Deliberate hypotension and anesthetic requirements of halothane

Following determination of minimum alveolar anesthetic concentration (MAC) of halothane during normotension, 18 dogs were divided into three groups. Dogs in each group then received either pentolinium, trimethaphan, or sodium nitroprusside, and mean arterial pressure was decreased to 60% of control values for 1 hour. The MAC of halothane was determined during hypotension and then arterial pressure was returned to control values and MAC was redetermined at normotension. The MAC of halothane decreased significantly during hypotension in all three groups: in the pentolinium group from a control MAC of 1.2 +/- 0.14 (SEM) to a MAC of 0.7 +/- 0.11 during hypotension, in the trimethaphan group from a control MAC of 1.02 +/- 0.06 to a MAC of 0.72 +/- 0.07 during hypotension, and in the nitroprusside group from a control MAC of 1.02 +/- 0.07 to a MAC of 0.73 +/- 0.07 during hypotension. Upon return of mean arterial pressure to normotensive levels, MAC of halothane returned to control values in dogs given pentolinium and trimethaphan, but in dogs given nitroprusside MAC remained at the hypotensive value. There was no correlation between the changes in MAC and carotid blood flow or cardiac index. These data indicate that anesthetic requirements decrease during deliberate hypotension irrespective of the drug used to produce hypotension.