Is the AV Block Complete?
By Ken Grauer, MD, Professor Emeritus in Family Medicine, College of Medicine, University of Florida . Dr. Grauer is the sole proprietor of KG-EKG Press, and publisher of an ECG pocket brain book.
Scenario: Interpret the rhythm strip shown above. Does it represent complete (3rd degree) AV Block? How would you proceed clinically?
Interpretation: The ventricular rhythm in the Figure is regular at a rate just over 50/minute (since the R-R interval is slightly less than 6 large boxes in duration). The QRS complex is narrow, indicating a supraventricular etiology. P waves are present however, they are not consistently conducting. Instead, the PR interval is changing. The PR interval preceding beats #3 and #4 (arrows) is clearly too short to conduct.
In the ECG Review from July 15, 2011 (see page 104), we defined the three degrees of AV block as follows:
1st degree AV block in which all atrial impulses are conducted to the ventricles, albeit with delay (so that the PR interval exceeds 0.20 second).
2nd degree AV block in which some (but not all) atrial impulses are conducted to the ventricles.
3rd degree (or "complete") AV block in which none of the atrial impulses are conducted to the ventricles, despite having adequate opportunity for conduction to occur.
The key to the diagnosis of complete AV block is in the last part of the definition: No atrial impulses are conducted to the ventricles "despite having adequate opportunity for conduction to occur." Although beats #3 and #4 in the Figure (and possibly also beat #2) are not conducted to the ventricles none of these beats has a "chance" to conduct, since the PR interval is simply too short. Thus, we have no idea if any degree of AV block is present since we cannot tell from this tracing if P waves could conduct were they given the opportunity to do so. We therefore interpret this tracing as showing "AV dissociation," since some P waves are unrelated to the QRS complexes that follow them. AV dissociation is never a "diagnosis" per se. Instead, it is the result of the underlying rhythm on the tracing. In this case the underlying rhythm is sinus bradycardia at a rate of 50/minute (the P-P interval is precisely 6 large boxes in duration for each of the P waves on this tracing). AV dissociation occurs by "default." That is, due to the relatively slow sinus rate, a nodal rhythm (at ~52/minute) takes over. This rhythm variant is not uncommonly seen in otherwise healthy, young adult individuals. It may well be that there is no degree of AV block present, and that normal conduction will resume whenever the sinus node speeds up to a normal rate. Diagnosis of 3rd degree AV block should be reserved for when no P waves conduct despite having adequate opportunity to do so.