This 67-year-old woman with history of mild stable angina for the past three years is in the Emergency Dept. with palpitation of two hours duration, associated with moderate dyspnea and mild retrosternal discomfort:

• Supraventricular tachycardia, 178/min
• Tachycardia terminates at end of ECG

Comment: Negative P waves are seen in the inferior leads, preceding the QRS (likely retrograde). The arrhythmia terminates with a P wave (very unlikely to be atrial tachycardia). The termination of the arrhythmia with a P wave favors the diagnosis of AV node-dependent tachycardia rather than atrial tachycardia. Atrial tachycardia ends when the ectopic atrial rhythm stops. Therefore, atrial tachycardia terminates with a QRS, not with a P wave. Atypical AVNRT is the most likely diagnosis (anterograde conduction through the fast nodal pathway, retrograde conduction through the slow pathway).