By Ken Grauer, MD

Professor Emeritus in Family Medicine, College of Medicine, University of Florida

The dual lead rhythm strip in the figure below was obtained from an elderly patient with syncope. Is there AV dissociation? Is there complete AV block? This is a challenging tracing because a different diagnosis is suggested by the first five beats in the rhythm strip, compared to the last four beats.

Looking first at the initial five beats in the tracing, the QRS complex in lead V1 is notched and appears to be slightly widened. Although some P waves are partially (or totally) hidden by the QRS complex, the underlying atrial rhythm appears to be regular at ~100-110 beats/minute. However, the PR interval preceding each of the first five beats is changing constantly. This tells us there is AV dissociation because none of the P waves for this first portion of the tracing are related to neighboring QRS complexes.

Things change for the last four beats in the tracing. The underlying regular atrial rhythm continues and is easier to appreciate because P waves do not fall so close to neighboring QRS complexes. The QRS itself has narrowed, and QRS morphology has changed. The most important difference is the PR interval preceding each of the last four beats now is constant, which tells us beats 6, 7, 8, and 9 are sinus-conducted. The P wave that falls near the middle of the R-R interval of beats 6-7, 7-8, and 8-9 is not conducted, which defines the conduction defect for these last four beats as second-degree AV block with 2:1 AV conduction.

Importantly, AV dissociation is not the same as complete AV block. Although there is complete AV dissociation for the first five beats on this tracing, this does not represent complete AV block. Proof that this is not complete AV block is forthcoming by clear evidence of conduction for the last four beats. In addition to complete AV dissociation, the diagnosis of complete AV block requires P waves to receive adequate opportunities to conduct, yet still fail to do so. The first five beats on this tracing represent less than six seconds of ECG monitoring, which simply was not a long enough period to allow P waves to appear in all parts of the cardiac cycle. With a few more seconds of monitoring, that “magic point” in the cardiac cycle where conduction is possible was found.

For more information about and further discussion on this case, please click here.

ECG Review