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Preload changes modify systolic pressure-diameter relations in the conscious dog.

The behavior of the end-systolic ventricular diameter was examined during control state, atrial pacing, and acute volume loading (VL) followed by methoxamine infusion in conscious dogs instrumented with a left ventricular micromanometer and ultrasonic crystals measuring internal diameter and wall thickness. Heart rate and systolic pressure were markedly increased by maximal VL but for moderate VL were minimally modified. Moderate VL increased end-diastolic diameter by 5% and end-systolic diameter by 2.9% with a significant increase of systolic stress. There was no common final pathway of shortening in the force-velocity-length diagram for different preloads. During methoxamine infusion, the end-systolic pressure-diameter points described a linear regression but points obtained during atrial pacing before VL were significantly shorter. We conclude that VL increases the total loading of the heart producing a lengthening of the end-systolic diameter when end-diastolic diameter is increased. This reduces the expression of the Starling mechanism and suggests caution when interpreting end-systolic pressure-diameter relations and derived indices for significantly different end-diastolic diameters.