TIME-VARYING WALL STRESS - AN INDEX OF VENTRICULAR VASCULAR COUPLING

Previous work in the isolated heart and intact circulation has suggested that the relationship between wall stress and time during left ventricular (LV) ejection is linear and that the slope, which will be referred to as time-varying wall stress, increases in response to augmentation in afterload. However, the etiology of the increase in slope has not been determined in an intact animal. Magnetic resonance imaging coupled with high-fidelity LV pressure measurement using a nonferrous catheter-tip manometer generates a detailed assessment of wall stress in an animal model where the thorax and pericardium have never been disturbed. Accordingly, six anesthetized dogs were studied during autonomic blockade with atropine and propranolol during angiotensin infusion, producing three widely disparate left ventricular systolic pressures (87 +/- 7 vs. 124 +/- 13 vs. 152 +/- 10 mmHg, P < 0.001). Time-varying wall stress did not change from low to medium load (-42.4 +/- 9.5 to -27.3 +/- 22.3 g.cm-2.ms-1) but increased significantly at high load (-21.7 +/- 14.9 g.cm-2.ms-1, P < 0.05). Analysis of the relative contribution of pressure, chamber radius, wall thickness, and long-axis dimension to the changes in time-varying wall stress demonstrated only the pressure component to change its relative contribution at medium (P < 0.001) and-high load (P < 0.001). Therefore, we conclude that the increase in time-varying wall stress results from augmentation of pressure in the latter one-half of systole that is incompletely offset by shortening and wall thickening. This increase of pressure in late systole as afterload increases would be due to an early return of pressure waves reflected from the periphery and could have an adverse effect on left ventricular ejection dynamics.