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Partial Preservation of Cerebral Vascular Responsiveness to Hypocapnia during Isoflurane-Induced Hypotension in Dogs

Received from the Department of Anesthesiology, University of Washington School of Medicine, Seattle, Washington.

Abstract

This study was undertaken to determine whether the cerebral vascular response to hypocapnia is preserved during isoflurane-induced hypotension. In six dogs (group 1) cerebral vascular resistance and cerebral blood flow were determined at normocapnia (PaCO₂ 40 mm Hg) and at hypocapnia (Paco₂20 mm Hg) while mean arterial pressure was normal, and then again during isoflurane-induced hypotension to a mean arterial pressure of 50 mm Hg. Hypocapnia increased cerebral vascular resistance and decreased cerebral blood flow during both normotension and isoflurane-induced hypotension. However, the magnitude of these responses was greater when mean arterial pressure was normal. In another six dogs (group 2), CO₂ responsiveness was examined during isoflurane-induced hypotension without prior determination of CO₂ responsiveness at normal mean arterial pressure and during sodium nitroprusside-induced hypotension to a mean arterial pressure of 50 mm Hg. As in group 1, partial preservation of CO₂ responsiveness was observed during isoflurane-induced hypotension; the magnitude of the response in group 2 during isoflurane-induced hypotension was similar to that in group 1. In contrast, in group 2 during sodium nitroprusside-induced hypotension, hypocapnia caused no significant change of cerebral vascular resistance or cerebral blood flow. It is concluded that cerebral vessels respond to changes in Paco₂ differently during isoflurane-induced hypotension than during hypotension with other commonly used hypotensive treatments. Hypocapnia decreases cerebral blood flow during isoflurane-induced hypotension and, therefore, may also decrease cerebral blood volume, brain bulk, and intracranial pressure.

Key Words: ANESTHETICS, VOLATILE—isoflurane • ANESTHETIC TECHNIQUES—hypotension • BRAIN—blood flow, cerebral vascular resistance • CARBON DIOXIDE—hypocapnia

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999