Rate of change of cerebral blood flow velocity with hyperventilation during anesthesia in humans

Purpose: Although it has been suggested that the rate at which the cerebral circulation responds to changes in PaCO2 is different with differing anest hetics, there have been no attempts to measure this. Transcranial Doppler a llows the continuous measurement of cerebral blood flow velocity (CBN) and any changes over time. Our aim was to compare the rate of change of CBN whe n end-tidal CO2 (PETCO2) was rapidly altered during halothane or isoflurane anesthesia. Methods: Twenty-eight unpremedicated healthy patients were randomly assigne d to receive air/O-2 and either 1 - 1.5 MAC halothane or isoflurane as the primary anesthetic. After 15 min of steady state, PETCO2, was rapidly reduc ed from 45 mmHg to 30 mmHg. CBN and PETCO2 were recorded every 30 sec for t he next 10 min. Results: The rate of change of normalized CBFV (Delta CBN vs Delta time) wa s more rapid in the isoflurane group (P < 0.0001) especially in the initial few minutes. In all patients anesthetized with isoflurane, and in all but two patients anesthetized with halothane, the reduction in PETCO2 produced a corresponding decrease in CBFV. However, there were no differences in the magnitude of cerebrovascular CO2 reactivity (Delta CBFV vs Delta PETCO2) b etween the two groups. Conclusions: The rate of change of CBN was faster in the isoflurane than in the halothane group especially in the initial few minutes. Indeed, for two patients in the halothane group Vmca did not change despite a change in PE TCO2. This may be of clinical importance when cerebrovascular tone needs to be changed rapidly.