By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
How might the reader interpret the two-lead rhythm strip shown in the figure below? Is there AV block? If so, is this complete (i.e., third-degree) AV block?
Trying to fully explain the two-lead rhythm strip is challenging. That said, appreciation of a few basic concepts in arrhythmia interpretation should allow accurate diagnosis of the essentials of this complex arrhythmia in less than a minute.
First, step back from this tracing. Doing so should suggest there is a pattern that repeats throughout. There is “group beating,” with alternating shorter-then-longer R-R intervals. Although there are several arrhythmias that may produce this type of bigeminal pattern, recognition of “group beating” should suggest the possibility of some type of Wenckebach conduction.
Although only two leads are visible, it appears the QRS is narrow. This suggests the rhythm is supraventricular. Although QRS morphology looks similar for all beats in lead I, every other beat in lead II looks different. Intermittent aberrant conduction probably accounts for this difference in QRS morphology.
Regular P waves are present. These are best seen in lead II (there is too much artifact for identification of all P waves in lead I). Although some P waves are partially hidden within the early part of the QRS, regularity of the atrial rhythm can be confirmed by setting calipers to the P-P interval that is clearly evident from the P wave just before the second beat in lead II and the P wave occurring just after this second beat in lead II. Doing so allows one to “walk out” regular P waves throughout the rhythm strip.
There are more P waves than QRS complexes. For each of the two-beat groupings, there are three P waves. This means at least one of the three on-time P waves in each group is not conducted, which means some form of AV block is present.
Look at the P waves in front of each even-numbered beat in lead II (i.e., beats 2, 4, 6, 8, 10, and 12). Taking into account there is some slight distortion caused by angling of this tracing, does it appear as if the PR interval in front of each beat is the same? This indicates each even-numbered beat is conducted, which means this rhythm cannot possibly represent complete AV block.
Finally, look at the far-away P waves in front of each odd-numbered beat in lead II (i.e., in front of beats 3, 5, 7, 9, and 11). Although prolonged, these PR intervals also repeat.
The finding of a supraventricular rhythm with group beating and regular P waves throughout the tracing, in which at least some of the PR intervals repeat, is not by chance. The fact that one out of every three P waves fails to conduct strongly suggests some form of AV Wenckebach (i.e., second-degree AV block of the Mobitz I type) is present. Details of the specific type of Wenckebach conduction are admittedly complex, but do not alter the essentials of rhythm diagnosis, which is that this tracing clearly represents some form of second-degree AV Wenckebach.
For more information about and further discussion on this case, please click here.
How might the reader interpret the two-lead rhythm strip shown in the figure? Is there AV block? If so, is this complete (i.e., third-degree) AV block?
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