Load dependence of cardiac output in biventricular pacing: right ventricular pressure overload in pigs.

Background: The effect of biventricular pacing on stroke volume is believed to be dependent on right ventricular/left ventricular delay, but effects in individual patients are unpredictable. This variability may reflect relative right and left ventricular volume and/or pressure overloads. Accordingly, we tested the hypothesis that the relation of cardiac output to right ventricular/left ventricular delay is load dependent in a pig model of pulmonary stenosis.
Methods: After median sternotomy in 6 anesthetized, domestic pigs, complete heart block was induced by ethanol ablation. During epicardial, atrial tracking DDD biventricular pacing, atrioventricular delay was varied between 60 and 180 ms in 30-ms increments. Right ventricular/left ventricular delay was varied at each atrioventricular delay from +80 ms (right ventricle first) to -80 ms (left ventricle first) in 20-ms increments. Aortic flow, right ventricular pressure, peripheral arterial pressure, and electrocardiogram were measured in the control state and during pulmonary stenosis, created by tightening a snare around the pulmonary artery until cardiac output decreased by 50%.
Results: Atrioventricular and right ventricular/left ventricular delay had no effect on cardiac output during the control state, but during pulmonary stenosis there was a statistically significant (P =.0001, repeated-measures analysis of variance) right ventricular/left ventricular delay-related trend toward higher cardiac output with right ventricular pacing first. This effect was more pronounced when the optimal atrioventricular delay was determined first, resulting in a 20% increase in cardiac output when the optimal right ventricular/left ventricular delay was compared with simultaneous biventricular pacing.
Conclusions: Optimized biventricular pacing in swine is associated with increased cardiac output during acute pulmonary stenosis, but not during the control state. Further studies are needed to determine whether specific types of right ventricular and left ventricular overload predictably affect the relation between right ventricular/left ventricular delay and cardiac output.