Write a short note on respiratory failure.

Definition

Failure of the respiratory system to maintain adequate gas exchange: PaO2 < 60 mmHg and/or PaCO2 > 50 mmHg at sea level on room air.

By time course: ACUTE (low pH, no renal compensation); CHRONIC (near-normal pH with raised bicarbonate); ACUTE-ON-CHRONIC (raised bicarbonate with an acutely low pH). The distinction is made from the pH and bicarbonate and determines management.

Causes

TYPE I — problems within the lung: V/Q mismatch (commonest) — pneumonia, pulmonary oedema, PE, asthma, COPD; shunt — ARDS, consolidation, right-to-left cardiac shunt; diffusion impairment — ILD; reduced inspired oxygen — altitude.

TYPE II — problems with the ventilatory pump:

Clinical Features

Hypoxaemia: dyspnoea, tachypnoea, tachycardia, CENTRAL CYANOSIS (absent in severe anaemia), restlessness, agitation and confusion — often the earliest signs and frequently misread as anxiety; later bradycardia, hypotension, seizures, coma. Chronic: polycythaemia, pulmonary hypertension, cor pulmonale.

Hypercapnia (CO2 narcosis): WARM peripheries with a BOUNDING PULSE, FLAPPING TREMOR (asterixis), morning headache and papilloedema, drowsiness progressing to coma, conjunctival suffusion.

Also: accessory muscle use, intercostal recession, paradoxical abdominal movement (diaphragmatic fatigue — ominous), inability to speak in sentences, exhaustion.

Investigations

• ARTERIAL BLOOD GAS — the definitive test. Interpret systematically: pH → PaCO2 → PaO2 → bicarbonate → A–a gradient
• A–a gradient — normal ≈ (age/4) + 4 on air. A NORMAL gradient with hypoxaemia and hypercapnia indicates pure HYPOVENTILATION (drug overdose, neuromuscular disease); a RAISED gradient indicates intrinsic lung disease. This single calculation localises the problem
• Pulse oximetry — for monitoring; unreliable in poor perfusion and falsely reassuring in carbon monoxide poisoning and methaemoglobinaemia, and gives no information about CO2
• Chest X-ray, CBC, electrolytes, glucose, ECG, and cause-directed tests
• Serial FVC and single-breath count in neuromuscular causes

Management

1. Treat the cause — bronchodilators and steroids, antibiotics, diuretics, naloxone, immunotherapy, drainage.

2. Oxygen

• TYPE I: high-concentration oxygen, target SpO2 94–98%
• TYPE II or at risk of retention: CONTROLLED oxygen via Venturi mask, target 88–92%, with a repeat gas at 30–60 minutes
• Devices: nasal cannulae, simple mask, Venturi (fixed predictable FiO2 — the device of choice where control matters), non-rebreathing reservoir mask, and high-flow nasal oxygen

3. Ventilatory support

• NON-INVASIVE VENTILATION (BiPAP) — the treatment of choice for hypercapnic failure with acidosis (pH < 7.35) in COPD, and CPAP for cardiogenic pulmonary oedema. Reduces intubation, complications and mortality
• INVASIVE VENTILATION — for failure of or contraindication to NIV, coma, inability to protect the airway, haemodynamic instability, exhaustion, or refractory hypoxaemia
• Contraindications to NIV: arrest, unprotected airway, facial trauma, vomiting, severe agitation, undrained pneumothorax, instability

4. Supportive: treat bronchospasm and secretions, correct potassium, magnesium and phosphate (deficiency weakens respiratory muscles), nutrition, thromboprophylaxis, avoid sedatives.

5. Long-term: long-term oxygen therapy; domiciliary NIV for chronic hypercapnia in COPD, obesity hypoventilation and neuromuscular disease; rehabilitation; vaccination.