Effects of volatile anesthetics on force interval relationships in guinea-pig atrial preparations

Depressant effects of halothane, enflurance and isoflurance on isolated guinea pig right atrial muscles bathed in Tyrode's solution at 30℃ were examined. Contractions were elicited by stimulation through external field electrodes while tension was recorded continuously. Frequency-force relationships at stimulation rates of 0.1, 0.2, 0.5 and 1Hz were studied. The pD_2 values of these volatile anesthetics at each stimulation rates were not different significantly, suggesting the frequency-independent depressions of these anesthetics. Interval-strength relationships at time intervals of 0.3～20 sec during 1Hz-stimulation were also studied. Mechanical restitution curves after converting to logistic function reached a peak at 8～20sec and were fitted well to double exponential functions with time constants of 200～600 msec (k_1) and 2～6sec (k_2). All volatile anesthetics depressed the magnitude constants for fast response in a dose dependent manner. Accordingly, at high concentrations, mechanical restitution curves were fitted to single exponential functions. Halothane and enflurane did not alter time constants k_1, k_2 significantly, while isoflurane increased k_2 significantly. These results suggest that the mechanisms of myocardial depressant effects are different between these anesthetics.