The simultaneously-recorded 3-lead rhythm strip in the figure was obtained from a 75-year-old woman who presented to the emergency department with syncope. She was diagnosed as being in complete (third-degree) AV block. Do you agree with this interpretation?
If all you saw were the first five beats on the tracing below, would you then diagnose complete AV block?
Interpretation: The advantage of viewing an arrhythmia in several simultaneously recorded leads is that it provides more than a single vantage point. For example, atrial activity is best seen in lead II of this figure, but a change in QRS morphology is best appreciated by QRS appearance in lead V1. We highlight the following features in our interpretation:
- Regular atrial activity is seen throughout this tracing at a rate of about 100/minute (red arrows in lead II).
- QRS morphology changes after the first five beats. While best seen in lead V1, a look at simultaneously recorded leads II and V6 confirms beyond doubt that QRS morphology is different for the last four beats on the tracing. That said, the QRS is no more than minimally widened for the first five beats that probably represent an escape focus arising from below the AV node, but still within the conduction system (most likely from the bundle branch system).
- P waves “march through” the QRS (with continually varying PR intervals) for the first five beats in the tracing. None of the P waves preceding the first five QRS complexes are conducting to the ventricles. Therefore, there is AV dissociation during the initial part of this tracing. However, the degree of AV block cannot (by definition) be complete, because a constant and normal PR interval is seen to precede the last four beats on the tracing. Every other P wave conducts during these last four beats, so that there is second-degree, 2:1 AV block for beats six through nine.
Conclusion: It turns out this patient did receive a permanent pacemaker for her high-grade, second-degree AV block. Nevertheless, this case is illustrative of a number of important diagnostic points. The reason we cannot make the diagnosis of complete AV block from the first five beats on the tracing is that despite AV dissociation, this initial period of monitoring is simply too short. P waves don’t occur at all points in the cardiac cycle, so they never have full opportunity to conduct, yet fail to do so. In fact, when a strategically placed P wave with a PR interval of 0.19 second occurs, sinus conduction (albeit with 2:1 AV block) occurs. Additional findings in support of conduction during the latter part of this tracing include: 1) recognition that beat number six occurs slightly early, and 2) that this earlier-than-anticipated sixth beat manifests a different (slightly narrower) QRS morphology than was seen during the AV dissociation present during the first five beats.
NOTE: Further discussion of this tracing (with laddergram illustration) is available on an ECG video found here.