Cerebral Blood Flow Determined by Saturation and Desaturation with Krypton 85 :

Abstract

Cerebral blood flow was studied in 11 subjects using the krypton 85 method during a 10-minute saturation period followed by a 10-minute desaturation period. The calculated cerebral blood flow averaged 8.5% higher during desaturation than during saturation for the eight cases with blood flow values in the normal range. At higher flow (one case) the deviation was less pronounced, while at lower flows (two cases) the percentage difference increased. It is suggested that this systematic difference is due to the failure, at 10 minutes, of reaching equilibrium between average tissue and mixed venous blood in the human brain. The studies disclose a systematic error inherent in the Kety-Schmidt technique. According to observations in the literature this error is, however, not of such magnitude that it affects seriously the method as applied to the brain. Employing an inert gas saturation period of 10 minutes, an overestimation of the flow by about 10% probably occurs in normal subjects, while at subnormal flow levels the magnitude of this error increases somewhat. The error mentioned may be counteracted by prolonging the saturation period to 15 or 20 minutes and by extrapolation of the curves to infinity.