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Cardiology Unit, University of Vermont College of Medicine, Burlington, Vermont 05405
Restoring forces, which are generated when the
left ventricle contracts below its equilibrium volume
(Veq), are responsible for
diastolic suction. Their magnitude is inversely related to end-systolic
volume (ESV). In previous studies in which the mitral valve was
replaced with a prosthesis, increased contractility was shown to
augment restoring forces independently of ESV. In the present study, we
quantified restoring 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 by a
servomotor system that clamped left atrial pressure below LVP. A
negative FRP indicated a restoring force was present. We related FRP to ESV during control, intravenous, and left anterior descending coronary artery (intracoronary) administration of dobutamine. With intravenous dobutamine, we observed an approximately parallel downward and rightward shift of the FRP-ESV relation, indicating increased restoring forces at any ESV less than
Veq. The downward shift averaged
2.6 ± 1.6 (SD) mmHg at the control
Veq. A similar shift occurred with
intracoronary dobutamine. In additional experiments (n = 2), we found that over a common
range of ESV dobutamine slightly increased wall thickness (<10%)
during nonfilling diastoles, consistent with an increase in coronary
blood volume. We conclude that dobutamine increases restoring forces
independently of changes in ESV in conjunction with an increase in
Veq. This effect may partly be related to increased coronary blood volume.
ventricular suction; isovolumic relaxation; diastolic function
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