Single-lead electrocardiogram (ECG) showing sinoatrial (SA) nodal reentrant tachycardia

Electrocardiogram showing SA nodal reentrant tachycardia. The first three beats are normal sinus beats at a rate of about 107 beats/min; the fourth beat is an atrial premature beat that is followed by a return to sinus rhythm. The eighth beat (arrow) represents the sudden onset of SA nodal reentrant tachycardia at a rate of about 145 beats/min. Since the P waves are similar to the sinus beats, the diagnosis is suggested only by the abrupt onset of the tachycardia.

Normal rhythm strip in lead II. The PR interval is 0.15 sec and the QRS duration is 0.08 sec. Both the P and T waves are upright.