Influence of pacing site on canine left ventricular force-interval relationship

We tested the hypothesis that while absolute extrasystolic (ES) and postextrasystolic (PES) dP/dtmax would be influenced by pacing site, if these values were normalized by the steady-state dP/dtmax obtained with the same pacing site, they would be independent of pacing site. We employed an isovolumetrically contracting isolated heart preparation with a balloon placed in the left ventricle. Pacing electrodes were placed on the atrium and four different epicardial ventricular sites. While pacing from each site separately, a steady mechanical state was established at a constant rate, and the interval between beats was then perturbed. The same pacing sequence was used while pacing from each of the five sites. On each ES and PES beat, dP/dtmax was expressed as a percentage of the steady-state dP/dtmax. The normalized responses measured with each ventricular pacing site (DPventr) were plotted as a function of the normalized responses obtained with atrial pacing (DPatr). On average, DPventr = 1.02 DPatr - 3.2% (n = 6, with r2 = 0.989), indicating that pacing site did not significantly influence the "normalized" force-interval relationship. Thus the basic information retrievable from measurements of the ventricular force-interval relationship is independent of the site from which the heart is paced.