A 58-year-old man with hypertension presents with palpitations and is found to have a regular narrow-complex tachycardia at exactly 150 bpm on ECG. The rhythm strip shows the characteristic pattern marked **A** in the diagram — regular, repetitive waves at approximately 300 bpm with no isoelectric baseline between them. Which of the following best describes the electrophysiological mechanism underlying this atrial rate?

Explanation

The sawtooth flutter waves marked **A** at ~300 bpm with no isoelectric baseline represent atrial flutter, which is caused by a macro-reentrant circuit typically involving the cavotricuspid isthmus in the right atrium. In typical (Type I) atrial flutter, counterclockwise rotation produces the characteristic negative sawtooth pattern visible in inferior leads (II, III, aVF). The atrial rate of ~300 bpm is pathognomonic for this mechanism. The ventricular rate of 150 bpm in this patient reflects 2:1 AV conduction, which is the most common presentation and should raise immediate suspicion for atrial flutter. This macro-reentrant mechanism is the fundamental electrophysiological basis of typical atrial flutter, as established in Harrison's Principles of Internal Medicine. The regular, organized nature of the atrial activity (unlike the irregular fibrillation in atrial fibrillation) is the hallmark of reentrant flutter.