A second example of a non-sustained ectopic atrial tachycardia is shown here. This tracing is from a 72 year female. The first two and last two beats are sinus with a rate of 83/min. In between, there is a 15 beat run of a narrow complex tachycardia with a rate of 170/min. It starts and stops abruptly, without an initial acceleration or terminal deceleration and is initiated by an atrial premature beat. Each QRS complex of the tachycardia is preceded by a P wave that is located in the preceding T wave. The sudden onset and offset of this arrhythmia is in contrast to that seen on the previous page and suggests that reentry may be the underlying mechanism.