Factors reducing left and right ventricular output during simultaneous atrioventricular activation in the pig heart

Forward stroke volume fell by 26% (23-30%) (median and 95% confidence interval) when simultaneous atrioventricular (AV) pacing was induced at constant heart rate in 10 anaesthetized open-chest pigs. To assess the relative importance of factors which could cause this reduction in right and left ventricular (RV and LV) output, we compared cardiac dynamics when either ventricular filling or forward stroke volume was equally reduced by caval constriction and simultaneous AV pacing. We estimated the degree of ventricular filling by recording segment lengths (SL) in the free walls of both ventricles. Our analysis revealed that abolished active LV filling by the left atrium reduced forward stroke volume by 11% (8-14%). The remaining fall in output could be attributed to mitral regurgitation. In the right side of the heart the response was different. The drop in RV filling during simultaneous AV pacing accounted for approximately one-half of the fall in forward RV stroke volume. Estimates based on SL recordings demonstrated that forward RV stroke volume fell by 7% (2-25%) because of tricuspidal regurgitation. Pulmonary artery pressure was 4.5 (3.4-5.7) mmHg higher during simultaneous AV pacing than during caval constriction, representing a relative rise in afterload that reduced the RV stroke volume by 6-8%. Thus, reduced ventricular filling during simultaneous AV pacing accounted for approximately one-half of the drop in forward output from both ventricles. Slightly more than one-half of the reduction in forward LV stroke volume could be attributed to mitral regurgitation. In the right side of the heart tricuspidal regurgitation and a relative rise in pulmonary artery pressure each accounted for about one-quarter of the fall in forward RV output.