Three-minute blood flow index for assessment of cerebrovascular reserve

Regional cerebral blood flow studies with xenon-133 are useful in the functional assessment of cerebrovascular diseases. Conventional models for cerebral blood flow calculation employ 11 minutes of data collection. However, in many circumstances it is not possible to maintain steady-state physiologic conditions for 11 minutes. We compared a monocompartmental model that requires only 3 minutes of data collection with the bicompartmental model that requires 11 minutes of data collection. The correlation between the absolute values for global cerebral blood flow (initial slope index, intravenous method) in 72 anesthetized patients was r = 0.88; for 54 awake patients inhaling xenon-133, the correlation was r = 0.77. Cerebral blood flow was determined with intravenous xenon-133 at baseline and during a CO2 challenge in 50 patients during cerebrovascular surgery under general anesthesia. Reactivity to a 10-mm Hg rise in PaCO2 was calculated in absolute terms and as a percentage change from baseline using both the 3-minute and the 11-minute models. The correlation of CO2 reactivity calculated with the two models was r = 0.9 for the absolute values and r = 0.8 for the relative change. Cerebral blood flow calculated with the two models correlated well in both awake and anesthetized patients. In addition, there was a good correlation between CO2 reactivity calculated with the two models. In situations in which physiologic conditions cannot be held stable for 11 minutes, the 3-minute initial slope index may be used to quantitatively assess cerebrovascular reserve with a CO2 challenge.