The lungs are exposed to a variety of insults during routine cardiac surgery ( HachenbergJ995). These include loss of functional residual capacity with patchy atelectasis, ischemia-reperfusion injury, and exposure to bypass-activated neutrophils and inflammatory mediators. Increased pulmonary capillary permeability results in increased extravascular lung water, which may be further aggravated by elevated left atrial pressure. Ventilation-perfusion mismatch occurs and pulmonary compliance may markedly decrease. The patient may be hypoxemic, the chest radiograph often demonstrates lobar collapse, and the work of breathing is increased. Pulmonary dysfunction is maximal at around 24 h postoperatively and may persist for several weeks. Despite this, relatively few patients (less than 5 per cent) develop frank postoperative respiratory failure.
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