Pulmonary vascular responses to moderate changes in PaCO2 after cardiopulmonary bypass

Clinical observation by the authors suggests that small changes in PaCO2 cause significant alterations in pulmonary hemodynamics after cardiac surgery. To examine this, the authors induced moderate ventilatory hypocarbia (PaCO2 = 30.7 +/- 0.4 mmHg, mean +/- SD) in eight patients early after coronary artery bypass surgery. Normocarbia (40.6 +/- 0.5 mmHg) and hypercarbia (51.5 +/- 0.5 mmHg) were then induced by adding CO2 to the inspired gas. Standard hemodynamic measurements were performed at each stage. In four of the patients, CO2 exposure was subsequently withdrawn in reversed order. There were no clinically significant changes in systemic hemodynamics. Despite unchanged pulmonary blood flow, in all patients pulmonary artery pressure increased with increasing PaCO2. An almost two-fold and three-fold increase was observed in the mean pulmonary vascular resistance (PVR) and pulmonary diastolic gradient (pulmonary artery diastolic pressure-pulmonary capillary wedge pressure), respectively, when PaCO2 was changed from hypocarbia to hypercarbia. The changes in the pulmonary diastolic gradient correlated (r = 0.77, P less than 0.001) with the changes in the PVR. The pulmonary vasoconstrictor response was reversible with CO2 washout. Avoidance of even moderate hypercarbia, therefore, seems advisable in the early postperfusion stage because of a further potential impedance to right ventricular ejection. Frequent measurement of pulmonary diastolic pressure gradient provides a useful method of rapid estimation of the resistance to flow in the pulmonary vascular bed.