Demonstration of an "excitation-contraction recoupling" mechanism in mammalian ventricular myocardium

Mechano-electric coupling, heterogeneity in repolarization and the electrocardiographic T-wave

Stretch influences repolarization by mechano-electric coupling (MEC) and contributes to arrhythmogenesis. Although there is an abundance of research on electrophysiological effects of MEC, it is still unclear how MEC translates to the ECG. We aim to provide an overview of the MEC research focused on the ECG and the underlying changes in electrophysiology. In addition, we present new data on the effect of left ventricular pressure on the electrocardiographic T-wave. We show that an increase in left ventricular pressure leads to prolonged QT-intervals with increased amplitudes of the STT-segment. This corresponds to a prolongation in repolarization and an increased interventricular dispersion of repolarization. MEC is dependent on timing, intensity and modality of stretch and these three factors should be taken into account to analyse the effects of MEC on the heart and on the ECG. In addition, the deformation of the heart itself should be considered, since it influences the amplitude of the STT-segment. Because the electrocardiographic T-wave represents heterogeneity in repolarization, left ventricular pressure increases may have significant influence on the inducibility of (re-entrant) arrhythmias.

A simple automated stimulator of mechanically induced arrhythmias in the isolated rat heart

Transient stretching of the ventricle can trigger arrhythmias and evoke ventricular fibrillation, especially when the stimulation occurs in the vulnerable period. To explore the sensitivity of small hearts we used a commercial pressure servo to study the kinetic relationship of left ventricular pressure to excitability and arrhythmias in the rat heart. Stimulation protocols were readily composed on the computer and programmed to vary the stimulus amplitude and timing relative to pacing. The pressure-induced premature ventricular excitations were similar to those observed in larger hearts, but the convenience of using small hearts allows the use of inexpensive transgenic animals to explore the molecular basis of transduction.

Effect of myocardial shortening velocity on duration of electrical and mechanical systole. S2T interval as measure of shortening rate

To determine the effect of myocardial shortening velocity on the duration of electrical and mechanical systole, five healthy men, ages 32 to 41, were studied. Carotid message and atropine were used to define the effects of changes in heart rate without changes in shortening rate. The effects of amyl nitrite, which produced approximately the same degree of tachycardia as atropine but a significantly higher maximum shortening rate, were compared with those of atropine to assess the effects of faster shortening velocities. The increased heart rate produced by both agents caused a substantial decrease in both the QT and the S1S2 intervals, but the QT interval was longer and the S1S2 interval shorter with the amyl nitrite. Thus, the S2T interval was substantially longer after amyl nitrite. The results are in keeping with the finding in isolated cardiac muscle that active shortening increases action potential duration and decreases the duration of mechanical activation. These observations raise the possibility that longer QT intervals and shorter S1S2 intervals would also be seen in patients with hypokinetic ventricular segments when the healthy muscle contracts more vigorously to compensate for the weaker segments.