The electrophysiology of the atrioventricular node in normal and failing rabbit hearts

Abstract

Conduction abnormalities affect prognosis in chronic heart failure (CHF). Previous investigators have observed abnormal delay in atrioventricular (AV) conduction in a rabbit model of left ventricular dysfunction (LVD) due to apical myocardial infarction. In this model, AV conduction time increased with increasing pacing rates, suggesting the most likely site of delay is the AV node. The mechanisms by which this occurs are not fully understood. The purpose of this thesis was to confirm that the abnormal prolongation of AV conduction time originates at the AV node in a rabbit model of LVD due to apical myocardial infarction, and explore possible mechanisms underlying the observation. Using surface electrogram recording and standardised pacing techniques in an isolated AV node tissue preparation I confirmed that there is abnormal prolongation of AV nodal conduction in this rabbit model of LVD, as evidenced by prolongation of atrio-hisian (AH) interval and Wenckebach cycle length (WCL) in LVD compared to control. Furthermore, using optical mapping of electrical activation using voltage sensitive dye I observed that the prolongation of the AH interval is predominantly a consequence of conduction delay between the inputs of the AV node and the compact nodal region. Neuro-hormonal derangement in chronic heart failure has a central role in the pathogenesis of the disease, with evidence of downregulation of beta ()-adrenoceptors in the left ventricular myocardium. I therefore explored the possibility of β-adrenoceptor downregulation in the AV node as a mechanism underlying the abnormal AH interval prolongation in LVD. There was no evidence of β-adrenoceptor downregulation in the AV node in LVD compared to control to account for the observed abnormal conduction delay. Adenosine is known to have profound effects on AV nodal conduction and the possibility of tonic excess of adenosine in LVD was explored as a possible mechanism for the prolonged conduction delay. Using an exogenously applied adenosine A1 receptor antagonist there was no evidence of excess endogenous adenosine in LVD compared to control. There was, however, an increase in the sensitivity of the LVD samples compared to control to exogenous adenosine, with a significant increase in AH interval and WCL with increasing concentrations. This thesis also investigates the effect of acidosis on AV nodal conduction. There was significant prolongation of the spontaneous sinus cycle length, AH interval and WCL, as well as the AV nodal functional and effective refractory periods, proportional to the degree of acidosis. These effects were reversible with return to normal pH. Optical mapping studies showed that the spatiotemporal pattern of AV nodal delay during acidosis was similar to that observed in LVD, with the predominant delay in conduction between the AV nodal inputs and the compact AV node. In summary this thesis has confirmed that even in the absence of a direct ischaemic insult to the AV junction, conduction abnormalities in the AV node may still occur as a pathophysiological response to a myocardial infarction resulting in LVD. The mechanisms underlying this response are likely to be complex and multiple, and are not yet clear. Establishing the electrophysiological basis and the effects of neuro-hormonal modulators of atrioventricular nodal function may lead to development of targeted therapeutic strategies to improve overall survival and improve symptom control for patients with CHF