The Effect of the Atrioventricular Interval During Atrioventricular Sequential Pacing on the Hemodynamics in Dynamic Obstruction of the Left Ventricular Outflow Tract in Dogs

The present study was performed to clarify the mechanism involved in the reduction of the pressure gradient in the left ventricular outflow tract of patients with hypertrophic obstructive cardiomyopathy when treated with atrioventricular (AV) sequential pacing. The effect of AV sequential pacing with variable AV intervals on the hemodynamics and dyssynchronous wall motion was experimentally studied using echocardiography in the dynamic obstruction of the left ventricular outflow tract created by dobutamine infusion in 17 dogs. The pressure gradient of the left ventricular outflow tract decreased with shortening of the AV interval during AV sequential pacing. Also, the dyssynchrony time, defined as the difference in the time between the intraventricular septum and posterior wall during the systolic phase recorded with M-mode echocardiography, increased with shortening of the AV interval during AV sequential pacing. However, very short AV intervals produced a significant decrease in the aortic pressure and increase in the pulmonary capillary wedge pressure. The dyssynchrony time showed a positive liner correlation with the percentage reduction in the pressure gradient of the left ventricular outflow tract (R=0.794, p<0.0001). It was concluded that dyssynchronous wall motion in the left ventricle was produced by pacing from the right ventricular apex and resulted in a reduction in the pressure gradient of the left ventricular outflow tract. Optimization of the AV interval during AV sequential pacing may play an important role in improving the hemodynamics in dynamic obstruction of the left ventricular outflow tract.