Relationships between beat-to-beat interval and the strength of contraction in the healthy and diseased human heart.

Twenty-six adult patients, classified by clinical and catheter criteria into groups of those with normal and abnormal left ventricular function, were studied during cardiac catheterization. Right heart pacing was established, and left ventricular dP/dt was measured with end-catheter manometers. By varying the interval preceding a test beat after periods of steady pacing it was confirmed that recovery of left ventricular mechanical function (maximum dP/dt) occurs approximately 800 msec (optimum interval) after a beat. The augmentation of maximum dP/dt of the first 2 beats after an extrasystole, each spaced at the optimum interval, was also studied; the amount of potentiation was varied by alterations in extrasystolic interval. Potentiation decayed from the first to the second postextrasystolic beat with a ratio that was fixed in each individual patient. The ratio (recirculation fraction) was higher in patients with normal than in those with abnormal left ventricular function (mean +/- SD 0.52 +/- 0.10 vs 0.37 +/- 0.11, p less than .005). There was an inverse relationship between this ratio and the degree of potentiation of the first postextrasystolic beat (r = .80, p less than .001). We postulate a disturbance of excitation-contraction coupling mechanisms to explain these effects.