Nitrous oxide method of measuring cerebral blood flow during hypothermic cardiopulmonary bypass

Background. Determination of cerebral blood flow and oxygenation is a means of evaluating our cardiopulmonary bypass (CPB) practice. Because much of CPB is hypothermic, our measurement technique must be valid over a range of temperatures. In this study we evaluate the validity of N₂O washin for measurement of cerebral blood flow and oxygenation at three temperatures. Methods. Cerebral blood flow and oxygenation were measured in 7 dogs undergoing CPB at 37°, 32°, and 27°C using simultaneous direct (sagittal sinus outflow) and indirect (nitrous oxide washin) techniques. Animals underwent CPB with a whole blood prime and α-stat pH management. Results. In the absence of hemodilution, cerebral blood flow and oxygenation were reduced by approximately 38% and 55% at 32°C and 27°C, respectively, by both techniques. Direct and indirect methods showed an excellent correlation (R = 0.87) during CPB between 27.5°C and 37.8°C (21 paired measurements). Conclusions. This investigation demonstrates that the correlation between a direct measure of global cerebral blood flow and that obtained by the N₂O saturation method is excellent during CPB over the range of common CPB temperatures.