Hypoxic Pulmonary Vasoconstriction in the Human Lung: the Effect of Prolonged Unilateral Hypoxic Challenge during Anaesthesia

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Abstract
The influence of time on the pulmonary vasoconstrictor response to hypoxia was studied in 6 subjects during general anesthesia and artificial ventilation prior to elective surgery. The lungs were intubated separately with a double-lumen bronchial catheter. After preoxygenation of both lungs for 30 min, the test lung was rendered hypoxic for 60 min by ventilation with 5% O2 in N2, with the control lung still being ventilated with 100% O2. Cardiac output was determined by thermodilution and the distribution of blood flow between the lungs was assessed from the excretion of a continuously infused poorly soluble gas (SF6). The fractional perfusion of the test lung decreased from 53% to 25% of cardiac output within the first 15 min of unilateral hypoxia. The pulmonary artery mean pressure increased by 14% and the pulmonary vascular resistance (PVR) of the test lung increased by 54%. Venous admixture increased from 21% to 39% of cardiac output, while the true shunt was maintained at .apprx. 15%. Arterial oxygen tension (PaO2) fell from 45 kPa [kiloPascal] to 12 kPa. Prolonging the unilateral hypoxic challenge caused no further change in the redistribution of the pulmonary blood flow, but cardiac output and pulmonary artery mean pressure continued to increase to 40%-50% above control values after 1 h of hypoxia. The PVR of the test lung remained unchanged. There is an immediate vasoconstrictor response to hypoxia in the human lung and there is no further potentiation or diminution, of the response during a 60-min period of hypoxia.