Evaluation of left ventricular contractile performance utilizing end-systolic pressure-volume relationships in conscious dogs

Abstract

The relationship between left ventricular end-systolic pressure and volume has been proposed as a model of left ventricular contraction which may be useful for quantifying inotropic state independent of preload and afterload. Although the model has been well-validated in isolated hearts, systematic evaluation in conscious animals with an intact peripheral circulation has been limited. Accordingly, we derived end-systolic pressure-volume relationships in twelve conscious dogs, chronically instrumented to measure left ventricular pressure and dimensions from endocardial ultrasonic crystals in three orthogonal axes. We examined the linearity of the end-systolic pressure-volume relationship, its response to alterations of inotropic state and the peripheral circulation, and the influence of β-adrenergic reflexes. End-systolic pressure-volume relationships were constructed by linear regression of end-systolic pressure-volume coordinates produced by transient inferior vena caval occlusions during atrial pacing. The relations were highly linear; of 127 inferior vena caval occlusions, the correlation coefficient was 0.96 ± 0.05 (mean ± SD). The slope of the end-systolic pressure-volume relationship was not significantly altered either by a moderate resistive afterload induced by angiotensin II infusion, or by a moderate increase in preload produced by dextran, but was increased from 4.7 ± 2.3 to 6.5 ± 2.2 mm Hg/ml (P <0.05) in response to the positive inotropic stimulus of dobutamine. The volume intercept at zero end-systolic pressure was unaffected by dextran or dobutamine, but was decreased from 12 ± 8 to 5 ± 11 ml (P <0.01) by angiotensin II infusion, indicating a leftward shift of the end-systolic pressure-volume relationship. Pretreatment with propranolol to block β-mediated adrenergic reflexes did not affect the response to angiotensin or dextran. We conclude that linear end-systolic pressure-volume relationships can be derived in conscious dogs with intact sympathetic reflexes. The slope appears to reflect left ventricular contractile function and is independent of the level of afterload and preload. However, the relation is shifted leftward by high levels of arterial resistance, indicating that end-systolic pressure is a function, not only of end-systolic volume and inotropic state, but also of the peripheral circulation, in this intact animal model.