CARDIAC PERFORMANCE DURING PROLONGED HALOTHANE ANAESTHESIA IN THE CAT Isolated heart muscle study

The contractile response of papillary muscles, isolated from normal cats, to prolonged administration of halothane at minimum alveolar anaesthetic concentration (MAC) was studied. Average values of maximal velocity of shortening (V^^) and maximal developed force (Fm) obtained in 12 muscles during the 1st, 2nd and 3rd hour of exposure to halothane anaesthesia were significantly less than those obtained during the control period and after recovery from halothane. There were no significant differences in values over a 3-h period. When post-extra-systolic potentiation was induced by paired electric stimulation, average values of Vmax and Fm increased significantly. We conclude that prolonged administration of halothane at a constant concentration resulted in a sustained depression of myocardial contractility without tissue tolerance. The myocardium depressed by halothane, however, still retains the ability to respond to additional inotropic stimulation. McGregor and associates (1958) observed that initial decreases in cardiac output during a constant level of halothane anaesthesia in children were followed by a gradual return toward normal. A similar return of cardiac output towards norma