Modeling triggered cardiac activity: an analysis of the interactions between potassium blockade, rhythm pauses, and cellular coupling

Arrhythmogenesis by single ectopic beats originating in the Purkinje system

Cells in the Purkinje system (PS) are known to be more vulnerable than ventricular myocytes to secondary excitations during the action potential (AP) plateau or repolarization phases, known as early afterdepolarizations (EADs). Since myocytes have a lower intrinsic AP duration than the PS cells to which they are coupled, EADs occurring in distal branches of the PS are more likely to result in propagating ectopic beats. In this study, we use a computer model of the rabbit ventricles and PS to investigate the consequences of EADs occurring at different times and places in the cardiac conduction system. We quantify the role of tissue conductivity and excitability, as well as interaction with sinus excitation, in determining whether an EAD-induced ectopic beat will establish reentrant activity. We demonstrate how a single ectopic beat arising from an EAD in the distal PS can give rise to reentrant arrhythmia; in contrast, EADs in the proximal PS were unable to initiate reentry. Clinical studies have established the PS as a potential substrate for reentry, but the underlying mechanisms of these types of disorder are not well understood, nor are conditions leading to their development clearly defined; this work provides new insights into the role of the PS in such circumstances. Our findings indicate that simulated EADs in the distal PS can induce premature beats, which can lead to the tachycardias involving the conduction system due to interactions with sinus activity or impaired myocardial conduction velocity.

Accelerated onset and increased incidence of ventricular arrhythmias induced by ischemia in Cx43-deficient mice

BACKGROUND

Myocardial ischemia causes profound changes in both active membrane currents and passive electrical properties. Because these complex changes develop and progress concomitantly, it has not been possible to elucidate the relative contributions of any one component to arrhythmogenesis induced by acute ischemia. Cx43+/- mice express 50% of the normal level of connexin43 (Cx43), the major ventricular electrical coupling protein, but are otherwise identical to wild-type (Cx43+/+) mice. Comparison of arrhythmogenesis in Cx43+/- and +/+ mice can provide insights into the role of changes in electrical coupling as an independent variable in the complex setting of acute ischemia.

METHODS AND RESULTS

Acute ischemia was induced in isolated perfused mouse hearts by occlusion of the left anterior descending coronary artery. Spontaneous ventricular tachyarrhythmias (VT) occurred in more than twice as many Cx43+/- hearts than Cx43+/+ hearts. VT was induced in nearly 3 times as many Cx43+/- hearts. Multiple runs and prolonged runs of spontaneous VT were more frequent in Cx43+/- hearts. Onset of the first run of VT occurred significantly earlier in Cx43+/- hearts. Premature ventricular beats were also more frequent in Cx43+/- hearts. The size of the hypoperfused region was equivalent in both groups.

CONCLUSIONS

Reduced expression of Cx43 accelerates the onset and increases the incidence, frequency, and duration of ventricular tachyarrhythmias after coronary artery occlusion. Thus diminished electrical coupling per se plays a critical role in arrhythmogenesis induced by acute ischemia.