DETERMINANTS OF LOW SYSTEMIC VASCULAR-RESISTANCE DURING CARDIOPULMONARY BYPASS

Although low systemic vascular resistance occurs during normothermic a nd hypothermic cardiopulmonary bypass, the determinants of depressed s ystemic vascular resistance and its effect on outcomes are unknown. To assess the predictors and clinical effects of low systemic vascular r esistance, 555 patients undergoing isolated coronary artery bypass gra fting were evaluated prospectively. The extent of low systemic vascula r resistance during bypass was estimated by the amount of the vasocons trictor phenylephrine administered: group 1, 0 to 160 mu g; group 2, 1 61 to 800 mu g; group 3, more than 800 mu g. Multivariate analysis ide ntified bypass temperature, bypass time, and ventricular function as d eterminants of low systemic vascular resistance. Patients on normother mic bypass accounted for 65% of the patients in group 3 and only 34% o f the patients in group 1 (p < 0.0001). The bypass time was longer in the patients in group 3 (97 +/- 28 minutes) than in the patients in gr oup 1 (89 +/- 24 minutes p < 0.006). Patients with a preoperative left ventricular election fraction of 0.40 or less required less phenyleph rine during cardiopulmonary bypass (498 +/- 68 mu g) than did patients with a fraction exceeding 0.40 (1,087 +/- 88 mu g; P < 0.001). By mul tivariate analysis, advanced age and the presence of peripheral vascul ar disease were found to decrease the likelihood of low systemic vascu lar resistance during normothermic bypass. Diabetes, the left ventricu lar ejection fraction, the bypass time, and the total cardioplegia inf used were found to influence the likelihood of low systemic vascular r esistance during hypothermic bypass. Patients in group 3 had a higher cardiac index and lower mean arterial pressure and systemic vascular r esistance postoperatively. In those patients who received a left inter nal mammary artery graft, the incidences of the low-output syndrome (g roup 1, 4.9%; group 3, 2.7%; p = not significant) and myocardial infar ction (group 1, 1.4%; group 3, 1.8%; p = not significant) were not inf luenced by the amount of phenylephrine infused during cardiopulmonary bypass. In those patients who were at high risk of suffering a stroke preoperatively, the hypotension induced by the low systemic vascular r esistance and its treatment with phenylephrine was not associated with an increased incidence of stroke (group 1, 5.8%; group 3, 2.816; p = not significant).