Diastolic myocardial wall stiffness and ventricular relaxation during partial and complete coronary occlusions in the conscious dog.

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Abstract
Seven dogs were instrumented with a left ventricular micromanometer, pairs of ultrasonic crystals to measure wall thickness in normal and ischemic regions, as well as both short and long axes of the left ventricle; cuff occluders were placed around the left circumflex coronary artery and the inferior vena cava. Left ventricular volume, mass, circumferential wall stress, midwall strain and strain rate were calculated at rest, at 1 and 2 min of partial and complete coronary occlusion, and 1, 2, and 10 min after release of partial and complete coronary occlusions. Reference values at zero pressure were obtained during inferior vena cava occlusion. Relaxation was assessed from the exponential fall of left ventricular pressure and from the exponential fall of left ventricular stress (myocardial relaxation) of the control and the ischemic wall regions. Passive diastolic wall stiffness was calculated from the exponential diastolic stress-strain relationship. At zero diastolic pressure, the left ventricular short axis increased from 35-37 mm during complete coronary occlusion but remained unchanged during partial occlusion. One minute after release of partial and complete occlusion, left ventricular short axis decreased significantly compared to control and returned to control 10 min after release of coronary occlusion. The time constant of ventricular pressure fall increased during complete coronary occlusions, whereas myocardial relaxation of the control wall region increased, and, of the ischemic wall region, decreased, during both partial and complete occlusions. Passive diastolic wall stifness increased significantly from 22-27 (23% increase) at 2 min of complete coronary occlusion but was unchanged during partial occlusion. One minute after release of both partial and complete coronary occlusion, wall stiffness decreased significantly. The diastolic pressure-volume relationship of the left ventricle was shifted significantly upward during complete coronary occlusion without inferior vena cava occlusion, but no shift was observed during partial occlusion or in the experiments with inferior vena cava obstruction. Myocardial wall stiffness is normal during coronary stenosis when systolic wall thickening is reduced but maintained, whereas it is increased during complete coronary occlusion when there is systolic wall thinning. Resting muscle length at zero diastolic pressure is increased during complete coronary occlusion and decreased during reperfusion, demonstrating a genuine change in resting muscle length. Regional relaxation parameters show a decrease in the rate of relaxation of the ischemic wall but an increase in that of the nonischemic wall during partial and complete coronary occlusions. The upward shift of the diastolic pressure-volume curve during complete coronary occlusion is prevented by inferior vena cava obstruction and could be due to reductions in right ventricular pressure and volume (ventricular interaction).

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