ECG Review: What’s Going On? (Part II)

By Ken Grauer, MD

Figure. 12-lead ECG and lead II rhythm strip obtained from an 84-year-old man with acute dyspnea.

Clinical Scenario: The 12-lead ECG and accompanying rhythm strip in the Figure were obtained from an 84-year-old man who presented to the Emergency Department with acute dyspnea from pneumonia and heart failure. Can you account for the relatively slow heart rate despite his acute shortness of breath? (Hint: As was the case for the ECG review from November 15, the key to interpreting this tracing lies within QRST morphology of the lead II rhythm strip.)

Interpretation: QRS complexes occur at a regular rate of about 60 beats/minute in the rhythm strip. However, this is not simply a sinus rhythm. Close inspection of each T wave in the rhythm strip shows variable T wave packaging. This variation in T wave morphology is not the result of artifact. Instead, it represents slight variation in the time of occurrence of premature P waves that deform each T wave. The rhythm is therefore atrial bigeminy, in which each premature P wave occurs so early in the cycle that it is blocked. Thus, the primary problem responsible for the relatively slow heart rate despite this patient's acute dyspnea is atrial bigeminy. Correction of hypoxia and treatment of his heart failure addressed the substrate producing the frequent PACs, and the normal sinus rhythm at a more appropriate rate was restored.

Dr. Grauer, Professor, Assistant Director, Family Practice Residency Program, University of Florida, is Associate Editor of Internal Medicine Alert.