CHRONIC NONPULSATILE BLOOD-FLOW .1. CEREBRAL AUTOREGULATION IN CHRONIC NONPULSATILE BIVENTRICULAR BYPASS - CAROTID BLOOD-FLOW RESPONSE TO HYPERCAPNIA

To investigate the response of the carotid blood flow and general circ ulation to hypercapnia in chronic nonpulsatile blood pow, we performed 18 carbon dioxide gas inhalation studies on three calves undergoing a centrifugal biventricular bypass with ventricular fibrillation. An ul trasonic flow probe was put on the carotid artery during biventricular bypass pump implantation, and pump flows were maintained at 90, 100, and 120 ml/kg per minute for 1 week each. The carbon dioxide inhalatio n studies were performed twice a week. Hypercapnia was induced by admi nistering pure carbon dioxide gas through a nasal tube at flow rates o f 0, 5, 7.5, 10, 12.5, and 15 L/min for 5 minutes each at three differ ent nominal pump flow rates, and the resultant arterial blood gas and hemodynamic changes were recorded. No significant correlation existed between the carotid blood flow and mean aortic pressure, which varied from 70 to 140 mm Hg, but the carotid blood pow correlated significant ly (p < 0.01) with the systemic pump flow rate. A significant (p < 0.0 1) linear relationship was found between the carotid blood flow and ar terial carbon dioxide tension. For each 1 mm Hg change in arterial car bon dioxide tension, there was a 2.8% change in the carotid blood pow. The percent changes in the carotid blood pow in response to arterial carbon dioxide tension were calculated as 2.9%, 3.7%, and 2.5% for eac h 1 mm Hg change in arterial carbon dioxide tension at pump flows of 9 0, 100 and 120 ml/kg per minute. No significant differences in the car otid blood flow response to hypercapnia were detected among the three systemic pump flow rates. These results thus suggested that chronic no npulsatile blood flow had no detrimental effects on cerebral autoregul ation.