The correct interpretation includes marked sinus bradycardia with sinus arrhythmia, biatrial enlargement, and an RSR’ pattern in lead V1 suggestive of right ventricular conduction delay.
A heart rate below 60 beats/min is considered sinus bradycardia; below 50 beats/min, it may be called marked sinus bradycardia. Sinus arrhythmia is identified by comparing the RR intervals between the third/fourth and fourth/fifth beats on the rhythm strips with the others. This particular variation is most likely respiratory.
Biatrial enlargement is characterized by P waves ≥ 2.5 mm in lead II (right atrial enlargement), P waves > 120 ms in width in lead II, and a biphasic P wave in lead V1 (> 40 ms wide, 1 mm deep). The criteria aren’t crystal clear in this example, but given the large P wave in leads II and aVF and the biphasic P waves in lead V1, the finding is inferred.
Finally, the RSR’ pattern in V1 suggests right ventricular conduction delay but does not meet the criteria (QRS duration > 120 ms) for a right bundle branch block.