To the Editor:—In our opinion, two related issues in the article by Ohsumi et al. [1] need to be addressed. The cerebral blood flows (CBF) and metabolic rates for oxygen (CMRO2) they report using the Kety-Schmidt technique are appropriate and similar to reports by our laboratory [2] and that of Stephan et al., [3] with the exception of the CBF (19 ml *symbol* 100 g sup -1 *symbol* min sup -1) they document during hypothermic cardiopulmonary bypass. During hypothermia, the patients in the study were significantly hypocarbie (mean PaCO2= 29.7 mmHg). This degree of hypocarbia will decrease CBF by approximately 25–30% in the context of a opioid-benzodiazepine anesthesia. [4,5] Correcting for this hypocarbia, the CBF under normocarbic conditions (with alpha-stat management and hypothermia) would be closer to 25 ml *symbol* 100 g sup -1 *symbol* min sup -1. This value is similar to what our laboratory [2] and that of Stephan et al. [3] reported.

Second, and more importantly, Ohsumi et al. sell the Kety-Schmidt method short when comparing their results to those previously obtained with133Xenon clearance. [6,7] Ohsumi et al. suggest in their discussion that the CBF and CMRO2values they report are too high when compared to values reported with the133Xenon method. We would argue that the133Xenon values typically reported are too low, and the Ohsumi et al. values are correct. [2] The Ohsumi et al. Kety-Schmidt-derived CBF and CMRO2values are consistent with theoretical predictions and direct measurements in animals, unlike those commonly reported with133Xenon.

David J. Cook, M.D.; Assistant Professor Anesthesiology; John D. Michenfelder, M.D.; Emeritus Professor of Anesthesiology; Mayo Medical School; Mayo Clinic; Rochester, Minnesota 55905.

(Accepted for publication November 10, 1994.)

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