Response of Cerebral Blood Flow to Phenylephrine Infusion during Hypothermic Cardiopulmonary Bypass

Abstract

Twenty-eight adult patients anesthetized with fentanyl, then subjected to hypothermic cardiopulmonary bypass (CPB), were studied to determine the effect of phenylephrine-induced changes in mean arterial pressure (MAP) on cerebral blood flow (CBF). During CPB patients managed at 28.degree. C with either alpha-stat (temperature-uncorrected PaCO2 = 41 .+-. 4 mmHg) or pH-stat (temperature-uncorrected PaCO2 = 54 .+-. 8 mmHg) PaCO2 for blood gas maintenance received phenylephrine to increase MAP .gtoreq. 25% (group A, n = 10; group B, n = 6). To correct for a spontaneous, time-related decline in CBF observed during CPB, two additional groups of patients undergoing CPB were either managed with the alpha-stat or pH-stat approach, but neither group received phenylephrine and MAP remained unchanged in both groups (group C, n = 6; group D, n = 6). For all patients controlled variables (nasopharyngeal temperature, PaCO2, pump flow, and hematocrit) remained unchanged between measurements, Phenylephrine data were corrected based on the data from groups C and D for the effect of diminishing CBF over time during CPB. In patients in group A CBF was unchanged as MAP rose from 56 .+-. 7 to 84 .+-. 8 mmHg. In patients in group B CBF increased 41% as MAP rose from 53 .+-. 8 to 77 .+-. 9 mm Hg (P < 0.001). During hypothermic CPB normocarbia maintained via the alphastat approach at a temperature-uncorrected PaCO2 of .simeq. 40 mmHg preserves cerebral autoregulation; pH-stat management (PaCO2 .simeq. 57 mmHg uncorrected for temperature, or 40 mmHg when corrected to 28.degree. C) causes cerebraovascular changes (i.e. impaired autoregulation) similar to those changes produced by hpercarbia in awake normothermic patients.