PR prolongation with alternating wide and narrow QRS complexes: What is the mechanism?

The alternating wide and narrow QRS complexes following prolonged PR interval is a challenging issue. The scalar diagrams with the graphic representation of the electrical events and the careful analysis of their temporal relationship can help to better understand the surface electrocardiogram. A 52-year-old man affected by nonischemic cardiomyopathy with mild reduced left ventricular ejection fraction and arterial hypertension was referred to our Cardiology Unit for electrophysiological evaluation due the following 12-lead electrocardiogram (ECG) (Figure 1). His medical history was negative for chest pain, syncope, and/or dyspnea. At first glance, the ECG revealed sinus rhythm at 83 bpm, first-degree atrioventricular block with alternating narrow and wide QRS complexes. The PR interval ranged from 360 to 440 ms; when longer, it was followed by a narrow QRS complex, and when shorter, it was followed by a wide QRS complex with left bundle branch block (LBBB) morphology. Questions: What is the most likely diagnosis? What are the next steps in managing this patient? The interpretation of the present ECG was not univocal, and its nature has been debated: dual atrioventricular nodal pathway physiology with left bundle branch conduction recovery or ventricular ectopic beats (VEBs) determining a second-degree atrioventricular block. The 24-hour ECG Holter (24 h ECG Holter) did not show advanced degree atrioventricular node (AV) block or significant pauses. The electrophysiology study (EPS) revealed a marked prolongation of the AH interval (290 ms), whereas the PA interval (35 ms) and HV interval (50 ms) were within the normal range. The right ventricular para-Hisian origin of the wide QRS complexes was confirmed; the local electrogram at that site during the VEB preceded the onset of the surface QRS by 35 ms and showed a negative unipolar electrogram. The atrioventricular Wenckebach point was 160 bpm. No dual AV nodal pathway physiology was shown. Owing to the lack of symptoms and the absence of infra-Hisian block, the patient was discharged with no pacemaker implantation. Two most likely hypotheses may explain the mechanism of alternating narrow and wide QRS complexes which follow a prolonged PR interval with two alternating different durations. First, the wide QRS complexes are because of the left ventricular conduction delay; the narrow QRS complexes that follow the longer PR interval are the expression of the left bundle branch conduction recovery.1 The marked change in the PR interval duration, at least 50 ms, may be because of the pathway skipping and suggests the diagnosis of dual nodal pathway physiology.2 In most cases, the longer PR interval persists for many beats because the conduction through the slow pathway tends to be maintained by the "linking phenomenon"3. More rarely, the linking phenomenon does not occur, and a beat-to-beat alternation of long and short PR intervals can be observed, as in our case. Second, the wide QRS complexes are because of VEBs, which are retrogradely conducted to the AV node.4 The VEBs prolong the AVN refractoriness and determine a second-degree atrioventricular block because of interference (Figure 2). The presence of a fusion complex (sixth wide QRS) and the LBBB-like QRS morphology in the V1-V2 leads characterized by R wave duration >30 ms and onset-to-nadir QRS duration >60 ms suggest the diagnosis of VEBs, which was confirmed at EPS. The extremally prolonged first-degree AV block (PR >300 ms) should be treated with pacemaker implantation in patients with persistent symptoms, similar to those of pacemaker syndrome and attributable to the conduction disorder.5 Our patient was asymptomatic and no evidence of infra-Hisian conduction disorders was revealed by 24 h ECG Holter and EPS. Finally he was discharged with no pacemaker implantation. At 6-month follow-up, the patient was stable. The present case highlights the value of electrocardiography for the differential diagnosis in wide QRS complexes. In memory of Prof. Giuseppe Oreto who showed us the way to approach to the electrocardiogram. Not applicable. The authors declare no conflict of interest. This case report complied with the Declaration of Helsinki and its later amendments. The informed consent was obtained from the patient to publish this case report. Not applicable. The data are available from the corresponding author upon reasonable request.