Evidence for a link between mechanical and electrical alternans in acutely ischaemic myocardium of anaesthetized dogs

Electrical alternans induced by a brief period of myocardial ischemia during percutaneous coronary intervention: The characteristic ECG morphology and relationship to mechanical alternans

BACKGROUND

Electrical alternans (EA) has not been fully studied in the current percutaneous coronary intervention (PCI) procedure.

OBJECTIVE

The purpose of this study was to evaluate visible EA and the morphology of ST segment during PCI.

METHODS

The incidence of visible EA and ST-segment morphology were studied while the coronary artery was occluded for 20 seconds. When data were available, the relationship between EA and blood pressure was analyzed. The clinical and electrocardiographic data were compared with those of the age- and sex-matched controls.

RESULTS

During balloon inflation, visible EA was observed in 5 of 306 patients (1.6%) in the last 2 years. EA was limited to PCI in the proximal left anterior descending artery. The ST segment elevated to 10.1 ± 3.2 mm, followed by an alternating QRS complex with a lower ST segment (5.6 ± 1.9 mm; P = .0047) with characteristic ST-segment morphology, which is known as lambda pattern. The mean age of the 5 patients was 68 ± 20 years, and 4(80%). were men. After the release of inflation, the ST-segment level returned rapidly to baseline, followed by normalization of J point. Compared with controls, the maximal elevated ST segment was significantly higher in patients with EA (5.7 ± 2.7 mm; P = .0028). The occlusion of the proximal left anterior descending artery with more severe ischemia seemed to be a prerequisite for developing EA. A higher ST segment was associated with a lower blood pressure and vice versa.

CONCLUSION

A short period of ischemia during PCI may induce visible EA and alternating QRS complexes with a characteristic ST-segment morphology. A higher ST segment was associated with a lower blood pressure and vice versa.

Effects of pacing site and stimulation history on alternans dynamics and the development of complex spatiotemporal patterns in cardiac tissue

Alternans of action potential duration has been associated with T wave alternans and the development of arrhythmias because it produces large gradients of repolarization. However, little is known about alternans dynamics in large mammalian hearts. Using optical mapping to record electrical activations simultaneously from the epicardium and endocardium of 9 canine right ventricles, we demonstrate novel arrhythmogenic complex spatiotemporal dynamics. (i) Alternans predominantly develops first on the endocardium. (ii) The postulated simple progression from normal rhythm to concordant to discordant alternans is not always observed; concordant alternans can develop from discordant alternans as the pacing period is decreased. (iii) In contrast to smaller tissue preparations, multiple stationary nodal lines may exist and need not be perpendicular to the pacing site or to each other. (iv) Alternans has fully three-dimensional dynamics and the epicardium and endocardium can show significantly different dynamics: multiple nodal surfaces can be transmural or intramural and can form concave/convex surfaces resulting in islands of discordant alternans. (v) The complex spatiotemporal patterns observed during alternans are very sensitive to both the site of stimulation and the stimulation history. Alternans in canine ventricles not only exhibit larger amplitudes and persist for longer cycle length regimes compared to those found in smaller mammalian hearts, but also show novel dynamics not previously described that enhance dispersion and show high sensitivity to initial conditions. This indicates some underlying predisposition to chaos and can help to guide the design of new drugs and devices controlling and preventing arrhythmic events.

Premature beats elicit a phase reversal of mechanoelectrical alternans in cat ventricular myocytes. A possible mechanism for reentrant arrhythmias

BACKGROUND

Alternans of the ST segment of the ECG is an important risk factor for sudden cardiac death. Premature beats during alternans and the development of discordant alternans are associated with the onset of ventricular tachycardia and ventricular fibrillation. Moreover, premature beats can switch the pattern of alternans from discordant to concordant alternans. The mechanisms of how a premature beat can elicit a pattern shift in alternans and develop malignant ventricular arrhythmias are not clear. The purpose of this cellular study was to determine the electrical and mechanical restitution properties during cycle length-induced alternans and to determine how premature and delayed beats affect the resultant phase of alternans.

METHODS AND RESULTS

A perforated patch recording method and video-based edge detector were used to record action potentials and contractions, respectively, from single ventricular myocytes enzymatically isolated from the cat heart. Electrical and mechanical restitution curves were determined by programmed test beats delivered at different cycle lengths during mechanoelectrical alternans. At 35 degrees C, 97.8% of cells exhibited concordant cellular alternans (action potentials with the larger action potential duration [APD] were associated with the larger contraction, and action potentials with the smaller APD exhibited the smaller contraction). The sequence or phase of concordant cellular alternans could be systematically reversed by (1) early premature beats that followed only action potentials with the shorter APD and smaller contraction (type 1 phase reversal; n = 34) or (2) late delayed beats that followed only action potentials with the longer duration and the larger contraction (type 2 phase reversal; n = 14). A phase reversal point was defined as a threshold time interval that resulted in switching the sequence of the alternating beats. A test stimulus at the phase reversal point caused temporary suppression of mechanoelectrical alternans. Lower temperatures (32 degrees C) or decreases in the basic cycle length induced larger beat-to-beat changes in the magnitude of alternans (APD or contraction) and significantly shifted the phase reversal point to earlier premature intervals for type 1 phase reversal. The interval of the phase reversal point was a function of the contractile ratio (the magnitude of the larger contraction/smaller contraction for two consecutive beats, r = .93) and not the APD ratio (longer APD/shorter APD; r = .501). In cells stimulated at cycle lengths longer than the threshold of alternans, a single premature beat could elicit a damped form of concordant mechanoelectrical alternans. A critically timed second premature beat reversed the phase of the damped alternans.

CONCLUSIONS

Properly timed premature or delayed beats during cycle length-induced alternans consistently reversed the phase of cellular mechanoelectrical alternans. Reversal of the phase of alternans was dependent on recovery of mechanical activity, not electrical activity. The premature stimulus interval at the phase reversal point can be predicted by the magnitude of mechanical alternans. Thus, during cycle length-induced alternans, mechanical alternans governs the phase of electrical alternans. From the present results, a multi-cellular model is proposed that may explain how critically timed premature beats cause a regional change in the phase of mechanical alternans and thereby result in discordant electrical alternans or dispersion of refractoriness. Premature beats that induce phase reversal in mechanoelectrical alternans may contribute to the development of reentrant arrhythmias.