A 58-year-old man has palpitations. ECG shows a perfectly regular narrow-complex tachycardia at 150 bpm with inverted sawtooth-shaped atrial waves best seen in leads II, III and aVF, with two atrial waves per QRS. What is the diagnosis?

• A Sinus tachycardia
• B Focal atrial tachycardia
• C AV nodal re-entrant tachycardia
• D Typical atrial flutter with 2:1 AV conduction ✓

Explanation

An atrial rate near 300 bpm with fixed 2:1 conduction gives a ventricular rate close to 150 bpm, a classic trap because the flutter waves may be hidden within QRS or T waves. The sawtooth pattern in the inferior leads identifies counterclockwise cavotricuspid isthmus-dependent flutter. AVNRT and focal atrial tachycardia do not produce continuous sawtooth atrial activity at 300 bpm, and sinus tachycardia cannot exceed roughly 220 minus age reliably at this regularity.

Reference: Goldberger's Clinical Electrocardiography, 9th ed.

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