Invisible LAHB Causing 2:1 AV Block

Report:

Sinus tachycardia 120/min

2:1 AV block

Right axis deviation +140o

Right bundle branch block

Possible old anterolateral infarction

Comment:

The cause of the RAD may be RVH or old anterolateral MI but the former was certainly not present clinically and the latter was absent historically, with only the borderline Q waves in V4-6 and the RAD itself to suggest it. LPHB is quite likely, even though V6 has not become an RS complex (it’s not invariable).

Now it becomes apparent that the block is intraventricular; the conducted beats are there only through the remaining fascicle in the left side, the anterior-superior one. The conduction fails completely in alternate beats – it is, most likely, concurrent LAHB on top of pre-existing LPHB and RBBB: a 2:1 LAHB. Surface ECG cannot, of course, completely exclude AV node as the site of 2:1 block, but the normal PR interval makes it less likely.

Only 20 minutes earlier the patient was in complete AV block, with junctional escape rate 47/min (Fig 68a).

She received a pacemaker (Fig 68b), but to no avail (Fig 68c).