF = 10 μA (normal conditions). 'F' = floating (isolated from earth). Equipment with intracardiac connections must be type CF. IEC 60601-1.
Monophasic = single direction current, typically 360 J. Biphasic = bidirectional current in two phases, 150-200 J. Biphasic is superior: lower energy, less myocardial damage, higher success rate. Energy = ½CV². Most modern defibrillators are biphasic. Shock duration in milliseconds, not seconds.
Magnet on ICD = suspends shock/ATP therapy, preserves bradycardia pacing. Magnet on pacemaker = asynchronous mode. Not all ICDs respond to magnets - confirm preoperatively. External defibrillation must be available. Reactivate ICD postoperatively. Device interrogation pre- and post-surgery.
ODM = Doppler velocity in descending aorta via oesophageal probe. SV = VTI × CSA (nomogram). CO = SV × HR. FTc reflects preload (~330-360 ms). Measures ~70% of CO. Operator-dependent. Contraindicated: varices, strictures. NICE MTG3 (2011) recommends ODM for fluid optimisation in major surgery.
BIS = processed EEG → dimensionless number 0-100. Target for GA: 40-60. Uses bispectral analysis + burst suppression ratio. May reduce awareness risk. Limitations: EMG interference (falsely high), ketamine (falsely high), electrocautery.
Clark electrode = polarographic O₂ sensor. Platinum cathode (-0.6V), silver anode, KCl electrolyte, O₂-permeable membrane. Current ∝ PO₂. In blood gas analysers. Consumes O₂. Severinghaus electrode = CO₂ (modified pH electrode).
Lead II: best for P waves (rhythm) + inferior ischaemia. Along cardiac axis. Leads II + V5 detect ~80% of ischaemia. Add V4 → 96%. Inferior leads: II, III, aVF. Lateral: I, aVL, V5, V6. Anterior: V1-V4. CM5 configuration approximates V5 with 3 electrodes.
Phase I (depolarising) = no fade, no PTC facilitation, uniform twitch reduction, preceded by fasciculations. Phase II (dual block) = fade, PTC facilitation, similar to non-depolarising block. Phase II after prolonged/high-dose suxamethonium (>5 mg/kg).
Fast flush test: assesses damping and natural frequency. Optimal: 1-2 oscillations after square wave. Underdamped: >2 oscillations, overestimates systolic. Overdamped: no oscillations, underestimates systolic. MAP most accurate. Common overdamping causes: air bubbles, clots, kinks.
Thermodilution: cold saline via proximal PA catheter port → thermistor at tip measures temperature change. Modified Stewart-Hamilton equation. Large area under curve = low CO. Small area = high CO. Average 3 measurements. Errors: TR, shunts, arrhythmias, injectate inconsistency.
Pulse contour = CO from arterial waveform analysis. SV ∝ area under systolic curve (corrected for compliance). Beat-to-beat, continuous monitoring. Calibrated (PiCCO, LiDCOplus) more accurate than uncalibrated (FloTrac). Requires good arterial waveform.
Antistatic measures prevented ignition of flammable anaesthetics (ether, cyclopropane) by static sparks. Conductive flooring, footwear, rubber, humidity >50%. Largely historic since non-flammable agents became standard.
Diathermy uses 300 kHz-3 MHz (radiofrequency range). Above ~100 kHz, neuromuscular stimulation ceases. Heating effect (P = I²R) predominates. Current density determines tissue effect - high at active electrode, low at dispersive. RF interference with monitoring and pacemakers is a recognised hazard.
Oscillometry: MAP = maximum oscillation amplitude (most accurate parameter). Systolic and diastolic derived by algorithms (less accurate). Cuff width ~40% arm circumference. Too-small cuff → overestimates BP. Too-large → underestimates. Errors: arrhythmias, movement, obesity, extreme BP.
TOE: piezoelectric ultrasound transducer in oesophagus, 3-7 MHz. Superior cardiac imaging (close to heart, no lung/chest wall). Assesses: valves, function, volume, air, thrombus, dissection. Essential in cardiac surgery. Complications: oesophageal perforation/trauma (rare).
Dispersive pad burn = poor contact → high current density → local heating. Causes: detachment, folding, bony prominence, scar tissue, hair, prep fluid pooling. REM system monitors impedance and disables generator if contact is poor. Place on well-vascularised, hairless, flat skin near surgical site.
Monopolar = active electrode (small, surgical site) → through patient → dispersive electrode (large, return pad). High current density at active electrode = heating. Low current density at dispersive = minimal heating. REM system monitors dispersive electrode contact.
Earth leakage = current flowing to earth during normal operation via capacitative coupling and insulation leakage. Must be limited: <500 μA (type B) to <10 μA (type CF). Broken earth → leakage may flow through patient. RCD detects imbalance and disconnects supply at 30 mA.
E = ½CV². Typical values: C ≈ 32 μF, V ≈ 5000 V → 400 J stored. Delivered energy < stored energy (impedance losses). Transthoracic impedance ~70-80 Ω. Inductor shapes the waveform. Reduce impedance: good pad contact, correct size, conductive gel, expiration.
Capacitative coupling = current transfers across intact insulation (dielectric) to adjacent conductor. Xc = 1/2πfC - low reactance at high frequency allows current flow. Dangerous with hybrid trocars (metal + plastic). Prevention: all-metal or all-plastic trocars, AEM systems.
RCD detects live-neutral current imbalance (leakage to earth). Trips at 30 mA in 30 ms. Uses differential transformer. Protects against macroshock but NOT microshock (100 μA << 30 mA trip threshold). Does not replace need for type CF equipment in cardiac patients. Also called ELCB or GFCI.
Let-go threshold = 10-15 mA at 50 Hz. Above this, victim cannot release conductor → prolonged exposure. 1 mA = perception, 5 mA = pain, 10-15 mA = let-go, 50-100 mA = VF, >1 A = burns/asystole. Women ~10 mA, men ~15 mA. These values underpin equipment safety standards.
Double square symbol = Class II = double insulated, no earth needed. Two independent insulation layers. Two-core cable only. Does not rely on earth integrity. Practical for smaller equipment. Class I = earth, Class II = insulation, Class III = low voltage. IEC 61140 / BS EN 60601-1.
Current density (I/area) determines tissue effect, not total current. Active electrode: small area → high density → cutting/coagulation. Dispersive: large area → low density → no effect. >100°C = vaporisation (cutting). 60-100°C = denaturation (coagulation).
Δf = 2f₀v cos θ / c. Doppler shift ∝ velocity × cos(angle) × transmitted frequency. Maximum shift at 0° (parallel to flow). No shift at 90°. Angle >60° = significant error. Positive shift = towards transducer. Oesophageal Doppler minimises angle. Double Doppler = factor of 2.
Mainstream: sensor at airway, fast response (<100 ms), no sampling delay, adds dead space/weight, CO₂ only, heated sensor.
Doppler: Δf = (2f₀ × v × cos θ) / c. cos θ is greatest error source. θ = 0° → accurate. θ = 60° → only 50% detected. θ = 90° → no signal. Keep angle <60° (ideally <20°). Oesophageal Doppler assumes ~45-60° angle. Velocity underestimation → gradient underestimation (ΔP = 4v²).
Standard site: ulnar nerve at wrist → adductor pollicis (thumb adduction). Negative electrode distal. Supramaximal stimulation (30-70 mA). Free thumb movement needed for AMG. Facial nerve/orbicularis oculi: recovers earlier (more resistant to NMB) - may overestimate recovery.
Zeroing: open stopcock to air at phlebostatic axis (mid-axillary, 4th ICS) → set to zero. Establishes atmospheric reference. 1 cm height error ≈ 0.74 mmHg pressure error. Factory-calibrated disposable transducers → only zeroing needed. Silicon strain gauge (Wheatstone bridge).
Biphasic = lower energy needed (150-200 J vs 360 J monophasic). Higher first-shock success. Less myocardial damage. Recommended as standard by Resuscitation Council UK. Two phases: current flows forward then reverses. BTE and RLB are the two main biphasic waveform types.
Transthoracic impedance ~70-80 Ω. Reduce with: correct pad size, gel/adhesive pads, firm pressure, shaved skin, expiration. Self-adhesive pads preferred for safety and consistent contact. Lower impedance = more energy to myocardium. Air (inspiration) increases impedance.
Metal implants = low-resistance pathway for monopolar current. Risk of thermal injury at tissue-metal interface. Position dispersive pad to avoid current crossing implant. Bipolar preferred (localised current). Lowest power, short bursts. Longer implants = greater risk.
O₂ is paramagnetic (2 unpaired electrons → attracted to magnetic field). Most other gases diamagnetic. Paramagnetic analyser: measures O₂ in breathing circuit. Types: dumbbell magnetometer, differential pressure. Accurate, fast response. N₂O causes small negative error.
O₂ cannot be measured by IR (homonuclear diatomic, no dipole change). N₂ also cannot. Use paramagnetic or fuel cell for O₂. All volatile agents, CO₂, and N₂O are measurable by IR. Collision broadening: N₂O widens CO₂ absorption → potential overestimation.
Air bubbles = most common error → overdamping → underestimates systolic, overestimates diastolic (MAP OK). Other overdamping causes: clot, kink, long tubing, compliant tubing, multiple connectors. Underdamping → overestimates systolic.
Normal ETCO₂ = 4.5-5.5 kPa (35-45 mmHg). ETCO₂-PaCO₂ gradient normally 0.3-0.8 kPa. Low ETCO₂: hyperventilation, low CO, PE, cardiac arrest, disconnection. High ETCO₂: hypoventilation, ↑CO₂ production, rebreathing. Sudden loss to zero = disconnect/extubation/arrest. Sloping plateau = bronchospasm.
Oesophageal Doppler: ultrasound Doppler shift from descending aortic RBCs. Δf = (2f₀ × v × cosθ) / c. Measures: peak velocity (contractility), FTc (preload/afterload, normal 330-360 ms), SV, CO. FTc <330 = hypovolaemia. Only measures ~70% of CO. Minimally invasive, real-time.
PTC used when TOF = 0 (intense block). Method: 50 Hz tetanus × 5s → 3s pause → single twitches at 1 Hz. Count visible post-tetanic twitches. PTC 0 = very deep. First TOF response returns at PTC ~10-15. Mechanism: post-tetanic ACh mobilisation.
Best core temperature sites: tympanic membrane, nasopharynx, distal oesophagus, PA catheter (gold standard), bladder. Share hypothalamic blood supply. Axilla: intermediate (slow, may underestimate). Skin: peripheral. Core-peripheral gradient: >4°C = vasoconstriction, <2°C = vasodilation.
MetHb → SpO₂ trends to ~85% (R ratio → 1.0). If true SaO₂ >85% → SpO₂ falsely low. If true SaO₂ <85% → SpO₂ falsely high. COHb → SpO₂ falsely high (reads COHb as HbO₂). Anaemia → SpO₂ unaffected if Hb saturated (measures % saturation, not absolute O₂). Jaundice → minimal effect.
Macroshock = current through body surface (>100 mA for VF at 50 Hz). Microshock = current via intracardiac conductor (100 μA for VF). Current pathway through the heart is the critical danger. Equipment faults can make staff an inadvertent conductor to the patient.
Class I = basic insulation + protective earth. Earth carries fault current → fuse blows. Relies on intact earth connection. PAT testing checks earth continuity (<0.1 Ω). Green/yellow striped earth wire. Most large medical equipment is Class I. Three-pin plug: earth first on, last off.
Channelling = high current density through narrow tissue (pedicle, digit, penis). Current density = I/area - small area = high density = burn risk. Use bipolar for narrow structures. Position dispersive pad to avoid narrow pathway. Tourniquet does NOT protect.
Low CO = wide curve, large area under curve. High CO = narrow curve, small area. CO inversely proportional to area under curve. Modified Stewart-Hamilton equation. Tricuspid regurgitation causes biphasic curves and overestimates the area → underestimates CO. Average 3 measurements for accuracy.
Wet areas: skin resistance drops from ~100 kΩ to ~1 kΩ → much higher current for same voltage. Protection: 30 mA RCD + SELV + equipotential bonding. IPX-rated equipment for splash/immersion protection. BS 7671 requirements for special locations.
All-metal trocars: coupled current disperses safely to abdominal wall (large contact area). All-plastic: no coupling target. Hybrid (metal + plastic anchor) = dangerous: coupled current cannot disperse, concentrates at cannula tip → bowel burn. Active electrode monitoring (AEM) also reduces risk.
TOF ratio ≥0.9 = safe for extubation. T4/T1 ratio. Subjective fade detection unreliable below 0.4. Quantitative monitoring recommended (acceleromyography). Residual paralysis (TOF 0.5-0.9) commonly missed clinically. NAP5 and AAGBI recommend quantitative monitoring for all NMBA use.
Capnography uses infrared absorption at 4.26 μm (Beer-Lambert law). Normal ETCO₂ 4.5-5.5 kPa. Most reliable confirmation of intubation. ETCO₂ reflects cardiac output and ventilation. Phases: I (dead space), II (rising), III (plateau), 0 (inspiration). Mainstream vs sidestream designs.
Wheatstone bridge: 4 resistors in diamond, detects small resistance changes in strain gauge → voltage output ∝ pressure. Used in invasive pressure transducers. Balanced bridge = zero output. Pressure deforms diaphragm → strain gauge resistance changes → bridge unbalanced → voltage generated.
SVV = (SVmax - SVmin) / SVmean × 100%. >12-13% = fluid responsive. From pulse contour analysis (PiCCO, LiDCO, FloTrac). Requires: controlled ventilation, TV ≥8 mL/kg, sinus rhythm, closed chest. Similar to PPV (pulse pressure-based). Superior to CVP/PAWP.
Beer-Lambert law: A = εcl (absorbance ∝ concentration × path length). Transmitted intensity I = I₀e^(-εcl). Fundamental to: pulse oximetry, capnography, agent analysers, co-oximetry. Assumes: monochromatic light, dilute solution, no scattering. Deviations at high concentrations.
Acceleromyography: piezoelectric sensor on thumb measures acceleration ∝ force (F=ma). Most common quantitative NMB monitor. Ulnar nerve stimulation → thumb adduction. May overestimate TOF ratio vs gold standard (mechanomyography). Requires free thumb movement.
MAC = alveolar concentration preventing movement in 50% to skin incision. Measure of potency (lower = more potent). Additive. ↓by: age, hypothermia, opioids, pregnancy. ↑by: hyperthermia, chronic alcohol. MAC-awake ~0.3-0.5 MAC. MAC-BAR ~1.5 MAC.
Microshock threshold = 100 μA (0.1 mA) directly to myocardium → VF. Imperceptible to skin. At-risk patients: those with intracardiac conductors (CVCs, pacing wires, PA catheters). Type CF equipment limits leakage to <10 μA. 1000× less current needed than macroshock.
50-60 Hz = most dangerous for VF (mains frequency). VF threshold lowest at this frequency. DC less dangerous for VF but causes burns/electrolysis. Above 100 kHz, heating predominates and neuromuscular stimulation ceases - basis of diathermy.