Figure 4. Effect of propofol on stimulus-to-His bundle (S-H) interval during rapid atrial pacing of a guinea pig isolated perfused heart. Atrial cycle length was abruptly shortened from 300 ms to 7% greater than the Wenckebach cycle length (WCL) for a 60-s period between 30 and 90 s. An example of progressive prolongation of the S-H interval (Delta S-H) in the absence of propofol and in the presence of 10 and 25 micro Meter propofol as a function of time after an abrupt increase in rate of pacing is shown. In comparison with control, in the presence of 10 micro Meter propofol, Delta S-H was greater and did not reach a steady state. In the presence of 25 micro Meter propofol, second-degree atrioventricular block ensued immediately after initiation of fast atrial pacing.