An abnormal ventilation-perfusion ratio (Qs/Qr) in the postoperative patient has been associated with

Abnormalities of ventilation-perfusion ratio result from the shunting of blood to a hypo ventilated lung or from the ventilation of hypo perfused regions of lung tissue. When this imbalance is extreme, as following massive pulmonary thromboembolism, the effect is life-threatening hypoxemia. Other common predisposing factors in the postoperative patient that contribute to this maldistribution include the assumption of a supine position, thoracic and upper abdominal incisions, obesity, atelectasis, and reduced cardiac output.