Narrow complexes tachycardia/arrhythmia are from supraventricular origin (i.e. discharge causing it comes from foci above ventricle.
First see whether the rhythm is regular or not. Secondly see whether P is present or not. Then, if P is present, whether all the P are in the same morphology or not.
If rhythm is regular and all the P are the normal morphology, it is likely sinus rhythm.
P due to atrial foci has different morphology to sinus P wave.
If P is absent, rhythm is irregular and base linehas fibrillatory waves, it is atrial fibrillation, commonest persistent arrhythmia.
If P is absent and rhythm is regular, it is supraventricular tachycardia.
If P is present but have more than 3 different morphology, rate is not rapid, it is called wandering pacemaker. If rate is rapid, it is called multifocal atrial tachycardia. (In this case, baseline between P is flat, differentiating it from atrial fibrillation.)
