ECG Review-Any Marriages in the Tachycardia?
By Ken Grauer, MD
Clinical Scenario: The telemetry tracing shown in the Figure was obtained from an acutely ill 49-year-old man who had just returned to the intensive care unit following valvular heart surgery. How would you interpret the rhythm in this tracing? What are the three most common causes of this rhythm disturbance?
Interpretation: This is a difficult tracing to interpret. However, use of a deductive approach greatly facilitates arriving at the correct answer. The five key parameters for rhythm interpretation are assessing for P waves (the presence and nature of atrial activity); determining QRS width (distinction when possible between ventricular and supraventricular rhythms); calculating heart rate; determining regularity; and when P waves are present, assessing for a relationship ("marriage") between P waves and QRS complexes (to determine if P waves are conducting). The memory aid, "Watch your P’s and Q’s—and the three R’s," facilitates recall of these five key parameters. The rhythm in the Figure is clearly supraventricular (as determined by the fact that the QRS complex is narrow). The rate is rapid and the rhythm almost (but not completely) regular. Atrial activity is present—however, the PR interval continually changes, suggesting AV dissociation. Putting together these findings results in an interpretation of junctional tachycardia (at a rate of about 125/minute) with AV dissociation. The most common causes of accelerated junctional rhythm or junctional tachycardia are digitalis toxicity, inferior infarction, and post-operative state, especially when the patient has undergone cardiovascular surgery.
A subtle additional point about this rhythm revealed by close inspection (measured with calipers) is slight variation in the R-R interval. Despite this, the atrial rhythm remains regular throughout the rhythm strip (use calipers to verify this, beginning with the 3 short vertical lines). It is likely that the slightly early occurring beats in this tracing (the 4th, 9th, and 14th QRS complexes) are "capture" beats being conducted by P waves that occur during a non-refractory portion of the cardiac cycle.