Borderline or established failure of gas exchange preoperatively

Unexpected perioperative complications (e.g. fluid overload, haemorrhage)

Intercurrent respiratory infection (with urgent surgery)

Inadequate or ineffective regional analgesia with deterioration in respiratory function

Chest disease productive of large amounts of secretions (e.g. bronchiectasis)

Unexpectedly prolonged procedure

Major abdominal or thoracic surgery

Acidosis

Major surgery not amenable to regional analgesia and necessitating systemic opioids

Hypothermia

Long duration of surgery

Depressed conscious level/ slow recovery from anaesthetic/ poor cough