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

Kwan Y. Chong, MB BS^*, Rosemary A. Craen, MB BS^*, John M. Murkin, MD^*, Donald Lee, MD, Michael Eliasziw, PhD and Adrian. W. Gelb, MB CHB^*

^* From the Departments of Anaesthesia and
Diagnostic Radiology, London Health Sciences Centre, University Campus and
Department of Epidemiology and Biostatistics, The University of Western Ontario, London, Ontario, Canada.

Dr. A.W. Gelb, Department of Anaesthesia, London Health Sciences Centre, University Campus, 339 Windermere Rd, London, Ontario, N6A 5A5 Canada. Phone: 519-663-3828; Fax: 519-663-3161; E-mail: agelb{at}julian.uwo.ca

Purpose: Although it has been suggested that the rate at which the cerebral circulation responds to changes in PaCO₂ is different with differing anesthetics, there have been no attempts to measure this. Transcranial Doppler allows the continuous measurement of cerebral blood flow velocity (CBFV) and any changes over time. Our aim was to compare the rate of change of CBFV when end-tidal CO₂ (P_ETCO₂) was rapidly altered during halothane or isoflurane anesthesia.

Methods: Twenty-eight unpremedicated healthy patients were randomly assigned to receive air/O₂ and either 1 - 1.5 MAC halothane or isoflurane as the primary anesthetic. After 15 min of steady state, P_ETCO₂ was rapidly reduced from 45 mmHg to 30 mmHg. CBFV and P_ETCO₂ were recorded every 30 sec for the next 10 min.

Results: The rate of change of normalized CBFV ( CBFV vs time) was 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 P_ETCO₂ produced a corresponding decrease in CBFV. However, there were no differences in the magnitude of cerebrovascular CO₂ reactivity ( CBFV vs P_ETCO₂) between the two groups.

Conclusions: The rate of change of CBFV 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 P_ETCO₂. This may be of clinical importance when cerebrovascular tone needs to be changed rapidly.

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