Atypical Wenckebach AV Block in the Infra-Hisian Region: Clinical Implications and Management

Introduction

Atrioventricular (AV) block is one of the common cardiac causes of syncope. Patients presenting with second-degree AV block on surface electrocardiogram (ECG) can be evaluated in detail with an electrophysiological study whenever there is doubt about the level of block. In patients with AV node-level block, the AH interval can be prolonged, but the His-ventricular (HV) interval is narrow (normal), ie, 35 to 55 msec. However, an infra-nodal type of second-degree AV block is rare and has a normal AH interval with a prolonged HV interval. This second-degree infra-His block is indicative of impending high-grade or complete infra-His block. In our case, we discuss this rare form of infra-His level AV block in a patient with a history of recurrent syncope.

Case Report

A 65-year-old female patient presented to our hospital with recurrent episodes of syncope over the past 6 months. All episodes occurred during her usual household work. There was no history of abnormal body movements, fever, diarrhea, shortness of breath, bowel or bladder involvement, nor any postural relation of symptoms. The patient had no comorbidities, such as diabetes mellitus, hypertension, or thyroid dysfunction. There was no history of any illicit drug use.

She had been evaluated at another hospital by a neurologist. Her carotid Doppler, electroencephalogram, and magnetic resonance imaging of the brain were normal, as was her 2-dimensional echocardiogram. As ECG showed a bi-fascicular block (right bundle branch block [RBBB] + left anterior fascicular block [LAFB]), she was referred to our hospital for further evaluation. During our electrophysiology consultation, the rhythm strip revealed second-degree AV block (Mobitz type 1), as shown in Figure 1A and 1B. Her general physical and systemic examination was unremarkable. Her blood investigations, including complete blood count, liver function tests, kidney function tests, serum electrolytes, erythrocyte sedimentation rate, C-reactive protein, urine routine and microscopy, thyroid profile, and cardiac markers, including troponin I and creatine phosphokinase-MB, were unremarkable. Her chest X-ray was within normal limits. Her 24-h Holter monitoring revealed no other significant pauses or conduction abnormalities apart from intra-ventricular conduction defect, as described above.

The patient was advised to undergo pacemaker implantation in view of her syncope and ECG findings. She insisted on an electrophysiological study prior to pacemaker implantation. During the study, one quadripolar catheter was positioned at the right ventricular apex, and another CRD-2 quadripolar catheter was positioned at the bundle of His. The electrograms revealed that she had infra-Hisian AV block, with infra-Hisian Wenckebach block and 2: 1 infra-Hisian AV block, as shown in Figures 2 and 3A. Also, there were features of atypical infra-Hisian Wenckebach, characterized by shortening of HV during the Wenckebach sequence, as shown in Figure 3A. She did not exhibit any intra-Hisian AV disease, as shown in Figure 3B.

A diagnosis of infra-Hisian AV block was made, and the patient underwent permanent pacemaker implantation. The procedure was uneventful, and she was discharged the next day. She was symptom-free at her 4-week follow-up.

Discussion

The Mobitz type 1 block manifests on the surface ECG as progressive prolongation of the PR interval before failure of an atrial impulse to conduct to the ventricle. The PR interval immediately after the non-conducted P wave returns to its baseline value, and the sequence begins again. This phenomenon typically occurs at the AV node level. On a surface ECG, it appears as a non-conducted P wave followed by a shorter PR interval than the preceding conducted P wave. On electrophysiology study, it is primarily observed as a prolonged AH interval. However, in rare cases, an infra-nodal block is seen, where the HV interval progressively prolongs till there is a non-conducted His potential. This infra-nodal Wenckebach is a rare cause of syncope, as documented in previous studies [1–6].

Our patient during presentation had a bi-fascicular block (RBBB+LAFB) with Wenckebach AV block, as shown in Figure 1B. Patients with BBB can have block within the His-Purkinje system. A prolonged baseline PR interval in a patient with second-degree AV block generally indicates an AV nodal block, whereas a PR interval less than 160 msec indicates that the block is most likely below the level of the AV node. Also, it is known that the PR interval on surface ECG is a poor predictor of HV interval [1–4]. A narrow QRS complex might indicate an AV node block that improves with exercise. If the block does not improve with exercise, it suggests an infra-Hisian block [2,7].

Our patient’s 24-h Holter monitoring was inconclusive. Therefore, we proceeded with an electrophysiology study to differentiate the level of AV block. An electrophysiology study helps to differentiate the level of block, namely whether an AV nodal level block is present with prolonged AH interval and normal HH’ and HV intervals, or if an infra-nodal level of block is present with prolonged HH’ and HV intervals. In our case, the patient had no splitting of His potential, with a normal intra-Hisian interval, and the HV interval was prolonged, indicating an infra-nodal second-degree AV block and the need for pacemaker implantation [8].

A prolonged HV interval is almost always associated with an abnormal QRS complex. As long as one of the fascicle is conducting normally, the HV interval should not exceed 55 msec [2,4,5,8]. However, in our case, the HV interval was 71 msec, accompanied by a bi-fascicular block, indicating an infra-Hisian block.

There are 2 types of infra-nodal type-1 second-degree AV block. In the first one, the block occurs due to intra-Hisian disease, which can be seen as prolongation of duration of His potential or as splitting of the His potential. The intra-Hisian type of Wenckebach block can occur between 2 His deflections, which is characterized by progressive conduction delay, until the first His deflection is not followed by the second His deflection. In the other type of infra-nodal type 1 second-degree AV block, the block occurs at the level of the bundle branch and is characterized by progressive prolongation of the HV interval, which is followed by His deflection, and there is no associated ventricular activation. The HV interval is minimally influenced by autonomic tone and typically remains stable despite varying levels of autonomic influence [2–4,5,8,9]. Lacharite-Roberge et al [10], reported an unusual case of alternating LAFB and left posterior fascicular block and a fixed RBBB that was found to be at the infra-Hisian level on electrophysiology study. The patient underwent permanent pacemaker implantation as per the current guidelines [11].

It is likely that our patient experienced syncope due to transient complete heart block from her infra-Hisian conduction abnormality. Dhingra et al [8] found that 12% of patients with bi-fascicular block and prolonged HV intervals progressed to complete heart block within a 7-year follow-up period. Therefore, pacemaker implantation was beneficial in our patient, as supported by other studies [12].

Conclusions

Understanding electrophysiological findings is essential for the precise diagnosis and treatment of second-degree AV block. Several ECG indicators can aid in the diagnosis of second-degree AV block. For example, RP-dependent PR interval or RP-PR reciprocity is characteristic of Wenckebach AV block. A short PR interval at baseline and minimal amount of PR interval lengthening before block in type 1 second-degree AV block suggest involvement of the His-Purkinje system.