Bigeminy in Sino-Atrial Exit Block

Report:

Sinus rhythm approximately 90/min

Left atrial abnormality (LAA)

3:2 Sino-atrial exit block, Möbitz 1 (Wenckebach)

Bigeminy

Left axis deviation –35o

Comment:

Bigeminy like this one has to be differentiated from atrial ectopic bigeminy: half the P waves may then be P’ waves. If the morphology of the P waves is constant – as seems to be the case here, allowing for rate and respiratory changes - it can still be sinus node “extrasystolic” bigeminy rather than 3:2 exit block. The proof requires observed transition to either shorter or longer cycle rate. If all the subsequent beats have the rate of the shorter cycle, the case for previous exit block is made.

The only other trace, taken a day later, is shown below (Fig 108a) . It does not help the differential diagnosis. It is slow enough (52/min) to make one consider a 2:1 S-A block in view of the previous day’s bigeminy. This is unlikely – zebras are more alluring than horses. Leads V2 and V3 are reversed.

This case is quite similar to another patient’s in this collection, also with a brain-stem infarction in ICU. Like then, I could not bring myself to give some atropine and find out. Next time – if there is one - I may think of an excuse to do it.