A Fascinating Electrocardiogram with an Apparent Alternating PR Interval

Abstract

A man in his 60s presented with occasional giddiness to the emergency department. Clinical examination and echocardiogram were within normal limits. His electrocardiogram was reported as having sinus arrhythmia with alternate PR interval prolongation. Does he require pacemaker? The discussion guides the reader with the explanation.

Case Report

Case: A sexagenarian patient presented with occasional giddiness to the emergency department. Clinical examination and echocardiogram were within normal limits. His electrocardiogram (ECG) is shown in Figure 1. The ECG was reported as having sinus arrhythmia with alternate PR interval prolongation. What is the rhythm diagnosis?

Figure 1.

12-lead electrocardiogram of the index patient.

Answer: The ECG shows sinus node dysfunction and atrial escape rhythm with complete heart block and right bundle escape rhythm. The first P wave is negative in lead III (arrow in Figure 1) and has a different morphology than the sinus P waves. Hence, this is an ectopic atrial beat that comes before the scheduled sinus P; thus, it can be called an atrial escape beat. The first P wave is exactly similar to the third, fifth, and seventh P waves (Figure 1). The sinus P waves are the second, fourth, and sixth P waves in the ECG with a fixed sinus interval of 2840 ms, indicating a sinus node dysfunction (Figure 2A). Importantly, the intervals between the ectopic atrial beats (first, third, fifth, and seventh waves) demonstrate consistency, suggesting their origin from a singular ectopic focus (Figure 2A). ¹ On close examination, it appears that the PR interval is decreasing for each apparently conducted sinus P wave and each PR interval for the atrial ectopic. This is a classic finding of the ventricular escape rhythm “marching on” independent of any atrial rhythm (Figure 2B). A narrower ectopic P wave than sinus P along with positivity in leads II, I, and aVL and negative in lead III suggests a low septal ectopic origin.

Figure 2.

A. Lead II rhythm strip magnified with all intervals shown. B. The PR interval is decreasing for each apparently conducted sinus P wave and each PR interval for the atrial ectopic. This is a classic finding of the ventricular escape rhythm “marching on” independent of any atrial rhythm. PR intervals are mentioned in the red/upper panel for ectopic atrial rhythm and in the blue/lower panel.

Electrocardiographically, complete heart block is defined by atrioventricular (AV) dissociation manifested by 3 golden rules: (a) the fixed RR interval, (b) the fixed PP interval (except ventriculophasic arrhythmia), and (c) the variable PR interval. Although the first and the third rules are met, the second rule was not met because of atrial escape at a fixed coupling interval with the sinus P wave giving a false impression of alternating short-long PR interval with conducted QRS. However, meticulous analysis of the ECG reveals that the PR intervals, while seemingly alternating at first glance, are not truly alternating. This distinction becomes particularly evident upon close examination of the final complex (Figures 1 and 2).

This case underscores the pivotal role of the “golden rules” in diagnosing complete heart block while highlighting a potential pitfall. Specifically, it emphasizes the importance of meticulous scrutiny of P wave morphology, particularly in scenarios where a seemingly variable PR interval might be present. This is a fascinating case of atrial escape masquerading both sinus and AV nodal dysfunction. The patient underwent dual-chamber permanent pacemaker insertion.

Summary

1. Meticulous examination of P wave morphology is essential in the evaluation of any arrhythmia.

2. Atrial escape occurring at consistent intervals in conjunction with a sinus rhythm has the potential to obscure the presence of both sinus bradycardia and complete heart block.

3. PR intervals that appear to alternate should be meticulously measured using calipers to definitively exclude atrioventricular dissociation.

Authors’ Contribution

SM (conceptualization: equal; formal analysis: lead; writing; original draft: lead; writing; review & editing: lead); JV (conceptualization: equal; formal analysis: lead; writing; original draft: lead; writing; review & editing: lead); NN (conceptualization: equal; formal analysis: lead; writing; review & editing: lead)

Data Availability Statement

All data are incorporated into the article and its online supplementary material.