An Unusual ECG: What is the Diagnosis?

A 26-year-old male with no medical history was brought by paramedics to the ED with ill-defined syncope after smoking illicit drugs. Reportedly, an indistinct tachycardia at 220 bpm was seen at some point, but no strips were recorded for review. Baseline ECG is shown (Figure 1). Given the ECG findings, in conjunction with ill-defined syncope and tachycardia, he was taken for diagnostic EP study (Figures 2-5). What is the diagnosis?

Discussion

The patient’s presenting 12-lead ECG showed very subtle pre-excitation with minimal QRS prolongation (Figure 1). During EP study, measurement of the baseline intervals revealed a short HV interval of 35 ms, proving the presence of a manifest accessory pathway (AP) (Figure 2).  

With progressively faster atrial pacing, the AH interval progressively lengthened without any change in the short HV interval (Figure 3) or in the QRS complex, demonstrating fixed pre-excitation (Figure 4). Even with aggressive premature atrial extrastimulus pacing at a short coupling interval of 220 ms, the QRS complex never changed (Figure 5). Aggressive attempts at inducing SVT and VT, including use of high-dose isoproterenol, were unsuccessful. These findings are diagnostic of a fasciculoventricular (FV) AP.

Anomalous connections between the atrium and ventricle were first described by Wolff, Parkinson, and White in 1930.¹ As such, Wolff-Parkinson-White (WPW) syndrome has been used to describe the condition of manifest pre-excitation seen on ECG with or without symptoms. The pathognomonic “delta wave” represents early direct ventricular activation as a result of antegrade conduction via AP fusing with normal antegrade AV node conduction. Typical APs in WPW consist of muscular connections between the atrium and ventricle in the AV groove spanning either the tricuspid, or more commonly, the mitral valve annulus. With typical APs capable of antegrade conduction, it is important to note that particularly in young patients, sometimes minimal pre-excitation is seen at baseline due to preferential antegrade conduction over a robust AV node. Furthermore, progressively faster incremental atrial pacing results in shortening of the AV interval (i.e., non-decremental conduction) as more of the ventricle activates via conduction from the AP as opposed to the AV node. Without protective decremental conduction properties, some APs are capable of conducting rapid rhythms arising from the atria (specifically atrial fibrillation) to the ventricle, thus conveying an increased risk of ventricular fibrillation (VF) and sudden cardiac death (SCD). 

In 1937, Mahaim and Benatt described APs with decremental properties, i.e., prolongation of AV time with faster pacing.² These APs were found to include normal His bundle tissue (thus accounting for the decremental conduction properties seen in normal AV node physiology) inserting into either distal conduction tissue or ventricular myocardium. “Mahaim pathways” are now more commonly and accurately referenced based on anatomic insertions, including atriofascicular (AF), nodofascicular (NF), nodoventricular (NV), and FV pathways. 

With respect to FV pathways, hallmark findings as discussed above include a fixed pre-excited QRS complex with normal decremental AV node conduction (manifesting as progressive prolongation of AH interval with faster atrial pacing) and constant short HV interval. The decremental properties of AV conduction in patients with FV pathways in particular are due to the anatomic location of the AP being distal to the normal AV node. Most FV pathways demonstrate fixed and short HV intervals; however, FV pathways with decremental conduction within the FV AP itself have also been described.³ With incremental atrial pacing or premature atrial stimulation at short coupling intervals, these FV pathways exhibit progressive prolongation of the HV interval with normalization of the QRS complex.

FV pathways in particular are reported to be exceedingly rare. However, the true prevalence is likely underreported as patients with FV pathways are typically completely asymptomatic.⁴ Because FV pathways are so close in proximity to the native conduction system, they cannot achieve the necessary conduction delay to foster reentrant tachycardia. As such, they also do not convey an elevated risk of VF or SCD. As a result, FV pathways have been more of an academic interest than clinical conundrum, as they are harmless and benign. No additional workup or therapies are necessary, and specifically, no ablation is indicated for patients with FV pathways.

In this particular patient, not surprisingly, neither SVT nor VT was inducible in spite of aggressive pacing attempts and use of high-dose isoproterenol. A follow-up 30-day monitor failed to demonstrate any significant dysrhythmia. The initially reported tachycardia of 220 bpm remains an unexplained mystery. He was ultimately found to have a persistent non-ischemic cardiomyopathy with an ejection fraction of 20% and high-risk features including a high scar burden with late gadolinium enhancement on cardiac MRI, suggestive of prior myocarditis. He ultimately opted to receive a primary prevention subcutaneous ICD, and has clinically done well.

References

1. Wolff L, Parkinson J, White PD. Bundle-branch block with short P-R interval in healthy young people prone to paroxysmal tachycardia. Am Heart J. 1930;5:685-704. 
2. Mahaim I, Benatt A. Nouvelles recherches sur les connexions superieures de la branche gauche du faisceau de His-Tawara avec la cloison interventriculaire. Cardiologia. 1937;1:61-73.
3. Dey S, Tschopp D, Morady F, Jongnarangsin K. Fasciculoventricular bypass tract with decremental properties. Heart Rhythm. 2006;3(8):975-976.
4. Josephson ME. Pre excitation Syndromes. In: M.E. Josephson (Ed.) Clinical Cardiac Electrophysiology. Third Edition. Lippincott Williams & Wilkins. Philadelphia; 2002:419-421.