Postextrasystolic contractility normally decays in alternans in canine in situ heart

Decay of postextrasystolic potentiation in the left and right ventricles of intact canine hearts

Intracellular regulation of myocardial Ca2+ has long been of interest to physiologists. The force-interval relationship provides a phenomenological approach that permits insight into aspects of calcium regulation. The response to an extrasystole is a potentiation in contractile force and the recovery in contractile force is described by the recirculation fraction (RF). The RF provides a gross estimation of calcium uptake by sarcoplasmic reticulum (SR), leading to myocardial relaxation. The current study focused on the relationship of right (RV) and left ventricular (LV) RF in canines under several contractile states. Anesthetized canines (n = 5) were catheterized for RV and LV pressure measurements. dP/dt(max) for the RV and LV was calculated for three baseline beats, one extrasystole and the first five postextrasystolic beats. The relationship between the LV dP/dt(max) and RV dP/dt(max) for all of the mentioned beats was then examined. Contractility was increased with calcium chloride and extrasystoles were delivered. Once cardiac function returned to a baseline level, contractility was reduced by increasing the concentration of isoflurane and the evaluation repeated. All ventricular contractions were controlled by RA pacing to maintain intrinsic conduction. A strong linear relationship between RV and LV dP/dt(max) (r = 0.94 +/- .06) existed for most canine's contractile states. These results build on findings in isolated hearts and demonstrate that biventricular response to extrasystoles and subsequent contractile recovery is both linear and correlated, suggesting that intracellular calcium regulation in a given heart across contractile state is static.

Alternans decay of postextrasystolic potentiation in human left ventricle

An organ-level assessment of the total Ca2+ handled in the excitation-contraction coupling in a beating heart has been accomplished in canine left ventricles (LVs). This approach combines the intramyocardial Ca2+ recirculation fraction (RF) with the cardiac O2 consumption for the excitation-contraction coupling. The RF has conventionally been obtained from the exponential decay of the postextrasystolic (PES) potentiation of myocardial contractility. However, in canine LVs, the PES contractility in terms of Emax (end-systolic pressure-volume ratio) has been shown to decay generally in alternans under both physiological and pathological conditions. Nevertheless, the RF can be obtained from the exponential decay component in the PES Emax alternans decay. We expected that the same Ca2+ assessment could be applied to the human heart. As the first step, we investigated whether the PES Emax would decay in alternans or exponentially in patient LVs. We retrospectively analyzed 13 patient cases that had stable regular beats unexpectedly interrupted by a spontaneous extrasystole followed by a PES compensatory pause during their diagnostic examination. These patients had either mitral regurgitation, old myocardial infarction, or dilated cardiomyopathy. Their LV Emax decayed consistently in alternans within the first several PES beats. These Emax alternans decays resemble those reported in canine LVs. This finding suggests for the first time the applicability of the same organ-level RF assessment method developed for canine hearts to human hearts.

Arterial and Left Ventricular Pressures Illude Transient Alternans of Contractility during Postextrasystolic Potentiation

We have previously found that the postextrasystolic (PES) potentiation (PESP) of the left ventricular (LV) contractility (Emax) decays typically in transient alternans even in the normally ejecting canine heart. This contradicted the general expectation that arterial pressure (AP) and LV pressure (LVP) usually decay exponentially during PESP. We hypothesized this contradiction to be due to the different cardiodynamic behaviors of AP and LVP from LV Emax during PESP. We tested this hypothesis by measuring AP, LVP, LV volume, Emax, effective arterial elastance (Ea) as an index of afterload, and pulse pressure (PP) during PESP in eight anesthetized open-chest dogs by using the conductance catheter system. We changed Ea by changing the total peripheral resistance (TPR) with methoxamine hydrochloride (iv) and repeated the measurements. Although the Emax alternans patterns during PESP were comparable between the normal and high afterloads, LVP and PP were slightly potentiated and alternated under the normal afterload, whereas LVP and PP were obviously potentiated and alternated under the high afterload. We also simulated the effects of Ea/Emax on the transient alternans of AP and LVP on a computer. Despite the same alternans pattern of Emax, a higher Ea/Emax, which is typical in heart failure, caused a larger PP alternans, whereas a lower Ea/Emax, which is typical in normal hearts, almost eliminated it. These results suggest that a transient alternans of LV contractility during PESP could be overlooked when AP and LVP are monitored in in situ normal hearts.