Key Facts

The respiratory system facilitates gas exchange between the atmosphere and blood, maintaining adequate oxygenation and CO2 elimination. Respiratory physiology underpins the understanding of common diseases such as asthma, COPD, pneumonia, and respiratory failure.

Epidemiology

Respiratory disease is the third leading cause of death in the UK. Asthma affects 5.4 million, COPD affects 1.2 million diagnosed patients, and pneumonia accounts for approximately 300,000 hospital admissions annually.

Aetiology

Ventilation depends on:

• Respiratory centre drive (medullary and pontine centres)
• Chest wall and diaphragm mechanics
• Airway patency
• Lung compliance and elastic recoil

Causes of respiratory failure:

• Type 1 (hypoxaemic): Pneumonia, PE, ARDS, pulmonary fibrosis, pulmonary oedema
• Type 2 (hypercapnic): COPD, neuromuscular disease, chest wall deformity, sedation, obesity hypoventilation

Pathophysiology

Lung mechanics:

• Compliance = ΔV/ΔP; reduced in fibrosis, ARDS, pulmonary oedema; increased in emphysema
• Surfactant (type II pneumocytes) reduces surface tension, preventing alveolar collapse — deficiency causes neonatal RDS
• Elastic recoil tends to collapse lung; chest wall tends to expand — functional residual capacity (FRC) is the balance point

Gas exchange:

• Fick's law: Diffusion ∝ (surface area × pressure gradient × solubility) / (membrane thickness × √molecular weight)
• CO2 is 20× more soluble than O2 — diffusion impairment causes hypoxia before hypercapnia

Oxygen transport: 98.5% bound to haemoglobin (1.34mL O2/g Hb), 1.5% dissolved

Type 1 Respiratory Failure

• Dyspnoea, tachypnoea, cyanosis
• Hypoxaemia responsive to supplemental O2 (unless shunt)
• Underlying cause symptoms: fever (pneumonia), pleuritic pain (PE), bilateral crackles (pulmonary oedema)

Type 2 Respiratory Failure

• CO2 retention: Headache, drowsiness, tremor, bounding pulse, papilloedema
• May have reduced respiratory drive — caution with high-flow O2 (target SpO2 88-92% in COPD)

Obstructive vs Restrictive Patterns

• Obstructive: Wheeze, prolonged expiration, hyperinflation, air trapping
• Restrictive: Reduced chest expansion, fine inspiratory crackles (fibrosis), reduced lung volumes

Red Flags

• Respiratory rate >25/min with rising PaCO2 — impending respiratory arrest
• Silent chest in asthma — life-threatening
• PaO2 <8kPa on high-flow O2 — consider ARDS, massive PE, or shunt
• Stridor — upper airway obstruction, urgent ENT assessment

Bedside

• Pulse oximetry: Continuous SpO2 monitoring
• ABG: Gold standard for assessing oxygenation, ventilation, acid-base
• Peak expiratory flow: Monitoring in asthma

Bloods

• FBC: Polycythaemia (chronic hypoxia), anaemia (reduced O2 carrying capacity)
• CRP/WCC: Infection markers
• D-dimer: PE exclusion (if low pre-test probability)

Imaging

• CXR: Consolidation, effusion, pneumothorax, hyperinflation, pulmonary oedema
• CT thorax (HRCT): Interstitial lung disease, bronchiectasis
• CTPA: Pulmonary embolism

Special Tests

• Spirometry: FEV1, FVC, FEV1/FVC ratio — obstructive vs restrictive pattern
• Full lung function tests: TLC, RV, TLCO (gas transfer)
• TLCO: Reduced in emphysema, fibrosis, PE, anaemia; increased in alveolar haemorrhage, asthma
• Cardiopulmonary exercise testing (CPET): Differentiates cardiac vs respiratory limitation

Non-pharmacological

• Oxygen therapy: Target SpO2 94-98% (88-92% in hypercapnic risk patients — BTS guidelines)
• NIV (BiPAP): First-line for acute hypercapnic respiratory failure in COPD (pH 7.25-7.35)
• Pulmonary rehabilitation: COPD, MRC dyspnoea ≥3
• Smoking cessation: Most important intervention in COPD

Pharmacological

• Bronchodilators: Salbutamol 5mg nebulised, ipratropium 500mcg nebulised (acute)
• Corticosteroids: Prednisolone 40mg OD for 5 days (acute exacerbation of COPD/asthma)
• Antibiotics: Amoxicillin 500mg TDS or doxycycline 200mg stat then 100mg OD for infective exacerbations
• LTOT: ≥15 hours/day if PaO2 ≤7.3 kPa on two occasions 3 weeks apart, when stable

Surgical

• Chest drain: Pneumothorax, large pleural effusion
• Lung volume reduction surgery: Selected emphysema patients
• Lung transplant: End-stage lung disease (COPD, IPF, CF)

Referral Criteria

• Unexplained dyspnoea — respiratory specialist
• FEV1 <50% predicted — consider specialist COPD services
• Suspected ILD — urgent respiratory referral for HRCT

• COPD: FEV1 decline 30-60 mL/year (vs 25-30 mL/year in healthy non-smokers); GOLD stage IV — 5-year survival ~20%
• ARDS: Mortality 30-40% (Berlin definition: mild 27%, moderate 32%, severe 45%)
• IPF: Median survival 3-5 years from diagnosis
• Smoking cessation reduces FEV1 decline to near-normal rates — the single most effective intervention
• NIV in acute hypercapnic COPD exacerbation reduces mortality from ~20% to ~10%

Lung Volumes and Capacities

Causes of Right-Shifted O2-Hb Dissociation Curve