Physiologic determinants of left ventricular strength-interval curve of the dog

Strength-interval curves, relating an index of the contractile vigor of premature beats to their coupling intervals, were elicited in anesthetized, open-chest dogs prepared for atrial or ventricular pacing and the high-fidelity measurement of left ventricular pressure. During inotropic interventions, changes of the curves were evaluated in terms of the parameters (slopes and intercepts) of their straight lines of best fit. These parameters were altered by isoproterenol, propranolol, and heart rate, and were correlated closely with paired values of isovolumic Vmax (of driven beats). The strength-interval curves were reproducible and were not affected by large changes of venous return or arterial blood pressure. These results provide a basis for the use of strength-interval curves for the evaluation of ventricular contractile state, using fluid-filled catheters for the measurement of left ventricular pressure.