End-systolic regional wall stress-length and stress-shortening relations in an experimental model of normal, ischemic and reperfused myocardium.

Assessment of left ventricular function is influenced by a number of hemodynamic factors. The purpose of this study was to evaluate the end-systolic regional wall stress-dimension relation in a series of 25 mongrel dogs. In Group A (n = 18) the regional wall stress-velocity of circumferential fiber shortening relation was measured before and after three interventions: volume infusion, metoprolol infusion and dobutamine infusion. The electrocardiogram, left ventricular pressure and its first derivative (dP/dt), arterial pressure and echocardiograms were recorded at baseline and after phenylephrine administration (to increase afterload). For each dog values for regional wall stress were plotted against the velocity of circumferential fiber shortening. For all dogs the relations were inversely linear (r = -0.65 to -0.98). Volume infusion increased end-diastolic volume (p less than 0.05) without shifting the slope or intercept of the relation, indicating its independence from preload. Dobutamine caused a shift in the intercept to the right but no change in slope and metoprolol shifted the relation to the left without altering the slope. The effect of ischemia and reperfusion on the end-systolic regional wall stress-length relation was examined in Group B (n = 7), at baseline, after 5 min of mid-left anterior descending coronary artery occlusion and after 10 min of reperfusion. Afterload was increased by hydraulic aortic occlusion. Regional wall stress was lowest at baseline, highest during ischemia and intermediate after reperfusion (100.2 +/- 32.1, 193.5 +/- 81.5, 141.9 +/- 67.6 kdyn/cm2, respectively, p less than 0.001). The end-systolic regional wall stress-length relation was linear during baseline, ischemia and reperfusion (r = 0.96, 0.95, 0.98, respectively, p less than 0.05). Ischemia caused an increase in the slope and a shift in the intercept to the right compared with baseline. Reperfusion represented an incomplete return toward baseline. This study demonstrates that the regional wall stress-velocity of circumferential fiber shortening relation is linear, independent of preload, incorporates afterload and is sensitive to changes in contractility. In addition, the regional wall stress-length relation can be measured in a nonhomogeneously contracting left ventricle and is predictably altered by ischemia and reperfusion. This relation may prove useful to determine whether alterations in regional systolic function result from changes in local load or contractility in the left ventricle with a regional wall motion abnormality.