Syncope and Complete AV Block?
September 15, 2023 2 minutes read
By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
The rhythm strip shown in the figure below was recorded from a patient with syncope. It was interpreted as consistent with complete AV block. Do you agree with that interpretation?
Interpretation: The short answer to the question as to whether this rhythm represents complete AV block is “probably not.” Most of the time when there is complete AV block, the ventricular escape rhythm will be regular. This clearly is not the case in this rhythm because beat 4 occurs early. When assessing an arrhythmia for the possibility of complete AV block, the best clue suggesting there probably is at least some conduction of atrial activity (and, therefore, not complete AV block) is when a beat occurs earlier than expected.
The more complete answer to the question regarding the severity of the AV conduction disturbance in the figure entails interpretation of this rhythm. I favor the systematic “Ps, Qs, and 3R” approach through which we assess five key parameters:
• The QRS complex is narrow in this single monitoring lead, so the rhythm appears to be supraventricular.
• Sinus P waves (upright in lead II) are present. Although difficult to prove without using calipers, atrial activity most likely is regular (with on-time sinus P waves probably hiding within the QRS complex of beats 2 and 7).
• The overall rhythm looks fairly Regular, except for beat 4, which occurs earlier than expected.
•The R-R interval never exceeds six large boxes in duration, so the overall heart Rate does not drop below 50 beats per minute.
• The last key parameter is the third R, which assesses whether P waves are Related to neighboring QRS complexes. As noted earlier, the fact that beat 4 occurs earlier than expected strongly suggests the P wave that occurs before this beat is conducted. Since the PR interval preceding beat 4 is of normal duration (i.e., 0.19 seconds), this suggests the degree of AV block may not be as severe as was initially thought.
Key Point: There is AV dissociation in this rhythm because many of the P waves appear to be unrelated to neighboring QRS complexes. Some of these P waves clearly have adequate opportunity to conduct yet fail to do so, which means some form of second-degree AV block is present.
That said, because we never see two P waves in a row that have a chance to conduct, yet fail to do so, we cannot determine from this single rhythm strip whether high-degree AV block is present.
Most of the beats on this tracing probably represent an appropriate AV nodal escape rhythm at ~50 beats per minute that prevents a more severe bradycardia. It is common to see this with the Mobitz I form of second-degree AV block.
Additional monitoring and clinical correlation will be needed to determine if permanent pacing is needed for this arrhythmia.
For more information about and further discussion of this case, please visit here.
The rhythm strip shown in the figure was recorded from a patient with syncope. It was interpreted as consistent with complete AV block. Do you agree with that interpretation?
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