His Heart’s Awkward Timing

ANSWER

The correct answer is atrial flutter with 2:1 atrioventricular (AV) conduction. The QRS complexes are narrow and regular, indicating the rhythm originates within the atria or AV node, with conduction down the normal His-Purkinje system, and not from the ventricles.

The regular rate of the P waves and QRS complexes rules out atrial fibrillation with a rapid ventricular response. If you look carefully, you’ll see a P wave immediately before each QRS complex, and you’ll also see a P wave at the onset of the T wave (best seen in leads II, V₃, and V₆) resulting in what looks like a notched T wave. If you measure the duration of the P at the onset of the T wave to the P wave prior to the QRS complex, you’ll see the intervals are regular and march through the QRS complexes.

With 2 P waves for every QRS complex, the atria are contracting at 310 beats/min (193 ms), a rate consistent with atrial flutter in a 2:1 conduction pattern, compared to the ventricular rate of 155 beats/min (387 ms).