This is a regular narrow-complex tachycardia at 130/minute. The differential of this rhythm includes:
1. sinus tachycardia
3. 2:1 flutter
The diagnosis depends on the P wave or atrial activity. Remember Marriott likes to say “cherchez le P.” In this tracing there is something midway between QRS complexes, seen best in the inferior leads, especially lead II. This is a flutter wave and the rhythm is 2:1 flutter! Notice the broad “S wave” at the end of QRS complexes in the inferior leads – actually this is a part of the other flutter wave (occurring at a rate of approximately 300/minute) superimposed on the QRS.