Alternating QRS Morphologies and PR Intervals After Transcatheter Aortic Valve Replacement

Abstract

Alternating bundle branch block pattern on electrocardiogram (ECG) is a concerning finding with important prognostic implications. This ECG challenge explores the electrophysiological mechanism of a case of alternating bundle branch block with alternating PR intervals. (Level of Difficulty: Beginner.)

Graphical abstract

Alternating bundle branch block pattern on electrocardiogram (ECG) is a concerning finding with important prognostic implications. This ECG challenge…

An 83-year-old man, whose baseline electrocardiogram (ECG) had a right bundle-branch block (RBBB) and left posterior fascicular block (LPFB), and who underwent transcatheter aortic valve replacement (TAVR) with a balloon-expandable valve 4 weeks before, was admitted to the hospital for symptomatic anemia. A 12-lead rhythm strip was reconstructed from telemetry (Figure 1A).

Figure 1.

Electrocardiogram

(A) Reconstructed 12-lead rhythm strip electrocardiogram. (B) Schematic diagram.

What is the mechanism of alternating bundle branch block?

• 1.Intermittent phase 3 block in the bundle branches
• 2.Intermittent phase 4 block in the bundle branches
• 3.Variable conduction delay in both bundle branches
• 4.Premature ventricular contractions with isorhythmic atrioventricular (AV) dissociation

Discussion

The best explanation for this patient’s ECG is variable conduction delay in both bundle branches. Bundle-branch block (BBB) patterns on ECG are typically caused by relative conduction delay in the left bundle (LB) or the right bundle (RB) rather than complete conduction block. Furthermore, conduction delay does not need to be fixed and can occur intermittently.

Before TAVR, the patient’s ECG showed sinus rhythm at a rate of 70 beats/min, with normal PR interval (170 ms), right-axis deviation (QRS axis 97°), RBBB (QRS duration 128 ms), and LPFB (Supplemental Figure 1). The RBBB and LPFB indicate that the left anterior fascicle (LAF) was the faster conducting fascicle. The patient likely had intermittent conduction in the RB, but at a slower conduction velocity than the LAF, producing a RBBB and LPFB pattern on the ECG. Following TAVR, the patient’s ECG showed sinus bradycardia at a rate of 54 beats/min, normal axis (QRS axis 64°), first-degree AV delay (PR interval 238 ms), and RBBB (QRS duration 128 ms) (Supplemental Figure 2). He had developed PR prolongation, and the pre-existing LPFB was no longer evident, likely because of balanced delay in the LAF, induced by mechanical injury.

On the reconstructed rhythm strip (Figure 1A), with sinus tachycardia (rate: 102 beats/min), the patient had primarily RBBB (QRS duration: 130 ms) with a normal PR interval (180 ms). When the RB is able to intermittently conduct, it now conducts faster than the LAF, resulting in a shorter PR interval than baseline (140 ms) and left bundle-branch block (LBBB) QRS duration 150 ms. Figure 1B provides a schematic diagram of the patient’s conduction system and our proposed mechanism for his ECG. The fixed sinus rate without variability and absence of atrial ectopy argues against phase 3 block and phase 4 block, respectively. The relatively fixed PR interval associated with the LBBB-morphology QRS complexes argues against a ventricular origin.

Thus, the patient was diagnosed with alternating BBB due to variable conduction delay in both bundle branches. In 1964, Lepeschkin described alternating bundle branch block and its tendency to progress to advanced heart block (1). In 2010, Massumi published a series of 16 patients with alternating BBBs and PR intervals, who all subsequently developed high-grade AV block requiring pacemaker placement (2). Most recently, in 2017, van Gils et al. found that, among patients with pre-existing RBBB who underwent TAVR, 8% developed alternating BBB within 24 h (3). Given the development of alternating BBB post TAVR, and the significant risk of progression to high-grade heart block, the patient ultimately underwent uncomplicated placement of a permanent pacemaker.