Seasonal changes of electrophysiological heterogeneities in the rainbow trout ventricular myocardium.

INTRODUCTION: Thermal adaptation in fish is accompanied by morphological and electrophysiological changes in the myocardium. Little is known regarding seasonal changes of spatiotemporal organization of ventricular excitation and repolarization processes. We aimed to evaluate transmural and apicobasal heterogeneity of depolarization and repolarization characteristics in the rainbow trout in-situ ventricular myocardium in summer and winter conditions. METHODS: The experiments were done in summer-acclimatized (SA, 18°C, n = 8) and winter-acclimatized (WA, 3°C, n = 8) rainbow trout (Oncorhynchus mykiss). 24 unipolar electrograms were recorded with 3 plunge needle electrodes (eight lead terminals each) impaled into the ventricular wall. Activation time (AT), end of repolarization time (RT), and activation-repolarization interval (ARI, a surrogate for action potential duration) were determined as dV/dt min during QRS-complex, dV/dt max during T-wave, and RT-AT difference, respectively. RESULTS: The SA fish demonstrated relatively flat apicobasal and transmural AT and ARI profiles. In the WA animals, ATs and ARIs were longer as compared to SA animals (p≤0.001), ARIs were shorter in the compact layer than in the spongy layer (p≤0.050), and within the compact layer, the apical region had shorter ATs and longer ARIs as compared to the basal region (p≤0.050). In multiple linear regression analysis, ARI duration was associated with RR-interval and AT in SA and WA animals. The WA animals additionally demonstrated an independent association of ARIs with spatial localization across the ventricle. CONCLUSION: Cold conditions led to the spatial redistribution of repolarization durations in the rainbow trout ventricle and the formation of repolarization gradients typically observed in mammalian myocardium.

Stretch-excitation correlation in the toad heart.

The activation sequence of the ventricular myocardium in ectotherms is a matter of debate. We studied the correlation between the ventricular activation sequence and the pattern of local stretches in 13 toads (Bufo bufo). Epicardial potential mapping was done with a 56-lead sock array. Activation times were determined as dV/dt (min) in each lead. Initial epicardial foci of activation were found on the left side of the ventricular base, whereas regions on the apex and the right side of the base demonstrated late activation. Video recordings (50 frames s-1) showed that the median presystolic stretch in left-side ventricular regions was greater than that in right-side regions [4.70% (interquartile range 3.25-8.85%) versus 1.45% (interquartile range 0.38-3.05%), P=0.028, respectively]. Intracardiac bolus injection elicited ventricular activation with a similar sequence and duration. Thus, ventricular areas of earliest activation were associated with greater presystolic stretch, implying the existence of a stretch-excitation relationship in ectotherm hearts.

Action potential duration gradients in the heart ventricles and the cardiac electric field during ventricular repolarization (a model study).

BACKGROUND: We simulated contributions of transmural, apicobasal, anteroposterior and interventricular action potential duration (APD) gradients to the body surface potential distribution (BSPD) with constant or varied magnitudes of the transmural and apicobasal gradients. METHODS: Simulations were done in the framework of the discrete computer model of the rabbit heart ventricles on the basis of realistic activation sequence and APDs. The APD gradients were set constant at 20 ms or varied in the range of ±80 ms. RESULTS: The apicobasal, transmural and interventricular APD gradients of 20 ms produced similar BSPDs, whereas the BSPD inversion was caused by the inverted apicobasal or transmural 80 ms gradients. The transmural APD gradient produced transversal and mainly apicobasal T-wave vectors due to wall curvature and cancellation effects. The "normal" transversal and apicobasal repolarization gradients were decreased and increased by activation sequence, respectively. CONCLUSION: The different APD gradients contributed consistently to the development of BSPD.

Load-induced changes in ventricular repolarization: evidence of autonomic modulation.

Augmented hemodynamic load increases the risk of arrhythmogenesis by modulating cardiac repolarization duration. We hypothesized that the intervention on the autonomic tone may affect the load-dependent changes in ventricular repolarization. Activation-recovery intervals were measured in unipolar electrograms simultaneously recorded from 64 ventricular epicardial leads, in a total of 26 chinchilla rabbits in resting conditions, and after 1 and 10 min of aortic stenosis. Eleven animals were given atropine and propranolol before the loading. The short-term stenosis decreased the activation-recovery intervals in the right ventricle, whereas the prolonged overload increased the repolarization duration in both ventricles. The treatment with the ß-adrenergic and M-cholinergic blockers prolonged the activation-recovery intervals, especially at the left ventricle, attenuating the apicobasal and interventricular gradients of repolarization duration seen in the baseline state. Further ventricular loading shortened the repolarization duration in both ventricles in animals with autonomic blockade. Thus, the autonomic tone was shown to be essential for the development of repolarization heterogeneity across the ventricles. The autonomic blockade transformed the biphasic changes of activation-recovery intervals into their monophasic shortening at aortic stenosis.

The contribution of ventricular apicobasal and transmural repolarization patterns to the development of the T wave body surface potentials in frogs (Rana temporaria) and pike (Esox lucius).

The study aimed at the simultaneous determination of the transmural and apicobasal differences in the repolarization timing and the comparison of the contributions of these two repolarization gradients to the development of the body surface T wave potentials in animals with the single heart ventricle (fishes and amphibians). Unipolar potentials were measured on the body surface, epicardium and in the intramural (subepicardial, Epi; midmyocardial; and subendocardial, Endo) ventricular layers of 9 pike and 8 frogs. Activation times, repolarization times and activation-recovery intervals were determined. A transmural gradient in repolarization durations in frogs (Endo>Epi, P<0.024) corresponds to the gradient in repolarization times. No significant transmural difference in repolarization duration is observed in pike that produces a repolarization sequence from Endo to Epi (Endo

Characterization of systolic intervals in healthy, conscious sheep.

OBJECTIVE: To characterize systolic intervals of the left ventricle and their relationship with heart rate in conscious sheep. Animals-11 healthy Romanov sheep (age range, 3 months to 10 years). PROCEDURES: Systolic intervals and indices of myocardial contractility of the left ventricle were measured in conscious sheep by use of polycardiography. RESULTS: The mean +/- SD pre-ejection period was 59 +/- 12 milliseconds, and the mean left ventricular ejection time was 194 +/- 34 milliseconds. The mean myocardial tension index was 0.22 +/- 0.05, and the mean ratio of the pre-ejection period to ejection time was 0.30 +/- 0.09. Total electromechanical systole, mechanical systole, and ejection time varied inversely with heart rate. The electromechanical delay and pre-ejection period were not correlated with heart rate, nor were the myocardial tension index and the ratio of the pre-ejection period to ejection time. The isovolumetric contraction index and isovolumetric contraction time were not significantly correlated with heart rate, although the values for the correlation coefficient were moderate (r = -0.561 and r = -0.482, respectively). CONCLUSIONS AND CLINICAL RELEVANCE: Although a larger study would be needed to provide reference intervals for healthy sheep, the results of the study reported here provided useful information for the cardiac evaluation of sheep.

Activation and repolarization patterns in the ventricular epicardium under sinus rhythm in frog and rabbit hearts.

Our study compared the contributions of activation sequence and local repolarization durations distribution in the organization of epicardial repolarization in animals with fast (rabbit) and slow (frog) myocardial activation under sinus rhythm. Activation times, repolarization times and activation-recovery intervals (ARI) were obtained from ventricular epicardial unipolar electrograms recorded in 13 Chinchilla rabbits (Oryctolagus cuniculus) and 10 frogs (Rana temporaria). In frogs, depolarization travels from the atrioventricular ring radially. ARIs increased progressively from the apex to the middle portion and finally to the base (502+/-75, 557+/-73, 606+/-79 ms, respectively; P<0.01). In rabbits, depolarization spread from two epicardial breakthroughs with the duration of epicardial activation being lower than that in frogs (17+/-3 vs. 44+/-18 ms; P<0.001). ARI durations were 120+/-37, 143+/-45, and 163+/-40 ms in the left ventricular apex, left, and right ventricular bases, respectively (P<0.05). In both species, repolarization sequence was directed from apex to base according to the ARI distribution with dispersion of repolarization being higher than that of activation (P<0.001). Thus, excitation spread sequence and velocity per se do not play a crucial role in the formation of ventricular epicardial repolarization pattern, but the chief factor governing repolarization sequences is the distribution of local repolarization durations.