Effects of oxygen on central haemodynamics and VA/Q distribution after coronary bypass surgery

Central haemodynamics and ventilation-perfusion (VA/Q) distribution were studied in 16 patients, 21 h after coronary bypass surgery, during ventilation with FIO2 = 0.3 and 1.0. VA/Q distributions were determined by the multiple inert gas elimination technique. In 15 patients with normal preoperative lung function, there was a significant shunt of 7.5% of cardiac output and perfusion of regions with VA/Q between 0.005-0.1 of 2.4% (FIO2 = 0.3). Mean VA/Q for the Q-distribution was 0.89 with log s.d. of 0.92. Ventilation of regions with VA/Q above 100 was 0.22. After 30 min of ventilation with FIO2 = 1.0, there was a slight increase in cardiac output while pulmonary arterial mean pressure and pulmonary vascular resistance showed slight decreases. There was a marked increase of the shunt in three patients but the increase for the whole group was not significant. Perfusion of regions with low VA/Q (0.005-0.1) increased significantly to 8.7% while the distribution of ventilation remained unchanged. The changes in distribution of Q were probably due to a release of hypoxic vasoconstriction in combination with complete or incomplete alveolar collapse. One patient with obstructive lung disease showed only minor changes in perfusion distribution.