Figure 5. Frequency-dependent prolongation of the S-H interval caused by anesthetics. Frequency-dependencence ratios ([SHdrug-SHcontrol)(fast)/(SHdrug-SHcontrol]slow) for propofol (n = 5), thiopental (n = 6), and ketamine (n = 3)(A) at drug concentrations that produced statistically-equivalent prolongations in the S-H interval at an atrial cycle length (ACL) of 300 ms (B). A ratio greater than one indicates a greater effect of an agent at a faster pacing rate than at a slower pacing rate. As per the simulated tachycardia experiments, ACL was abruptly shortened from 300 ms to an interval 10% above WCL for 90 s and then returned to 300 ms. Bars represent means+/-SEM. *P < 0.05 versus thiopental and propofol.