A Comparison of the Cerebrovascular and Metabolic Effects of Halothane and Isolflurane in the Cat

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Abstract
Halothane [HAL] is a well known cerebral vasodilator that can produce dangerous increases in intracranial pressure (ICP) in certain neurosurgical patients. Isoflurane [ISO] may be a less potent cerebral vasodilator. The effects of HAL and ISO on cerebral blood flow (CBF), cerebral vascular resistance (CVR), intracranial pressure and cerebral metabolic rate for O2 (CMRO2) were studied. Studies were carried out in normocarbic mechanically ventilated cats using the intracarotid 133Xe injection technique to measure CBF. The effects of 3 doses were examined: 0.5, 1.0 and 1.5 MAC [minimum alveolar concentration] studied in the continued presence of 75% N2O. Autoregulation also was tested at 1.0 MAC (plus 75% N2O) by recording CBF and CVR before and after elevation of blood pressure with angiotensin. Both agents has similar effects on blood pressure and ICP but while halothane produced significant increases in CBF at all doses, with values of 61 .+-. 5 ml/100 g per min (123 .+-. 8% of control, mean .+-. SE) at 1.0 MAC, isoflurane anesthesia caused no significant changes in CBF at any level, (e.g., 48 .+-. 8 ml 100 g per min or 94 .+-. 12% of control at 1.0 MAC). Both drugs produced dose-related decreases in CVR but the changes were greater with halothane, e.g., CVR at 1.0 MAC halothane = 1.46 .+-. 0.20 mm Hg/ml per 100 g per min (47 .+-. 7% of control) compared with 2.23 .+-. 0.40 mm hg/ml per 100 g per min (72 .+-. 9% of control). Isoflurane produced greater decreased in CMRO2 than did halothane and impaired autoregulation less. Apparently, ISO possesses cerebrovascular properties that are different from HAL, suggesting that ISO may come to play an important role in future neuroanesthetic practice.