A 62-year-old male 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, demonstrating

(c) Source: (Braunwald, 15/e, p 1297.) The rhythm strip in the question 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 due to the flutter waves. In the 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).