Contribution of Atrioventricular Synchrony to Left Ventricular Systolic Function in a Closed‐Chest Canine Model of Complete Heart Block: Implications for Single‐Chamber Rate‐Variable Cardiac Pacing

This study assessed the impact of atrioventricular (AV) synchrony on characteristics of left ventricular (LV) systolic function during ventricular pacing over a wide heart rate range in a conscious closed‐chest canine model of complete AV block. Ten healthy adult dogs underwent thoracotomy during which complete AV block was created by formaldehyde injection, and paired ultrasonic sonomicrometers were positioned on the LV anterior‐posterior minor axis. Following recovery from surgery, peak and end‐diastolic LV transmural pressure, maximum dP/dt, stroke work, end‐diastolic minor axis dimension, and maximum velocity of shortening, were quantitated at heart rates of 80, 100, 120, 140, and 160 beats per minute (bpm) during both ventricular pacing alone and AV sequential pacing with increasing AV intervals (0, 50, 100, 150, 200, 250, and 300 ms). Over the heart rate range tested, parameters of LV systolic function did not differ significantly during ventricular pacing with or without AV synchrony. For example, during ventricular pacing alone maximum LV dP/dt varied from 2110 ± 70 mmHg/s to 2463 ± 567 mmHg/s, a range essentially identical to that observed in the presence of AV synchrony. On the other hand, although the impact on LV performance of varying AV interval from 0 to 300 ms was small, differences tended to become more pronounced at higher pacing rates. At 80 bpm, neither stroke work nor maximum LV dP/dt were affected by change in AV interval, while at heart rates 120 bpm both stroke work and LV dP/dt tended to maximize at AV intervals of 50 and 100 ms and thereafter declined. Thus, findings in this study indicated that LV systolic function in the normal in situ heart is not adversely affected by absence of AV synchrony over a wide heart rate range.