ECG Review: Group Beating from AV Wenckebach?
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: The two tracings above were obtained from the same patient a few minutes apart. There is group beating in the top tracing. Does this represent second degree AV block of the Mobitz I type (AV Wenckebach)?
Interpretation: Recognition of the phenomenon of group beating should always suggest the possibility of a Wenckebach conduction disorder. That said, this case presents a wonderful example of why group beating does not always mean that a Wenckebach block is present.
Knowing that the two rhythm strips in the Figure were obtained from the same patient is helpful in our interpretation because findings present in one tracing will often be present later on. So it is here. The lower tracing clearly shows the underlying rhythm to be sinus arrhythmia. Beat #4 is early in this lower tracing. Note definite peaking of the T wave preceding beat #4 that is not seen in the much smoother T waves of beats #1, #2, and #5. This peaking in the T wave of beat #3 in the lower tracing is due to a PAC (premature atrial contraction) that is hidden within. The reason for the slightly different QRS morphology of beat #4, is that this early-occurring PAC is conducted with aberration.
Note the slight (1.6 second) pause at the end of the lower tracing (between beats #6-7). The commonest cause of a pause is a blocked PAC. T wave peaking of the T wave at the onset of this pause (in the T wave of beat #6) indicates that a blocked PAC is indeed the reason for this relative pause in the rhythm at the end of the lower tracing.
A similar phenomenon occurs repetitively in the top tracing. Despite group beating, the top tracing does not represent AV Wenckebach. This is because: 1) the atrial rhythm is not regular as it should be with a conduction disturbance due to AV block, and 2) the PR interval is not progressively increasing within groups of beats. Instead, there is subtle-but-real peaking of the T wave at the onset of each of the relative pauses in this top tracing. In the context of the lower tracing, it becomes much easier to appreciate that the T waves of beats #2, #5, and probably #8 in the top tracing are all taller than the T waves of beats #1, #3, #4, #6, and #7 that do not contain PACs within them. Thus, there is sinus arrhythmia with frequent PACs on these two sequential tracings. PACs are either non-conducted or conducted with aberration.