By Ken Grauer, MD
Professor Emeritus in Family Medicine, College of Medicine, University of Florida
Dr. Grauer reports no financial relationships relevant to this field of study.
The two rhythm strips in the figure below were recorded just a few minutes apart. There is group beating. Is the rhythm the result of atrioventricular (AV) Wenckebach (second degree AV block, Mobitz Type I)?
The presence of group beating often is a tell-tale clue the rhythm in question is the result of some type of Wenckebach conduction. In both rhythm strips shown in the figure, there is group beating, either in the form of two-beat or three-beat groups. Despite this, no form of AV block is present in either tracing.
Two of the most characteristic features of AV Wenckebach are: the atrial rate is regular (or at least almost regular) and there is progressive increase in the PR interval until a beat is non-conducted, after which the cycle begins again. Neither of these characteristic findings is present in the figure.
Using calipers greatly facilitates recognition of the mechanism of the arrhythmia in the figure. It should be readily apparent that the P-P interval is the same in rhythm strip A using the P waves prior to beats 3-4, 6-7, 9-10, and 11-12. Similarly, the P-P interval is the same in rhythm strip B, if one uses the P waves prior to beats 2-3, 4-5, 6-7, 8-9, and 10-11. By setting calipers to this P-P interval distance, it should be immediately apparent that sinus P waves are missing in multiple places in these two rhythm strips.
A key to the rhythm diagnosis is to determine the appearance of a “normal” ST-T wave. To do so, look at the ST-T wave of beats 3, 6, 9, and 11 in A and beats 2, 4, 6, 8, and 10 in B. Note how smooth the ST-T is for each beat.
Is the T wave of all other beats on these two tracings deformed (usually by a notch)? The reason for this deformity is that a premature P wave (i.e., a PAC) is hiding within (and notching) the T waves of beats 1, 4, 7, 10, and 12 in A and the T waves of beats 1, 3, 5, 7, and 9 in B. The PACs that notch the T wave of beats 1, 4, and 7 in A are conducted. However, none of the other PACs are conducted, which accounts for the slight pause that follows beats 10 and 12 in A and that follows beats 1, 3, 5, 7, and 9 in B.
It turns out that after every two sinus beats, a PAC occurs. This defines the underlying rhythm as atrial trigeminy in which some PACs are conducted, and others are dropped. There is no AV block.
For more information about and further discussion on this case, please click here.