Effects of change in frequency of stimulation on myocardial depression produced by thiamylal and halothane

The effects of change in the frequency of stimulation on the myocardial contractility depressed by thiamylal and halothane were studied in isolated dog heart muscle. An increase in the frequency of stimulation from 0.1 to 0.6 cps resulted in a progressive in increase in net-shortening (delta1) and maximum velocity of shortening at 0.4 g preload (V'max), namely a positive staircase. The myocardium previously depressed to a similar degree by thiamylal or halothane still showed a positive staircase. This result indicates that the mechanism to produce the myocardial depression by thiamylal or halothane is not a complete inhibition of Ca++ influx across the cell membrane. The time to reach a steady state of contraction following an increase in the frequency of stimulation was longer in the presence than in the absence of these anesthetics. The degree of recovery from the myocardial depression by increasing the frequency of stimulation was much higher in the presence of thiamylal than in the presence of halothane. This fact suggests that the mechanism to produce the myocardial depression may be different in there two anesthetics.