Dynamic Geometry of the Left Ventricle during Ventricular Pacing: Correlation with Cardiac Pumping Action

In five anesthetized open chest dogs, contraction patterns of the left ventricle induced by ventricular pacing were analyzed using cineangiocardiography. Left ventriculography was performed with dogs in the right anterior oblique position. The right atrial appendage (RA) and two ventricular sites (RVO: outflow tract of the right ventricle and LVA: apex of the left ventricle) were stimulated electrically at a rate of 150 per min. Contraction patterns and cardiodynamics in response to ventricular pacing were compared with those to RA pacing, which represents a normal sequence and extent of ventricular contraction. RVO pacing resulted in asyneresis of anterior wall and normal contraction of posterior wall, while LVA pacing produced a systolic expansion of the apex associated with enhanced contraction of the posterior wall. Ventricular pacing uniformly caused decreases in left ventricular end-diastolic volume (LVEDV), stroke volume (SV) and ejection fraction (EF), with relatively constant left ventricular end-systolic volume (LVESV). Blood pressure was significantly decreased with ventricular pacing. There were only minor differences of these parameters between RVO and LVA pacing. It was considered that a diminution of SV and EF during ventricular pacing resulted from the asynchronous contraction of the ventricle which was not related to decreased myocardial contractility.