Too Tired to Stop and Smell the Roses
ANSWER
Findings on this ECG include sinus rhythm at a rate of 60 beats/min, evidence of a second-degree atrioventricular (AV) block (Mobitz I), and a right bundle branch block (RBBB).
To understand the rhythm, it is best to focus on the rhythm strip, particularly lead I at the bottom of the ECG. If you measure the P-to-P interval, you will notice that it is consistent and constant at a rate of 60 beats/min, regardless of the QRS complex. If you look at the PR interval from the second to the sixth QRS complex, you will notice that it is regular until the QRS is dropped after the P wave that follows the sixth QRS complex. Following the pause, the PR interval on the seventh, eighth, and ninth QRS complexes gradually prolongs. Although this is not a classic example of Mobitz I block, it is indicative of an AV node with a conduction abnormality.
Subsequent rhythm strips documented multiple blocked PR intervals that corresponded to the patient’s dizziness. The RBBB is evident by the RSR’ pattern seen in lead V1 with a QRS duration ≥ 120 ms.