Carbon dioxide management and the cerebral response to hemodilution duringhypothermic cardiopulmonary bypass in dogs

Background. Increases in blood flow support oxygen (O-2) delivery with hemo dilution. However, with alpha -stat management, the cerebral response to he modilution is blunted. We tested the hypothesis that carbon dioxide (CO2) m anagement is a primary determinant of the cerebral blood flow (CBF) respons e to hemodilution during hypothermic bypass. Methods. Following Animal Care Committee approval, 15 dogs underwent bypass at 18 degreesC (pH-stat, n = 7 or alpha -stat, n = 8). Measurements were o btained after progressive hemodilution, and cerebral blood flow was determi ned by sagittal sinus outflow. Arterial pressure was maintained at 60 to 70 mm Hg. The CBF response to hemodilution and cerebral metabolic rate were c ompared in the two groups of animals. Results. In both groups, hemodilution increased CBF. At every hematocrit, C BF and O-2 delivery in the pH-stat, group exceeded that of alpha -stat grou p, although O-2 demand did not differ between groups. While absolute CBF in the pH-stat group was greater at every hematocrit, the relative change in CBF from control and the slope of the CBF-Hct relationship did not differ b etween groups. Conclusions. p H-stat management is associated with a greater absolute CBF and a greater ratio of cerebral O-2 supply to demand for any degree of hemo dilution. However, over the range of hematocrits common in practice, CO2 ma nagement per se does not determine the cerebral response to hemodilution. ( C) 2001 by The Society of Thoracic Surgeons.