Comparison of the  In Vitro Myocardial Depressant Effects of Isoflurane and Halothane Anesthesia

The myocardial depressant effects of isoflurane and halothane were compared using feline right ventricular papillary muscles bathed in Krebs-bicarbonate solution. In experiment 1 muscles were stimulated by field electrodes (0.2 Hz) to obtain control measurements of developed tension (dt) and maximal rate of tension development (dF/dt) prior to exposing the papillary muscles to four concentrations of either isoflurane (4.0%, 2.0%, 1.0%, 0.5%) or halothane (2.0%, 1.0%, 0.5%, 0.25%). Repeat measurements of dt and dF/dt were recorded after 20 min at each concentration. Isoflurane and halothane both caused dose-dependent depression of dt and dF/dt, but at 0.5%, 1.0%, and 2.0%, halothane was significantly more depressant than isoflurane (P less than 0.01 for dt and dF/dt). Quadratic equations were fitted to the dose-response data by least squares analysis (R2 greater than .985 for both anesthetics), and the isoflurane and halothane concentrations that decreased dt to 90%, 70%, 50%, and 30% of control were determined to compare the relative myocardial depressant potency of isoflurane and halothane by linear regression analysis. This potency relationship is described by the equation: isoflurane concentration = -0.005 + 1.445 (halothane concentration). In experiment 2 papillary muscle responses at two similar cardiodepressant concentrations of isoflurane (1.25% and 2.0%) or halothane (0.80% and 1.35%) were compared at stimulus frequencies of 0.05, 0.1, 0.2, 0.4, 0.8, 1.0, and 2.0 Hz. The concentrations of isoflurane and halothane were selected from the data obtained in experiment 1 and represent the anesthetic concentrations that diminish muscle function to approximately 70% and 50% of control.(ABSTRACT TRUNCATED AT 250 WORDS)