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

Central haemodynamics and ventilation-perfusion (VA/Q) distribution were studied in nine patients, 21 h after coronary bypass surgery, before and during nitroglycerin (TNG) infusion. VA/Q distributions were established with the multiple inert gas elimination technique of Wagner and West. Administration of TNG resulted in a decrease in mean arterial pressure, a slight reduction in cardiac output and stroke volume and a significant increase in heart rate, possibly explained by an initial relative hypovolaemia. Pulmonary arterial pressures and filling pressures for the right and left ventricles decreased significantly. There was a significant reduction in PaO2 and an increase in venous admixture (QVA/QT) from 11.3 to 16.5% of cardiac output. This was mainly due to an initial SF6-shunt of 6.4% increasing to 12.8%. Only 3.5% of cardiac output during TNG was due to perfusion of hypoventilated areas. The mean for the control Q-distribution was 0.88 with a mean log s.d. of +/- 1.14, indicating VA/Q-mismatch and did not change significantly. The reduction of pressures in the pulmonary vascular bed was accompanied by increased ventilation of areas with high VA/Q.