Cardiovascular Effects of Plasma Levels of Thiopental Necessary for Anesthesia

The cardiovascular effects of plasma levels of thiopental necessary for anesthesia were studied using systolic time intervals (STI). In ten healthy patients anesthesia was induced with thiopental, 2-2.5 mg/kg, intravenously, and maintained with an infusion of 1-1.5 mg/kg/min. STI and thiopental plasma levels were measured before induction and when corneal reflex and trapezius muscle response, indicators of anesthetic depth equivalent to response to surgical stimulation, were lost. Significant changes included: an increase in heart rate with induction of anesthesia; a decrease in 1/pre-ejection period2--indexed for heart rate (1/PEP2-I) at loss of corneal reflex; a decrease in systolic blood pressure and 1/PEP2-I at loss of trapezius muscle response. No other variable was significantly different from control. Control values for STI were in the high-normal range, indicating some sympathetic stimulation. With induction of anesthesia these values decreased to a normal range. Free and total plasma levels were 5.4 and 37.6 microgram/ml at loss of corneal reflex; 6.1 and 41.6 microgram/ml at loss of trapezius muscle response. In comparison with other studies, thiopental causes less cardiac depression than inhalational agents at approximately the same anesthetic depth. It is concluded from this study in healthy patients that plasma levels of thiopental producing surgical anesthesia result in minimal cardiac depression as determined by systolic time intervals.