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 rhythm in the figure below is challenging to interpret. More than one answer is possible. Unfortunately, no clinical information was available on the patient. What observations can one make about the cardiac rhythm?
This tracing was recorded at the standard 25 mm/second speed. Because the ECG grid is faded, I have indicated the size of a large box with a small gray square. Also, I show the duration of five large boxes in black numbers at the bottom of the tracing. I am not after a precise interpretation of this rhythm. Atrial activity is difficult to make out because of its small size. Unless one has calipers readily available, it is unlikely one can evaluate some details. What counts is appreciating the principal findings. So what are they? The rhythm in the figure is supraventricular because all QRS complexes are narrow. The overall heart rate is slow. There are several other important observations to make.
There is group beating for at least the first part of the tracing. That is, a short-long (or bigeminal) pattern is seen for the first six beats. Recognition of group beating is helpful because it suggests that some form of Wenckebach conduction may be present. Beat 9 appears to be sinus-conducted. In the long lead II rhythm strip, an upright P wave with normal PR interval precedes this QRS complex. Looking at the three long lead rhythm strips (taken from leads V1, II, and V5), no P wave precedes beats 1, 3, 5, 7, or 8. The R-R interval preceding each of these beats appears to be equal and approximately seven large boxes in duration. This corresponds to a heart rate just over 40 beats/minute. These are junctional escape beats. There appear to be other indications of periodic atrial activity. This is seen best in lead V1. Note the small, notched deflection in lead V1 that appears midway within the R-R interval between beats 1 and 2. In the long lead V1, does it look like there also is some subtle notching toward the end of the ST segment of beats 3, 5, and 7?
There is a phenomenon known as “escape-capture” bigeminy in which junctional escape beats are followed by atrial activity that “captures” (i.e., conducts to) the ventricles. This appears to be happening for the first six beats in the tracing. The subtle notching (P wave) that occurs within the ST segment of beat 7 is not conducted to the ventricles. There follows another junctional beat (beat 8). Finally, a sinus P wave conducts normally to the ventricles (beat 9). The fundamental rhythm disturbance in this tracing is marked bradycardia. Thus, a junctional escape rhythm arises with escape-capture until a P wave is nonconducted. This allows the sinus P wave that precedes beat 9 to regain control of the rhythm.