Key Facts

Mechanical ventilation is the most common organ support provided in the ICU. Understanding ventilator physiology, modes, and settings is essential for all ICU practitioners. The shift to lung-protective strategies has significantly reduced mortality in ARDS.

Epidemiology

Approximately 50-60% of ICU patients require mechanical ventilation. Average duration of ventilation is 5-7 days. Prolonged ventilation (>21 days) affects approximately 5-10% of ventilated patients. VAP incidence is approximately 5-15 per 1,000 ventilator-days.

Aetiology

Indications for mechanical ventilation:

• Acute respiratory failure (type 1 or type 2)
• Airway protection (GCS <8)
• Post-operative (major surgery, failed extubation)
• Neuromuscular disease (Guillain-Barré, myasthenia crisis)
• ARDS, pneumonia, COPD exacerbation (failing NIV)
• Shock (reduce respiratory work, redirect O2 to vital organs)

Pathophysiology

Normal spontaneous breathing: Negative intrathoracic pressure (diaphragmatic contraction) draws air into lungs. Venous return enhanced by negative pressure.

Positive pressure ventilation: Reverses normal physiology — positive pressure inflates lungs. This reduces venous return (may cause hypotension), increases intrathoracic pressure, and can cause barotrauma. Ventilator-induced lung injury (VILI) results from volutrauma (overdistension), atelectrauma (cyclic collapse and recruitment), and biotrauma (inflammatory mediator release).

Ventilator Modes

• CMV (Controlled Mandatory Ventilation): Full ventilator support; patient passive (used with paralysis)
• SIMV (Synchronised Intermittent Mandatory Ventilation): Set rate + patient-triggered breaths between; supports weaning
• Pressure Support (PS): Patient triggers all breaths; ventilator augments with set pressure; used for weaning
• BIPAP/APRV: Airway pressure release ventilation — maintains high CPAP with brief releases for CO2 clearance
• High-Frequency Oscillatory Ventilation (HFOV): Rescue in refractory ARDS (OSCAR/OSCILLATE trials — no benefit)

Non-Invasive Ventilation

• CPAP: Continuous positive airway pressure — cardiogenic pulmonary oedema, OSA
• BiPAP: Bilevel — COPD exacerbation with respiratory acidosis (pH 7.25-7.35)
• High-flow nasal oxygen (HFNO): Optiflow at 30-60 L/min — hypoxic respiratory failure, pre-oxygenation

Red Flags

• High airway pressures (>30 cmH2O) — risk of barotrauma; check for obstruction, bronchospasm, pneumothorax
• Sudden desaturation on ventilator — use DOPE mnemonic: Displacement, Obstruction, Pneumothorax, Equipment failure
• Rising CO2 despite adequate minute ventilation — consider increased dead space (PE), metabolic acidosis, or shunt
• Fighting the ventilator — assess sedation, pain, tube position, trigger sensitivity

Bedside

• ABG: PaO2, PaCO2, pH, lactate — guide ventilator adjustments
• P/F ratio: PaO2/FiO2 — classifies ARDS severity and guides prone positioning
• Ventilator graphics: Pressure-volume and flow-time waveforms — detect auto-PEEP, patient-ventilator asynchrony
• Peak vs plateau pressure: Difference indicates airway resistance (bronchospasm, secretions)

Bloods

• ABG/VBG: Serial for monitoring ventilation and oxygenation
• FBC, CRP, procalcitonin: If VAP suspected
• Sputum culture: Before starting antibiotics for suspected VAP

Imaging

• CXR: ETT position, pulmonary pathology, pneumothorax, line positions
• CT chest: ARDS characterisation, abscess, empyema
• Lung USS: Increasingly replacing CXR — pleural effusion, consolidation, pneumothorax, B-lines

Special Tests

• Recruitment manoeuvres: Sustained inflation or stepwise PEEP increases to recruit collapsed alveoli
• Transpulmonary pressure monitoring: Oesophageal manometry — optimise PEEP in severe ARDS
• SBT (Spontaneous Breathing Trial): T-piece or low PS (5-8 cmH2O) for 30-120 min to assess readiness for extubation

Non-pharmacological

• Lung-protective ventilation: VT 6mL/kg IBW, Pplat <30, PEEP 5-15, FiO2 titrated to SpO2 88-95%
• Prone positioning: 16+ hrs/day for moderate-severe ARDS (P/F <150) — PROSEVA trial showed 50% mortality reduction
• Conservative fluid strategy: After initial resuscitation — reduces ventilator days (FACTT trial)
• VAP prevention bundle: Head-up 30-45°, daily sedation hold, oral care with chlorhexidine, peptic ulcer prophylaxis, DVT prophylaxis, assess readiness for extubation daily

Pharmacological

• Sedation: Propofol 0.3-4mg/kg/hr or midazolam 0.03-0.2mg/kg/hr; target light sedation (RASS 0 to -2) — DahLIA and SPICE III trials
• Analgesia: Fentanyl 25-200mcg/hr or morphine 1-5mg/hr — analgesia-first approach
• Neuromuscular blockade: Cisatracurium 0.15mg/kg/hr for early severe ARDS (ACURASYS trial — benefit; ROSE trial — no benefit with current light sedation practice)
• Inhaled nitric oxide: Rescue therapy for refractory hypoxaemia — improves oxygenation, no mortality benefit
• Corticosteroids: Dexamethasone 6mg OD for COVID-19 ARDS (RECOVERY trial); role debated in non-COVID ARDS

Surgical/Interventional

• Tracheostomy: Consider day 7-10 for patients expected to require prolonged ventilation (TracMan trial — no mortality difference early vs late)
• ECMO: Extracorporeal membrane oxygenation for refractory respiratory failure (CESAR trial — refer to ECMO centre)
• Chest drain: For pneumothorax, empyema, significant pleural effusion

Referral Criteria

• Refractory hypoxaemia (P/F <80) despite optimal ventilation and prone positioning — consider ECMO referral
• Failed extubation ×2 — consider tracheostomy
• Difficult to wean — consider weaning centre referral for prolonged ventilation

• ARDS mortality: Mild 27%, Moderate 32%, Severe 45% (Berlin classification)
• Prone positioning: Reduces mortality from ~33% to ~16% in moderate-severe ARDS (PROSEVA)
• Duration of ventilation: Median 5-7 days; lung-protective ventilation and protocolised weaning reduce duration
• Ventilator-free days: Key outcome measure — more clinically meaningful than raw duration
• Post-ICU: ICU-acquired weakness, PTSD, cognitive impairment affect 25-50% of survivors

Berlin Criteria for ARDS

Key Ventilation Trials