MEASUREMENT OF CEREBRAL BLOOD-FLOW - COMP ARISON OF THE KETY-SCHMIDT TECHNIQUE AND THE INTRAVENOUS XE-133 CLEARANCE METHOD

In this study cerebral blood flow (CBF) was simultaneously measured wi th the Kety-Schmidt method and the intravenous Xe-133 clearance techni que. CBF, cerebral metabolic rate of oxygen (CMRO(2)), and CO2 reactiv ity of CBF were compared under fentanyl-midazolam anaesthesia and vary ing paCO(2) levels. Methods. Thirteen male patients were studied befor e they underwent coronary artery bypass surgery. For measurement of CB F with the Kety-Schmidt inert gas saturation technique, argon was used as indicator instead of nitrous oxide, because argon is less soluble in water and lipid such that arterial and hence organ saturation is at tained earlier. Wash-in periods of 10 min were used for all measuremen ts. For measurement of CBF with the intravenous xenon method 10 scinti llation detectors placed lateral to the skull and an air detector for calculation of tracer recirculation were used. 10-15 mCi of Xe-133 dis solved in physiological saline was injected via a peripheral i.v. cann ula. For comparison with the Kety-Schmidt technique CBF15-values repre senting the flow of the grey and white matter were chosen. CBF was mea sured simultaneously with both methods under normocapnic (paCO(2) 43+/ -3 mmHg), hypocapnic (paCO(2) 31+/-3 mmHg), and under hypercapnic (paC O(2) 54+/-4 mmHg) conditions. Results. All CBF15 values obtained with the intravenous xenon method were significantly lower than the corresp onding CBF-values measured with the Kety-Schmidt technique: by 36% und er normocapnic, 23% under hypocapnic, and 39% under hypercapnic condit ions, respectively. Hence, CMRO(2) values calculated from CBF values o btained with the xenon method were reduced to about the same degree as those derived from CBF values measured with the Kety-Schmidt techniqu e. There was no significant correlation between the CBF values of eith er method (y=1.82x-8.58,r=0.76 P=0.357). Non-linear curve-fitting proc edures yielded exponential CBF-paCO(2) relationships for both methods, although the relative carbon dioxide reactivity was higher with the K ety-Schmidt technique than with the xenon method (y=8.14 e(0.039x) e v s y=10,75 e(0.023x)). Conclusions. Most probably due to contamination with radioactivity from slowly perfused extracerebral tissues the intr avenous Xe-133-clearance technique underestimates CBF, CMRO(2), and ce rebrovascular CO2 reactivity, at least when CBF15 values are used as f low parameters.