The influence of respiratory acid-base status on adult pulmonary vascular resistance before and after cardiopulmonary bypass

Respiratory acid-base status has recently been shown to affect pulmonary vascular resistance (PVR) in adults following cardiac surgery. The purpose of this study was to examine what influence cardiopulmonary bypass has on the pulmonary vascular response to changes in respiratory acid-base status. Fifteen consecutive patients undergoing aortocoronary bypass were studied under general anesthesia both before and after cardiopulmonary bypass. Arterial PCO₂ was manipulated by the addition of 5 percent carbon dioxide to the breathing circuit. Both before and after bypass, PVR increased significantly as PCO₂ rose from 30 mm Hg to 50 mm Hg (p<0.05). The PVR returned to baseline as PCO₂ was returned to 30 mm Hg. These data suggest that increased PVR induced by hypercarbic acidemia is not simply a result of the effects of cardiopulmonary bypass on the pulmonary circulation. Instead, we conclude that respiratory acid-base status is an important determinant of adult PVR. We believe these data may be helpful in the treatment of mechanically ventilated patients, since patients are at particular risk of having abnormalities develop in respiratory acid-base status.