Cerebral blood flow during cardiac operations: Comparison of Kety-Schmidt and xenon-133 clearance methods

This study simultaneously compared the standard Kety-Schmidt and the modified xenon-133 (¹³³Xe) clearance techniques for measuring cerebral blood flow (CBF) and cerebral metabolic rate for oxygen (CMRO₂) during cardiac operations. The validity of the CBF method is important because our management of the patient during cardiopulmonary bypass (CPB) is based, in part, on our understanding of the cerebral hemodynamics during CPB. In 20 patients undergoing coronary artery bypass grafting, CBF and CMRO₂ were determined by both methods. Measurements were made before onset of CPB and once during CPB. Ten patients underwent CPB with systemic normothermia (37°C) and 10 with systemic hypothermia (27°C). Anesthesia consisted of fentanyl and midazolam. CPB pump flows were kept at 2.2 to 2.4 L · min^-1 · m^-2 and α-stat pH management was used. Xenon-133 clearance significantly underestimated CBF and CMRO₂ relative to the Kety-Schmidt technique before CPB and at both bypass temperatures. Values obtained by ¹³³Xe clearance were approximately 50% of that measured by the Kety-Schmidt method. The modified ¹³³Xe technique as typically used during cardiac operations does not appear to measure CBF accurately; this leads to corresponding errors in CMRO₂ calculations. Determination of CMRO₂ and cerebral autoregulatory function during cardiac operations appears to be more appropriate if based on the more direct Kety-Schmidt technique. Accordingly, our management of CPB with respect to cerebral perfusion as it has been determined by the modified ¹³³Xe clearance method may require reassessment.