Sinusoidal and Exponential Decays of Postextrasystolic Transient Alternans in Excised Blood-Perfused Canine Hearts.

We have recently reported that the postextrasystolic contractile potentiation decays in alternans after a compensatory pause in canine left ventricles even under normal coronary and contractile conditions. The transient alternans appears to consist primarily of a small-magnitude exponential decay and a large-magnitude sinusoidal decay. We, therefore, hypothesized that the contractility (y) of the postextrasystolic alternans beats (beat number x) could be expressed as y = a x exp[-(x-1)/b] + c x exp[-(x-1)/d] x sin[pi(x-0.5)] + yo, where a and c are the normalized magnitudes (relative to the preceding regular beat) of the two exponential terms in the first postextrasystolic beat, b and d are their time constants, and yo is the normalized magnitude of the post-alternans regular beat (approximately 1). The first exponential term represents the monotonic decay. The sine term multiplied by the second exponential term represents the alternating decay. Mathematical curve-fitting indicated: 1) the above equation very closely fitted the alternans data with a squared correlation coefficient of 0.9996 on average, 2) c was 7 times on average greater than a, indicating dominance of the sine component, 3) b and d were 2.5 and 1.0 beats on average, indicating a faster decay of the sine component, and 4) this b was comparable to the time constant of the exponential decay of the postextrasystolic potentiation after no compensatory pause. This study suggests that myocardium has a mechanism to switch the postextrasystolic potentiation between the exponential and alternans decays depending on the first postextrasystolic interval.