Termination of macro-reentrant tachycardia by a single extrastimulus delivered during the 'effective' refractory period: a computer modeled 'case report'

Fusion during entrainment of orthodromic reciprocating tachycardia is enhanced for basal pacing sites but diminished when pacing near Purkinje system end points

BACKGROUND

In the electrophysiological laboratory, orthodromic atrioventricular reciprocating tachycardia (ORT) can be distinguished from atrial tachycardia and atrioventricular node reentry tachycardia by identifying orthodromic and antidromic wavefront fusion during ventricular overdrive pacing (VOP). Previous work has shown that basal VOP near the accessory pathway (AP) increases the likelihood of observing fusion; however, in a third of cases, fusion is not appreciable regardless of VOP location.

OBJECTIVE

To explore the hypothesis that pacing near His-Purkinje system (PS) end points reduces fusion quality, which may explain patients with nonresponsive ORT.

METHODS

In a novel computer model of ORT, simulations were performed with a variety of AP locations and pacing sites; results were analyzed to assess factors influencing fusion quality in pseudo-electrocardiogram signals.

RESULTS

Entrainment by basal VOP near the AP was more likely to produce fusion visible on simulated electrocardiograms compared to entrainment by apical VOP, but this advantage was dramatically diminished when the pacing site was also near PS end points. Prediction of fusion quality based on AP proximity alone was dramatically improved when corrected to penalize for PS proximity.

CONCLUSIONS

These results suggest that basal VOP near the AP and far from the PS is optimal; this could be tested in patients. A denser basal ramification of PS fibers is known to exist in a minority of human hearts; our findings indicate that this unusual PS configuration is a plausible explanation for ORT cases where fusion is never observed in spite of entrainment by basal VOP near the AP.

Computer simulation of overdrive pacing during atrioventricular reentrant tachycardia

The study used a computer model of cardiac excitation to reproduce atrioventricular (AV) reentrant tachycardia and to evaluate the possibility of its termination by overdrive burst pacing. The model simulated activation waves radiating along a one-dimensional circular pathway, the portions of which represented the atrial, AV nodal, His-Purkinje, ventricular, and bypass parts of the tachycardia circuit. The pathway consisted of 289 elements. Only depolarised and resting states of elements were modelled. Differential refractoriness and conduction velocity for each element and the cycle length dependence of AV nodal decremental conduction were introduced. The experiments with the model examined the ability of overdrive 'on-circuit' pacing to terminate the tachycardia in order to determine the relevance of: (a) the coupling interval of the first beat in the burst; (b) the cycle length of the burst; (c) the number of stimuli in the burst; (d) His-Purkinje refractoriness; and (e) the degree of AV nodal decremental conduction. The results suggested that: (A) the general impression of a regular recovery wave and of a regular excitable window moving uniformly along the macro-reentrant circular path is incorrect; (B) the use of overdrive bursts of several stimuli with a short coupling interval has unpredictable effects; (C) the use of faster bursts with a cycle length only slightly shorter than the tachycardia cycle length is more safe (with respect to tachycardia reinitiation) and for certain combinations of the coupling interval and cycle length, prolonged bursts do not reinitiate the tachycardia; (D) the likelihood of tachycardia termination is increased by prolonging the refractoriness of the tachycardia circuit and by reducing AV nodal decremental conduction.

Computer simulation of electronic interactions during excitation and repolarisation of myocardial tissue

A three-dimensional model of myocardial tissue has been developed which incorporates electrotonic interactions between neighbouring myocytes. The algorithms of the model are based on asynchronous planning of discrete events. Each cellular element is described in a logical way and traces a predefined action potential which is dynamically modified depending on the electrotonic interactions. The model has a low computational complexity and has been implemented on personal workstations even for experiments investigating arrhythmogenic processes and simulating several tens of cycles in blocks composed of several thousand elements. The paper describes the algorithmic implementation of the model and presents three series of experiments examining the dependence of the accuracy of the model on the size of the modelled tissue, changes of the shape of simulated T-waves due to electrotonic interactions and arrhythmogenic processes caused by lowering the threshold of the electric flow which excites individual cells.