Acute Hypoxaemic Respiratory Failure

Acute hypoxaemic respiratory failure is arterial oxygen tension under 8.0 kPa (60 mmHg) on room air, or a ratio of PaO2 to inspired oxygen fraction (PF ratio) under 300 mmHg, with normal or low PaCO2. Its most severe form is the acute respiratory distress syndrome (ARDS).

Epidemiology. It is a leading reason for intensive care admission. ARDS accounts for about 10% of intensive care admissions and has mortality of about 35 to 45%, rising with severity.

Causes and associations.

• Direct lung injury: pneumonia (including viral), aspiration, inhalation injury, pulmonary contusion and drowning.
• Indirect injury: sepsis, pancreatitis, major trauma, transfusion and burns.
• Cardiogenic pulmonary oedema, pulmonary embolism, haemorrhage and interstitial disease.

ARDS definition. Onset within 1 week of an insult, bilateral opacities not explained by effusion, collapse or nodules, respiratory failure not explained by cardiac failure or fluid overload, and PF ratio 300 or less with PEEP of 5 cmH2O or more. Severity: mild 200 to 300, moderate 100 to 200, severe under 100.

Pathophysiology. Ventilation perfusion mismatch and shunt are the commonest mechanisms. In ARDS, alveolar capillary injury produces protein rich oedema, surfactant loss, atelectasis and reduced compliance.

Presentation is breathlessness, tachypnoea, tachycardia, cyanosis, confusion and use of accessory muscles. Chest signs depend on the cause.

Red flags.

• Saturation under 90% on high flow oxygen, or PF ratio under 150.
• Exhaustion, confusion or rising lactate.
• Rapid deterioration over hours.

Arterial gas with PF ratio, chest radiograph, lung ultrasound or CT, echocardiography (exclude left heart failure), natriuretic peptide, troponin, full blood count, cultures, respiratory viral testing, procalcitonin and lactate. Send bronchoalveolar lavage when the cause is unclear in the immunocompromised.

Differential diagnosis. Cardiogenic pulmonary oedema, pulmonary embolism, diffuse alveolar haemorrhage, acute eosinophilic pneumonia, acute interstitial pneumonia and organising pneumonia.

Oxygen and non-invasive support.

• Start with oxygen titrated to saturation 92 to 96% (88 to 92% if hypercapnic).
• High flow nasal oxygen 40 to 60 L/min with fraction adjusted to saturation lowers intubation rates in hypoxaemic failure. Review at 1 to 2 hours, and use the ROX index (saturation divided by inspired fraction, divided by respiratory rate). A value under 3.85 at 12 hours predicts failure.
• Helmet or face mask NIV in selected patients. Avoid delayed intubation.

Intubation criteria. Worsening work of breathing, PF ratio under 100 on support, confusion, shock or fatigue. Pre-oxygenate, use rapid sequence induction with ketamine 1 to 2 mg/kg or etomidate 0.3 mg/kg and rocuronium 1.2 mg/kg.

Lung protective ventilation.

• Tidal volume 6 mL/kg predicted body weight (4 to 8 mL/kg).
• Plateau pressure 30 cmH2O or less, driving pressure 15 cmH2O or less.
• PEEP set by a lower-PEEP and FiO2 table or by compliance titration. Higher PEEP is preferred in moderate or severe ARDS.
• Respiratory rate up to 35 breaths/min to keep pH over 7.25.

Prone position. For PF ratio under 150 on PEEP of 5 or more and FiO2 0.6 or more, prone for at least 16 hours daily, for as many days as the PF ratio stays under 150. Contraindications: unstable spine, open abdomen, raised intracranial pressure and unstable pelvic fracture.

Neuromuscular blockade. Cisatracurium 15 mg IV bolus then 37.5 mg/h for up to 48 hours in severe early ARDS with dyssynchrony. Use deep sedation.

Corticosteroid. Dexamethasone 20 mg IV daily for 5 days, then 10 mg daily for 5 days, in established moderate to severe ARDS. Avoid in influenza pneumonia unless another indication exists.

Fluid. Aim for a conservative fluid balance once shock has resolved. Consider diuretics (furosemide 20 to 40 mg IV) when haemodynamically stable.

Extracorporeal support. Venovenous ECMO for PF ratio under 80 for over 6 hours, or under 50 for over 3 hours, or pH under 7.25 with PaCO2 of 60 mmHg or more for over 6 hours, despite optimal care.

Cause directed treatment. Antibiotics for pneumonia within 1 hour of shock recognition, oseltamivir 75 mg orally twice daily for 5 days (dose adjusted to eGFR) in influenza, and drainage of source in sepsis.

Monitoring. Arterial gas, PF ratio, plateau and driving pressures, compliance, fluid balance and sedation depth. Wean when PF ratio is over 150 with PEEP under 10 cmH2O and sedation can be lightened.

Referral and escalation. Early intensive care review for all patients on high flow oxygen. Transfer to an ECMO centre before the patient is too unstable to move.

Special groups. In pregnancy, keep saturation over 95% and consider delivery when maternal oxygenation cannot be maintained. In children, specialist ventilation with tidal volume 5 to 8 mL/kg.