Regional Contraction patterns of the Left Ventricle during Ventricular Pacing

Ten mongrel dogs were used to assess regional myocardial function in response to electrical pacing from the inflow (RVI), outflow tracts (RVO), and apex (RVA) of the right ventricle and apex (LVA) and lateral wall (LVL) of the left ventricle. Straingauge arches were sutured to the epicardial segments of the apical and lateral walls of the left ventricle and their mechanical shortening was measured during right atrial (RA) and during ventricular pacing. The onset of shortening of two segments did not differ significantly in RA and RVI pacing, while the stimulation of RVA and LVA caused the initial contraction of the apical segment, and RVO and LVL pacing caused that of the lateral segment; i. e. RVA and RVO pacing resulted in left ventricular asynchrony of contraction similar to LVA and LVL pacing, respectively. Asynchrony of contraction was not accompanied by a uniform change in peak tension of regional myocardial segments, but pre-ejection tension rose and ejection tension fell at the pacing site. The reduction in mean aortic pressure inversely correlated with the prolongation of the time interval between the onset of shortening of two segments during left ventricular pacing. The decrease in cardiac performance observed during ventricular pacing was related to the severity of asynchrony rather than the direction of the ventricular depolarization or change in regional myocardial tension.