ECG Review: AV Block or Escape?
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.
Figure — Two tracings obtained from asymptomatic young adults.
Scenario: Tracings A and B in the Figure were each obtained from a hemodynamically stable and asymptomatic young adult. Is there evidence of AV block?
Interpretation: The first four beats in Tracing A are sinus conducted. The QRS complex is narrow throughout — although there is slight change in QRS morphology beginning with beat #5. Note gradual slowing in the rate of the sinus bradycardia for the initial four beats. No P wave is seen prior to beat #5. However, a retrograde P wave (negative in this lead II) is seen after the narrow QRS complex for beats #5, #6, and #7. These last three beats in Tracing A are junctional escape beats that manifest retrograde atrial conduction. This retrograde atrial conduction continually resets the sinus node, and serves in this way to perpetuate the junctional escape rhythm.
The important point is that there is no evidence of any AV block at all in Tracing A. On the contrary, in view of the fact that this tracing was obtained from a presumably healthy and otherwise asymptomatic young adult, there is not necessarily any abnormality at all. We simply see progressive sinus bradycardia with an appropriate AV node escape rhythm arising once the sinus rate drops below 40/minute. On the other hand, sinus bradycardia to this degree with need for an AV nodal escape rhythm to arise would be cause for concern if the patient was an older adult with a history of weakness or syncope. Clinical correlation is everything.
The underlying rhythm in Tracing B is sinus — as determined by the presence of an upright P wave with fixed PR interval preceding beats #1 and #2. Beat #3 occurs early. This beat is wide, different in morphology from the first two sinus-conducted beats, and not preceded by any P wave. Beat #3 is a premature ventricular contraction (PVC). Of interest — a retrograde P wave follows beat #3. Just like the situation in Tracing A, retrograde atrial conduction resets the sinus node. As a result, there is no P wave preceding beat #4 in Tracing B. Given how similar QRS morphology of beat #4 is to the first two sinus beats, this defines beat #4 as a junctional escape beat.
The remaining beats in Tracing B represent a fairly regular, albeit slightly accelerated, junctional escape rhythm at 65-70/minute. Sinus node activity gradually returns toward the end of the tracing. Before it does, there is a brief period of AV dissociation since the PR interval preceding beats #7 and #8 is clearly too short to conduct. That said, there is no evidence of any AV block since these non-conducting P waves do not have a chance to conduct (the PR interval preceding these beats is simply too short). Thus, the underlying rhythm in Tracing B is sinus. A short pause follows a PVC, which results in a brief period of AV dissociation by "default" due to transient sinus slowing. There is no evidence of any AV block — and no intervention is needed in this otherwise healthy young adult.
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