Postoperative Respiratory Failure

Postoperative respiratory failure is inability to maintain adequate gas exchange after surgery, usually defined as unplanned ventilation for more than 48 hours, reintubation within 30 days, or need for ventilatory support after extubation. Type 1 failure is PaO2 under 8 kPa (60 mmHg) with normal or low PaCO2. Type 2 failure is PaCO2 over 6.5 kPa (50 mmHg) with pH under 7.35.

Epidemiology. It occurs in about 2% to 5% of major surgery and up to 10% after emergency abdominal or aortic surgery. Mortality is about 25% to 40%.

Causes.

• Type 1: pneumonia, acute respiratory distress syndrome, pulmonary oedema, atelectasis, aspiration, pulmonary embolism, transfusion related lung injury.
• Type 2: opioid or sedative excess, residual neuromuscular block, diaphragm dysfunction, COPD exacerbation, obesity hypoventilation, pain splinting, neuromuscular disease, central nervous system event, pneumothorax.

Risk factors. Age over 60 to 70 years, COPD, heart failure, functional dependence, sepsis, low albumin, raised urea, emergency surgery, upper abdominal, thoracic, aortic and neurosurgery, duration over 3 hours, transfusion and smoking.

Pathophysiology.

Shunt and ventilation perfusion mismatch cause hypoxaemia. Reduced functional residual capacity, diaphragm dysfunction after upper abdominal surgery and respiratory muscle fatigue cause ventilatory failure.

Depressed central drive, increased dead space and inflammatory lung injury from surgery, transfusion or ventilation add to failure.

Dyspnoea, respiratory rate over 30/min or under 8/min, accessory muscle use, paradoxical abdominal movement, inability to complete sentences, cyanosis, agitation then drowsiness, flapping tremor, bounding pulse and headache with carbon dioxide retention.

Red flags.

• Exhaustion or falling conscious level.
• Silent chest.
• pH under 7.25 or rising PaCO2.
• Saturation under 90% on oxygen over 60%.
• Respiratory rate over 35/min.
• Haemodynamic instability.

Investigations.

• Arterial blood gas with PaO2/FiO2, PaCO2, pH, bicarbonate and lactate.
• Chest radiograph, lung ultrasound, ECG, troponin and natriuretic peptide.
• Echocardiography, full blood count, urea, electrolytes, C-reactive protein, cultures.
• CT pulmonary angiography if embolism is suspected.
• Train-of-four, vital capacity (under 15 mL/kg suggests need for support) and maximal inspiratory pressure (weaker than 20 to 25 cmH2O in magnitude) when muscle weakness is suspected.

Differential diagnosis.

• Pneumonia.
• Pulmonary oedema.
• Pulmonary embolism.
• Acute respiratory distress syndrome.
• Pneumothorax.
• Phrenic nerve palsy.
• COPD exacerbation.
• Opioid overdose or residual block.
• Central nervous system event.
• Sepsis.

First steps. Airway, oxygen titrated to saturation 94% to 98% (88% to 92% if hypercapnic risk), upright positioning, venous access, blood gas, and treatment of the cause. Call critical care early.

Hypoxaemic failure. High flow nasal oxygen at 40 to 60 L/min or CPAP 5 to 10 cmH2O. Intubate for worsening hypoxaemia, exhaustion or shock.

Hypercapnic failure. Non-invasive ventilation for pH 7.25 to 7.35 with PaCO2 over 6.5 kPa: start inspiratory pressure 10 cmH2O and expiratory pressure 4 to 5 cmH2O. Raise inspiratory pressure by 2 to 5 cmH2O steps to 20 to 25 cmH2O as tolerated, and repeat the blood gas at 1 hour. Use caution after upper gastrointestinal anastomosis. Intubate if pH falls under 7.25, conscious level drops or ventilation fails.

Invasive ventilation.

• Tidal volume 6 mL/kg predicted body weight (up to 8 mL/kg)
• Plateau pressure under 30 cmH2O, driving pressure under 15 cmH2O
• Positive end expiratory pressure 5 to 12 cmH2O raised in line with oxygen need
• Permissive hypercapnia with pH over 7.2
• Prone positioning for 12 to 16 hours when PaO2/FiO2 is under 150

Cause directed drugs.

• Opioid excess, naloxone: 40 to 100 microgram IV every 2 to 3 minutes titrated to respiratory rate over 10/min; reconsider the diagnosis after 400 microgram; observe for at least 2 hours; monitor respiratory rate and pain; use small increments in opioid dependence.
• Residual neuromuscular block, sugammadex: 2 mg/kg IV once (train-of-four count 2 or more), 4 mg/kg if post-tetanic count 1 to 2; maximum 16 mg/kg; confirm train-of-four ratio of 0.9 or more; avoid if creatinine clearance is under 30 mL/min.
• Cardiogenic oedema, furosemide: 40 mg IV slowly, repeat 80 mg after 1 hour if poor response; usual daily maximum 200 mg; monitor urine output, potassium and creatinine; larger doses in renal impairment.
• Suspected pneumonia with sepsis: broad spectrum antibiotics within 1 hour, selected by source and local resistance.

Weaning. Daily readiness check, then pressure support 5 to 8 cmH2O with positive end expiratory pressure 5 cmH2O for 30 to 120 minutes. Extubate high risk patients to high flow oxygen or non-invasive ventilation. Consider tracheostomy at 7 to 14 days.

Monitoring. Blood gas 30 to 60 minutes after each change, continuous saturation and end tidal carbon dioxide, fluid balance, delirium, nutrition and thromboprophylaxis.

Referral and escalation. Early critical care involvement for any patient needing ventilatory support. Cardiology, respiratory or neurology input as the cause dictates.

Special groups. Obesity needs ramped position and higher positive end expiratory pressure. In neuromuscular disease, vital capacity directs timing of ventilation. Children need weight based tidal volumes and earlier intervention.