EFFECTS OF DOBUTAMINE ON LEFT-VENTRICULAR RESTORING FORCES

Restoring forces, which are generated when the left ventricle contract s below its equilibrium volume (V-eq), are responsible for diastolic s uction. Their magnitude is inversely related to end-systolic volume (E SV). In previous studies in which the mitral valve was replaced with a prosthesis, increased contractility was shown to augment restoring fo rces independently of ESV. In the present study, we quantified restori ng forces in the presence of an intact mitral valve in open-chest dogs (n = 6) as the fully relaxed pressure (FRP) after completion of left ventricular pressure (LVP) fall during nonfilling diastoles produced b y a servomotor system that clamped left atrial pressure below LVP. A n egative FRP indicated a restoring force was present. We related FRP to ESV during control, intravenous, and left anterior descending coronar y artery (intracoronary) administration of dobutamine. With intravenou s dobutamine, we observed an approximately parallel downward and right ward shift of the FRP-ESV relation, indicating increased restoring for ces at any ESV less than V-eq. The downward shift averaged -2.6 +/- 1. 6 (SD) mmHg at the control V-eq. A similar shift occurred with intraco ronary dobutamine. In additional experiments (n = 2), we found that ov er a common range of ESV dobutamine slightly increased wall thickness (<10%) during nonfilling diastoles, consistent with an increase in cor onary blood volume. We conclude that dobutamine increases restoring fo rces independently of changes in ESV in conjunction with an increase i n V-eq. This effect may partly be related to increased coronary blood volume.