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 long lead II rhythm strip shown in the figure below was obtained from a hemodynamically stable patient. The rhythm was diagnosed as showing second-degree AV block, Mobitz Type II. Do you agree with that assessment?
Our systematic approach to the interpretation of any cardiac arrhythmia is to determine hemodynamic stability first. Then, we assess for five key parameters. These essential parameters are recalled easily by the saying, “Watch your Ps, Qs, and 3Rs.”
P waves. Sinus P waves are evident and recognized by the presence of upright P waves with similar morphology in this long lead II rhythm strip.
QRS width. The QRS complex is narrow. Although we would see all leads on a 12-lead ECG before committing to comment on QRS duration, the QRS complexes in this tracing clearly look to be narrow and supraventricular.
Rate. The rate of the rhythm varies, but it is neither excessively fast, nor excessively slow.
Regularity. The ventricular rhythm is not completely regular. That said, there is a pattern to this rhythm in that “group beating” is present, with three groups comprised of three beats each in a repetitive pattern.
Related. There does appear to be a consistent relation between several sinus P waves and neighboring QRS complexes. That is, the PR interval preceding beats 2/3, 5/6, and 8/9 appears to be constant, albeit slightly prolonged. Therefore, there is an underlying sinus rhythm.
What is the cause of the brief pauses between beats 3-4 and 6-7? Why is the PR interval at the end of each pause shorter than the PR interval preceding the second and third beats in each grouping? Why is the rate of sinus P waves not constant throughout this tracing?
Two clinical entities should come to mind as potential explanations for these ECG findings: some form of AV block and blocked PACs. There is no second- or third-degree AV block in this rhythm. Despite the apparent increase in PR interval between the first and second beats in each grouping, the rhythm is not AV Wenckebach. This is because the premise of AV Wenckebach (also known as the Mobitz I form of second-degree AV block) is that there should be an underlying regular sinus rhythm throughout the tracing. This is not present. The rhythm does not represent the Mobitz II form of second-degree AV block because the PR interval does not remain constant. Finally, this rhythm cannot be complete (i.e., third-degree) AV block because there is conduction of several sinus beats (i.e., beats 2/3, 5/6, and 8/9 are conducted with a constant PR interval.
The most common cause of an unexpected pause in a rhythm is a blocked PAC. This phenomenon occurs far more often than is appreciated. The reason it is so often overlooked is because of how subtle distortion of the T wave hiding the non-conducted PAC may be. Close scrutiny of the base of the T waves at the onset of each pause (i.e., the T waves of beats 3 and 6) reveals angulation that is not present in any other T wave. These blocked PACs reset the SA node and account for the pause that occurs after beats 3 and 6. Beats 1, 4, and 7 can be identified as junctional escape beats because they all manifest a slightly different QRS morphology (with taller R wave and smaller S wave) compared to the other six beats on the tracing that are sinus conducted. Further, the R-R intervals preceding beats 4 and 7 are identical, corresponding to the junctional escape rate. Finally, the reason the PR interval preceding beats 1, 4, and 7 is shorter than the PR interval preceding sinus-conducted beats is that the junctional escape focus fired before the sinus P waves preceding beats 1, 4, and 7 had a chance to conduct to the ventricles.