Determinants of end-systolic pressure-volume relations during acute regional ischemia in situ

The influence of extent and location of regional ischemia, baseline left ventricular systolic function, and autonomic reflexes on in situ left ventricular end-systolic pressure-volume relations (ESPVRs) during coronary occlusion were studied in 13 open-chest dogs. Circumflex or left anterior descending arteries were randomly occluded (at proximal or distal sites) for 3 minutes in reflex-blocked (n = 6, hexamethonium/vagotomy) and unblocked (n = 7) animals. Pressure-volume data were obtained by the conductance-catheter technique, with ESPVRs determined by transient inferior vena caval occlusion. Ischemic zone size was estimated for each occlusion by radiolabeled microspheres. The relative influence of each variable on ESPVR change with ischemia was determined by multiple regression analysis. As in previous studies, regional ischemia displaced ESPVRs to the right by an amount that varied directly with ischemic bed size (y = +0.48x, r = 0.76, p < 0.001). However, in contrast to previous data, coronary occlusion also reduced the ESPVR slope (end-systolic elastance, Ees) in the majority of cases. The extent slope change was primarily dependent on the baseline elastance [Ees(base)], such that the higher the initial elastance, the larger its subsequent reduction for any amount of ischemia (ΔEes = -0.78 Ees(base), r = 0.94, p < 0.001). Active reflexes added an offset constant to this relation (+3.15 mm Hg/ml, p < 0.001). In addition, Ees fell slightly more with larger ischemic regions. Thus, although previous studies have reported primarily rightward parallel shifts in ESPVR with regional ischemia, the present data demonstrate that the slope of the relation is often reduced. Greater baseline elastances typical of in situ, as opposed to isolated, ventricles probably explain the differences in apparent responses.