Pacemaker or LBBB?

Report:

Sinus rhythm 65/min 2

Atrial-sensing ventricular pacemaker 8

Comment:

This seems unimpeachable, except for lead V1 having the morphology of LBBB (steep descent, slow ascent) instead of expected right ventricular paced or ectopic beat (slow descent, sharp rise). This ECG is similar to Case 41, where V1 similarly remains ambiguous. Both may be examples of ventricular fusion, but there is rarely an opportunity to test it.

This patient, however, came to my ICU following colonic surgery, during which the ICD (and the magnet) function was turned off. When the technicians arrived to turn the ICD back on, I asked them to pace the patient for few seconds. This can be accomplished in two ways: speeding up the pacemaker or shortening the AV interval. The result is shown below (Fig 94a):

The first 4 beats are paced: note the shorter PR (AV) interval. There was no effect on blood pressure (bottom channel). The report above should therefore be amended to: Sinus rhythm. LBBB. Pacemaker pseudofusion beats.. This patient’s ECG prior to ICD implantation (not shown) had LBBB with left axis deviation – identical to the one seen in pseudofusion beats.

See also Case 15.