Author + information
- Nitish Badhwar, MD∗ ( and )
- Melvin M. Scheinman, MD
- Section of Cardiac Electrophysiology, Division of Cardiology, Department of Medicine, University of California San Francisco, San Francisco, California
- ↵∗Reprint requests and correspondence:
Dr. Nitish Badhwar, Section of Cardiac Electrophysiology, Division of Cardiology, Department of Medicine, University of California, San Francisco, 500 Parnassus Avenue, MU-East 431, Box 1354, San Francisco, California 94143-1354.
The presence of dual atrioventricular nodal conduction is usually seen clinically as AV nodal re-entrant tachycardia (AVNRT). Alternatively, patients can present with in-sinus rhythm as dual ventricular response to a single atrial beat (“double fire”). Another manifestation of duality of AV nodal conduction is described by Park et al. (1) in this issue of JACC: Clinical Electrophysiology. They describe 30 patients who showed episodic changes in the P-R interval with periods of sustained slow pathway conduction.
Schamroth and Perlman (2) were the first to describe dual AV nodal conduction in sinus rhythm with sustained slow pathway conduction. Fisch et al. (3) were the first to describe a large cohort of 21 patients showing evidence of dual AV nodal conduction in sinus rhythm. Change in the P-R interval was most often precipitated by specific events such as bursts of atrial tachycardia or premature atrial ventricular or junctional beats. It was felt that these premature beats blocked in the fast pathway and allowed slow pathway conduction. Only 2 patients showed spontaneous change from fast to slow pathway conduction in their series.
The report by Park et al. (1) adds to reports in a number of important ways. For example, they found a much higher incidence of spontaneous change from 1 conduction pathway to the other. In addition, they confirmed by invasive electrophysiologic study the weak bidirectional properties of fast pathway that sustains this phenomenon. They showed longer AV block cycle length and refractory period of the fast pathway with absence of retrograde conduction in most of the patients. This makes it more vulnerable to block with premature atrial and ventricular beats as well as changes in sinus cycle length. The paper further emphasizes the role of autonomic nervous system in transition from one to another pathway. Conduction over the slow pathway was often heralded by sinus slowing or sinoatrial block, while increased sinus rate (decreased parasympathetic activity or increased sympathetic activity) led to initiation of fast pathway conduction. Slow pathway conduction was not seen with exercise or isoproterenol challenge in their cases where such testing was applied. Initiation of slow pathway conduction with blocked P wave that was seen during sleep or early morning is explained by increased vagal tone, which causes simultaneous block in both pathways, followed by shift of conduction to slow pathway. This was also replicated by performing carotid sinus massage. The fast pathway appears to be differentially sensitive to increased vagal tone as well as adenosine (4). The study also confirms electrotonic interaction between the slow pathway and fast pathway where conduction over one pathway affects conduction properties over the other. Sustained conduction over one pathway may be possible because of sustained concealment in the other pathway. In addition, it has been long appreciated that ablation of the slow pathway leads to shortening of the fast pathway refractory period. This finding was also confirmed by electrophysiologic study in these patients.
It is important that clinician be aware of the diagnosis of duality of AV nodal conduction as well as reciprocal rhythms in order to explain a host of complex electrocardiographic findings, for example, termination of AV Wenckebach sequences by shift to slow pathway conduction and reciprocation to the atrium. In addition, sinus rhythm showing various degrees of conduction over each pathway may lead to complex and challenging arrhythmias (3). Rarely, the switch from fast to slow pathway allows for development of phase 4 conduction block in the infranodal conduction system.
The most important finding in this study was that duality of AV nodal conduction can lead to clinical symptoms; it is more than just a simple academic exercise. As much as 70% of patients were symptomatic with chest discomfort or fluttering sensation that correlated with intermittent sustained slow pathway conduction, as documented on ambulatory electrocardiographic monitoring. Prolonged P-R interval leads to loss of AV synchrony and simulates an accelerated junctional rhythm. Atrial contraction occurs against closed AV valves, causing troublesome venous pulsations in the neck, sudden increase in atrial pressure, and suboptimal ventricular filling. Twelve of these patients underwent radiofrequency catheter ablation of the slow pathway that was successful in 11 of them. The P-R interval was shortened in these patients, with no recurrence of symptoms or advanced AV block on long-term follow-up.
↵∗ Editorials published in JACC: Clinical Electrophysiology reflect the views of the authors and do not necessarily represent the views of JACC: Clinical Electrophysiology or the American College of Cardiology.
Both authors have reported that they have no relationships relevant to the contents of this paper to disclose.
- American College of Cardiology Foundation
- Park J.S.,
- Hwang H.,
- Joung B.,
- Lee M.-H.,
- Kim S.S.
- Fisch C.,
- Mandrola J.M.,
- Rardon D.P.
- Belhassen B.,
- Fish R.,
- Glikson M.,
- Glick A.,
- Eldar M.,
- Laniado S.,
- Viskin S.