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.
The ECG and long lead II rhythm strip in the figure below was obtained from a 58-year-old man who was admitted to the hospital with chest pain and weakness. How would you interpret this tracing? How many different types of conduction disturbances can you identify?
This is a complex tracing. That said, attention to some basic principles of arrhythmia interpretation go a long way toward deciphering the mechanism of this rhythm. We proceed as follows, focusing first on the long lead II rhythm strip at the bottom of the figure.
It is often easiest to begin by identifying atrial activity. Using calipers greatly facilitates this task. Red arrows in the figure reveal that the underlying atrial rhythm here is quite regular. In contrast, the ventricular rhythm is not regular. Instead, there is a pattern of “group beating,” with alternating long-short R-R intervals. Two different shapes of QRS complexes are evident. That is, the QRS complex of beats 1, 3, 5, and 7 looks similar. A different-shaped QRS is evident for beats 2, 4, and 6.
Some P waves are conducting. Note that the PR interval preceding beats 1, 3, 5, and 7 is constant, albeit slightly prolonged (i.e., slightly more than one large box = 0.20 second in duration). Therefore, the P waves preceding beats 1, 3, 5, and 7 are conducting with first-degree AV block. A look at simultaneously recorded lead I (for beat 1), lead V1 (for beat 5), and lead V6 (for beat 7) suggests that in addition to first-degree AV block, these P waves are conducting with left bundle branch block (LBBB). Beats 2, 4, and 6 occur later than expected and are not preceded by P waves. Since the R-R interval preceding each of these QRS complexes is similar and longer than the R-R interval preceding the sinus-conducted beats, this defines beats 2, 4, and 6 as escape beats arising from either the AV node or the bundle of His. A look at simultaneously recorded lead V1 (for beat 4) and lead V6 (for beat 6) suggests that these beats manifest as right bundle branch block (RBBB) morphology. Some form of second-degree AV block is present, because many of the P waves are not conducting. But because some P waves are conducting, this is not complete (i.e., not third-degree) AV block.
It is impossible to be certain from this single tracing whether this form of second-degree AV block tracing represents Mobitz I (i.e., AV Wenckebach) or Mobitz II because we never see two P waves in a row that conduct to the ventricles. Nevertheless, the conduction disturbance seen here is severe since there is first-degree AV block, some form of second-degree AV block, and alternating bundle branch blocks in the form of LBBB for conducting beats and RBBB for escape beats. Additionally, the deep, symmetric T wave inversion that is maximal in lead V3 (see beat 4) is more than one expects to see in lead V3 when there is RBBB. This means ischemia and/or recent infarction probably has occurred. The chances are good that a pacemaker will be needed.