1o AV Block Expressing BBBB

Report:

Sinus rhythm 77 – 82/min

Left atrial abnormality (LAA)

First degree AV block

PR 0.22”

Intermittent right bundle branch block + left anterior hemiblock

PR 0.16”

Nonspecific T wave changes

Comment:

At first, one would postulate a dissociated junctional or fascicular rhythm with RBBB/LAHB morphology, with rate-dependency in the former. This does not wash: the shorter PR interval remains constant, implying conduction.

Next, one would think of rate-dependent aberrancy consequent on acceleration in sinus rhythm – but no such acceleration occurs; in fact, the three-beat run of broad complexes is preceded by slight deceleration (best measured in V1), due to sinus arrhythmia.

The first two considerations would be automatic; most experienced Cardiologists “see” the diagnosis, confirm it, and proceed elsewhere. It’s at this stage, when two or three initial diagnoses fail to be confirmed, that the trace becomes interesting.

First of all, there may exist dual conduction20, with slight changes in sinus rate and vagal tone rendering one or the other pathway more or less refractory. This would be fine, except that additional diagnosis of rate-dependent aberrancy is required. Science favours the simplest hypothesis; medicine seeks a single cause.

That single cause would, in this case, be a bilateral first degree bundle branch block (BBBB). While congruous (equal in both bundle branches), it is manifest as a (typically modest) first degree AV block and normal QRS complex. With a slight change in refractoriness or conduction velocity in one of the bundle branches, a bundle branch block pattern appears, “with a different – usually shorter – PR interval21”. Here we have it! A matter of balance22.

The patient’s potassium was 6.6 mEq/L when this ECG was obtained; subsequent ECGs, taken at normal potassium levels, showed only 1o AV block and nonspecific ST/T changes. Seven years later, Wenckebach 2o AV block developed (Fig 42a below).