Sinus Tachycardia with Unusual Bigeminy

Report:

Sinus tachycardia 112/min

First degree AV block

Second degree AV block[!xe "AV block:second degree:2-1 advanced" \b!]

2:1 in top & bottom strips

Advanced in middle strip

Junctional escape beats

Escape-capture bigeminy

[!xe "Escape-capture bigeminy:mimicking alternating 2-1 3-1 AV block" \b!]

Comment:

First impression suggests an unusual alternation of 2:1 and 3:1 AV block in the middle strip. The common alternation, often seen in atrial flutter, would be 2:1 alternating with 4:1 conduction (cf). In that situation, if every second P or F wave is ignored, there would be clear evidence of Wenckebach 3:2 conduction: the second PR interval in each pair would be longer than the first.

Even without Wenckebach analogies, short cycles would generally be expected to have longer PR intervals than long cycles, due to the reciprocal R-P – P-R relationship.

Here, in each pair, the PR interval of the first beat is in fact longer than that of the second one. This means that, in all likelihood, that P wave is not conducted at all and the QRS following it is a junctional escape beat. A similar situation is seen in Case 42, where the longer PR interval precedes the pacemaker escape beats.

The strips below (Fig 44a), taken during an exacerbation of nausea (and increase in vagal tone), show the onset of complete AV block with junctional escape rhythm at 41/min.