Comparative myocardial depressive effects of propofol and thiopental

To compare the direct myocardial effects of thiopental and propofol, concentration-contractility curves were determined by using an isolated guinea pig left atrial preparation. Contractility was measured as peak twitch tension, maximum rate of contraction (dT/dt), and maximum rate of relaxation (-dT/dt). Drug concentrations ranged from 56 microM (10 micrograms/mL) to 2800 microM (500 micrograms/mL) propofol and 3.8 microM (1 microgram/mL) to 190 microM (50 micrograms/mL) thiopental. Data were fitted to a sigmoidal inhibitory pharmacodynamic model in which an increasing incremental drug concentration is needed to cause a consistent incremental effect. Comparisons were then made of the drug concentrations at which a 50% inhibition in contractility was noted (IC50). To compare drugs on an equi-anesthetic basis, comparisons also were made between IC50 values normalized to anesthetic concentrations needed to produce anesthesia (67.8 microM and 28.1 microM for thiopental and propofol, respectively). Data from seven atrial preparations for thiopental and seven atrial preparations for propofol were analyzed. The IC50 (microM) for peak twitch tension were 20.9 +/- 6.3 and 96.6 +/- 10.1 (P < 0.05), for dT/dt were 21.1 +/- 5.9 and 86.9 +/- 8.3 (P < 0.05), and for -dT/dt were 21.0 +/- 6.1 and 84.3 +/- 9.2 (P < 0.05) for thiopental and propofol, respectively. When the normalized IC50 were compared, the differences between anesthetics became greater. For guinea pig left atria in vitro, thiopental is a more potent myocardial depressant than propofol, especially when compared relative to anesthetic potencies; neither anesthetic preferentially affects peak twitch tension, maximum rate of contraction, or maximum rate of relaxation.