The functions of the lung as a pump and as an area of gas exchange are not independent. Disease commonly causes problems with both. A good example of this is seen postoperatively. Surgery causes atelectasis because of a combination of supine position, general anaesthesia, and pain. A reduction in functional residual capacity below closing volume also contributes to hypoxaemia. Good analgesia, chest physiotherapy, and early mobilisation help to prevent this. Excessive load from reduced compliance and increased minute volume in combination with opiates which depress respiration lead to hypercapnia. At risk patients are those with pre-existing lung disease, who are obese, or who have upper abdominal or thoracic surgery.
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