Figure 3. Atrial cycle length (ACL)-response relation for the negative dromotropic effect (increase in stimulus-to-His bundle interval) in guinea pig isolated perfused hearts in the absence of propofol and with 10 and 25 micro Meter propofol. As the concentration of propofol increased, the stimulus-to-His bundle prolongation (Delta S-H interval) was greater at any given ACL. ACL was inversely related to pacing rate (beats per minute). Therefore, as pacing rate increased, ACL decreased. ACL values of 200, 300, and 400 ms corresponded to heart rates of 300, 200, and 150 beats/min, respectively. Each point is the mean plus/minus SEM of data from six guinea pig hearts.