A 62-year-old man with underlying COPD develops a viral upper respiratory infection and begins taking an over-the-counter decongestant. Shortly thereafter he experiences palpitations and presents to the emergency room, where the following rhythm strip is obtained. What is the most likely diagnosis?

The rhythm strip reveals atrial flutter with 4:1 atrioventricular (AV) block. Atrial flutter is characterized by an atrial rate of 250 to 350/min; the electrocardiogram typically reveals a sawtooth baseline configuration characteristic of flutter waves. In this strip, every fourth atrial depolarization is conducted through the AV node, resulting in a ventricular rate of 75/min (although 2:1 conduction is more commonly seen). The rapid atrial rate excludes sinus rhythm (where the atrial and ventricular rates are the same) and junctional rhythm In SVT the atrial rate is around 150, and inverted P waves usually follow the QRS complexes because of retrograde atrial conduction from the AV node. Regular atrial depolarizations at a rate of 300/min (as in this case) would exclude SVT.