Alternation of atrioventricular nodal conduction time during atrioventricular reentrant tachycardia: are dual pathways necessary?

Universal mechanisms for self-termination of rapid cardiac rhythm

Excitable media sustain circulating waves. In the heart, sustained circulating waves can lead to serious impairment or even death. To investigate factors affecting the stability of such waves, we have used optogenetic techniques to stimulate a region at the apex of a mouse heart at a fixed delay after the detection of excitation at the base of the heart. For long delays, rapid circulating rhythms can be sustained, whereas for shorter delays, there are paroxysmal bursts of activity that start and stop spontaneously. By considering the dependence of the action potential and conduction velocity on the preceding recovery time using restitution curves, as well as the reduced excitability (fatigue) due to the rapid excitation, we model prominent features of the dynamics including alternation of the duration of the excited phases and conduction times, as well as termination of the bursts for short delays. We propose that this illustrates universal mechanisms that exist in biological systems for the self-termination of such activities.

Spontaneous Initiation and Termination of Complex Rhythms in Cardiac Cell Culture

INTRODUCTION

Complex cardiac arrhythmias often start and stop spontaneously. These poorly understood behaviors frequently are associated with pathologic modification of the structural heterogeneity and functional connectivity of the myocardium. To evaluate underlying mechanisms, we modify heterogeneity by varying the confluence of embryonic chick monolayer cultures that display complex bursting behaviors. A simple mathematical model was developed that reproduces the experimental behaviors and reveals possible generic mechanisms for bursting dynamics in heterogeneous excitable systems.

METHODS AND RESULTS

Wave propagation was mapped in embryonic chick myocytes monolayers using calcium-sensitive dyes. Monolayer confluence was varied by plating cultures with different cell densities and by varying times in culture. At high plating densities, waves propagate without breaks, whereas monolayers plated at low densities display spirals with frequent breaks and irregular activation fronts. Monolayers at intermediate densities display bursting rhythms in which there is paroxysmal starting and stopping of spiral waves of activity. Similar spatiotemporal patterns of activity were also observed as a function of the time in culture; irregular activity dominates the first 30 hours, followed by repetitive bursting dynamics until 54 hours, after which periodic target patterns or stable spirals prevail. In some quiescent cultures derived from older embryos, it was possible to trigger pacemaker activity following a single activation. We are able to reproduce all of these behaviors by introducing spatial heterogeneity and varying neighborhood size, equivalent to cell connectivity, in a spontaneous cellular automaton model containing a rate-dependent fatigue term.

CONCLUSION

We observe transitions from irregular propagating waves, to spiral waves that spontaneously start and stop, to target waves originating from localized pacemakers in cell culture and a simple theoretical model of heterogeneous excitable media. The results show how physiologic properties of spontaneous activity, heterogeneity, and fatigue can give rise to a wide range of different complex dynamic behaviors similar to clinically observed cardiac arrhythmias.

Autonomic blockade unmasks maturational differences in rate-dependent atrioventricular nodal conduction and facilitation in the mouse

INTRODUCTION

In large animals, rate-dependent AV nodal (AVN) properties of conduction are modulated by autonomic inputs. In this study, we investigated whether the properties of AVN conduction and facilitation are altered by autonomic blockade in the mouse and whether this effect is age dependent.

METHODS AND RESULTS

Young (age 4-6 weeks; n = 11) and adult (age 8-9 months; n = 11) female mice underwent in vivo electrophysiologic testing, before and after autonomic blockade. After autonomic blockade, the adult mice had significantly longer AVN effective refractory period (AVNERP; 67 +/- 14 msec vs 56 +/- 4 msec, P = 0.05) and functional refractory period (AVNFRP; 81 +/- 10 msec vs 72 +/- 4 msec, P = 0.05). With autonomic blockade, the increase from baseline of AVN Wenckebach cycle length (deltaAVW; 1.8 +/- 8.1 msec vs 8.8 +/- 3.3 msec, P = 0.04), as well as of AVNERP (deltaAVNERP; 3.5 +/- 3.5 msec vs 21.4 +/- 12.6 msec, P = 0.002) and AVNFRP (deltaAVNFRP; 2.3 +/- 3.2 msec vs 12.8 +/- 9.0 msec, P = 0.008), was significantly larger in adult than in young mice. Compared with young mice, adult mice were less likely to exhibit AVN facilitation (44% vs 90%, P = 0.03) and had smaller maximal shortening of AVN conduction times after the "test beat" for any coupling of the "facilitating beat" (4 +/- 4 msec vs 7 +/- 3 msec, P = 0.05).

CONCLUSION

Complete autonomic blockade significantly increases AVN conduction times and refractory periods in adult but not in young mice. Adult mice also exhibit less AVN facilitation. Our results confirm that, like in larger animals, rate-dependent murine AVN properties of conduction are under autonomic regulation. Adult mice have higher sympathetic AVN inputs at baseline, leading to slower conduction after autonomic blockade.

Complex AV nodal dynamics during ventricular-triggered atrial pacing in humans

In vitro experiments have shown that the complexity of atrioventricular nodal (AVN) conduction dynamics increases with heart rate. Although complex AVN dynamics (e.g., alternans) have been observed clinically, human AVN dynamics during rapid pacing have not been systematically investigated. We studied such dynamics during ventricular-triggered atrial pacing in 37 patients with normal AVN function (18 patients with dual AVN pathway physiology and 19 patients without). Alternans, which always resulted from single pathway conduction, occurred in 18 patients. In 16 patients (3 of whom also had alternans), quasisinusoidal AVN conduction oscillations occurred (mean frequency 0.02 Hz); such oscillations have not been previously reported. There were no significant differences in the dynamics for patients with or without dual AVN pathways. To illuminate the governing dynamic mechanism, a second atrial pacing trial was performed on 12 patients after autonomic blockade. Blockade facilitated alternans but inhibited oscillations. This study suggests that rapid AVN excitation in vivo can lead to autonomically mediated AVN conduction oscillations or single pathway alternans that are a function of inherent nonlinear dynamic AVN tissue properties.

Nonlinear-dynamical arrhythmia control in humans

Nonlinear-dynamical control techniques, also known as chaos control, have been used with great success to control a wide range of physical systems. Such techniques have been used to control the behavior of in vitro excitable biological tissue, suggesting their potential for clinical utility. However, the feasibility of using such techniques to control physiological processes has not been demonstrated in humans. Here we show that nonlinear-dynamical control can modulate human cardiac electrophysiological dynamics by rapidly stabilizing an unstable target rhythm. Specifically, in 52/54 control attempts in five patients, we successfully terminated pacing-induced period-2 atrioventricular-nodal conduction alternans by stabilizing the underlying unstable steady-state conduction. This proof-of-concept demonstration shows that nonlinear-dynamical control techniques are clinically feasible and provides a foundation for developing such techniques for more complex forms of clinical arrhythmia.

Restricted feedback control of one-dimensional maps

Dynamical control of biological systems is often restricted by the practical constraint of unidirectional parameter perturbations. We show that such a restriction introduces surprising complexity to the stability of one-dimensional map systems and can actually improve controllability. We present experimental cardiac control results that support these analyses. Finally, we develop new control algorithms that exploit the structure of the restricted-control stability zones to automatically adapt the control feedback parameter and thereby achieve improved robustness to noise and drifting system parameters.

Mechanism of atrioventricular nodal facilitation in rabbit heart: role of proximal AV node

The phenomenon of atrioventricular (AV) nodal "facilitation," described in traditional "black box"-functional studies, implies enhanced AV nodal dromotropic function. We investigated the role of atrial prematurities in the modulation of the nodal cellular responses in the mechanism of AV nodal facilitation. Atrial and His (H) bundle electrograms and microelectrode recordings from proximal AV nodal cells were analyzed in 15 superfused rabbit AV node preparations. The pacing protocol consisted of 30 basic beats (S1; coupling interval S1-S1 = 300 ms) followed by a facilitating prematurity (S2; coupling intervals S1-S2 of 300, 200, 150, and 130 ms) followed by the test beat (S3; coupling interval S2-S3 scanned in 5-ms steps). Conduction curves (S2-H2 vs. S1-S2, S3-H3 vs. S2-S3, and S3-H3 vs. H2-S3) were constructed. Facilitation (i.e., shortening of S3-H3 when S1-S2 was shortened) was demonstrated in all preparations using the H2-S3 (P < 0.001) but not the S2-S3 format. Microelectrode recordings revealed a causal relationship between the improved proximal AV nodal cellular responses in facilitation and the prolonged S2-S3 interval. There was no evidence for enhanced nodal dromotropic function directly resulting from the introduction of the facilitating beats. Thus facilitation is based on inherent cycle-length-dependent properties of the AV node during application of a complex pacing protocol and primarily reflects the uncontrolled modulation of the proximal cellular response.