CEREBRAL BLOOD-FLOW DURING CARDIAC OPERATIONS - COMPARISON OF KETY-SCHMIDT AND XE-133 CLEARANCE METHODS

This study simultaneously compared the standard Kety-Schmidt and the m odified xenon-133 (Xe-133) clearance techniques for measuring cerebral blood now (CBF) and cerebral metabolic rate for oxygen (CMRO(2)) duri ng cardiac operations. The validity of the CBF method is important bec ause 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 CMRO, were determined by both methods. Measurements were made before onset of CPB and once during CPB. Ten patients underwent CPB w ith systemic normothermia (37 degrees C) and 10 with systemic hypother mia (27 degrees C). Anesthesia consisted of fentanyl and midazolam. CP B pump hows were kept at 2.2 to 2.4 L . min(-1). m(-2) and alpha-stat pH management was used. Xenon-133 clearance significantly underestimat ed CBF and CMRO, relative to the Kety-Schmidt technique before CPB and at both bypass temperatures. Values obtained by Xe-133 clearance were approximately 50% of that measured by the Kety-Schmidt method. The mo dified Xe-133 technique as typically used during cardiac operations do es not appear to measure CBF accurately; this leads to corresponding e rrors in CMRO, calculations. Determination of CMRO, and cerebral autor egulatory function during cardiac operations appears to be more approp riate if based on the more direct Kety-Schmidt technique. Accordingly, our management of CPB with respect to cerebral perfusion as it has be en determined by the modified Xe-133 clearance method may require reas sessment.