Hi, Hasan,

To answer your question in the simplest terms, atrial tachycardia is a narrow-complex rhythm that is faster than 100 bpm, but usually between 140 and 240 bpm. The P waves are slightly different than the patient's sinus P waves, although they may be impossible to see, as they often fall in the preceding T waves. Those T waves will appear taller than the sinus T waves due to the addition of the P.  The QRS will be narrow unless a separate interventricular conduction delay, like bundle branch block, occurs. Depending on the origin of the rhythm, it may be very regular with a sudden onset and offset or regular to slightly irregular with a more gradual onset  

If you would like to delve deeper into the various forms of supraventricular tachycardia, search these terms: atrial tachycardia, multifocal atrial tachycardia, AVNRT, AVRT, Reentry, and paroxysmal supraventricular tachycardia. It is a very fascinating subject  

Hope this helps  

Dawn