A 58-year-old man with hypertension and chronic obstructive pulmonary disease presents with palpitations and dyspnea. His ECG shows a regular narrow-complex tachycardia at 150 bpm. The rhythm strip reveals the pattern marked **A** — regular atrial activity at 300 bpm with a characteristic sawtooth appearance in leads II, III, and aVF, and a 2:1 AV block. Which of the following best describes the underlying mechanism of this arrhythmia?

Explanation

## Why Option 1 is correct The sawtooth F-waves at 300 bpm with 2:1 AV block producing a regular ventricular rate of 150 bpm is pathognomonic for **typical (cavotricuspid-isthmus-dependent) atrial flutter**. This arrhythmia is a macro-reentrant circuit confined to the right atrium that obligately circulates around the tricuspid annulus with slow conduction through the cavotricuspid isthmus (CTI) — the anatomic bottleneck between the inferior vena cava and tricuspid annulus. The counterclockwise circuit (90% of cases) produces the characteristic negative (downward) sawtooth F-waves in inferior leads (II, III, aVF) seen in this patient. The regular atrial rate of 240–340 bpm (typically 300 bpm) and the 2:1 AV block are hallmark features. A regular narrow-complex tachycardia at 150 bpm should **always** prompt consideration of atrial flutter with 2:1 block (Harrison 21e Ch 247; Braunwald 12e Ch 38). ## Why each distractor is wrong - **Option 2 (Pulmonary vein ectopy)**: This describes atrial fibrillation or focal atrial tachycardia, which produce irregular atrial activity and irregular ventricular rates. The sawtooth pattern and regular 300 bpm atrial rate are incompatible with pulmonary vein ectopy. - **Option 3 (Disorganized atrial activity)**: This describes atrial fibrillation, which shows irregular fibrillatory waves with no organized atrial rate and an irregularly irregular ventricular response. The organized sawtooth pattern at 300 bpm rules out AF. - **Option 4 (Sinus tachycardia with Wenckebach)**: Sinus tachycardia produces a normal P-wave morphology, not sawtooth F-waves. Wenckebach block is progressive PR prolongation with occasional dropped beats, not a fixed 2:1 ratio. The atrial rate in sinus tachycardia would not exceed 150 bpm in a resting patient. **High-Yield:** Regular narrow-complex tachycardia at 150 bpm = atrial flutter with 2:1 block until proven otherwise; sawtooth F-waves in inferior leads confirm cavotricuspid-isthmus-dependent typical flutter; carotid massage or IV adenosine unmasks the flutter waves diagnostically. [cite: Harrison 21e Ch 247; Braunwald Heart Disease 12e Ch 38]