Narrow complex tachycardia

Narrow complex tachycardia
ECG shows rate of >100bpm and QRS complex duration of <120ms (<3 small squares on ECGs done at the standard UK rate of 25mm/s).

Differential diagnosis
Sinus tachycardia: normal P-wave followed by normal QRS.
Atrial tachyarrhythmias: Rhythm arises in atria, AV node is a bystander.
Atrial fibrillation (AF): absent P-wave, irregular QRS complexes.
Atrial flutter: atrial rate ~260-340bpm. Saw-tooth baseline, due to continuous atrial electrical activity. Ventricular rate often 150bpm (2 : 1 block).
Atrial tachycardia: abnormally shaped P-waves, may outnumber QRS.
Multifocal atrial tachycardia: 3 or more P-wave morphologies, irregular QRS complexes.
Junctional tachycardia: AV-node is part of the pathway. P-wave either buried in QRS complex or occurring after QRS complex.
AV nodal re-entry tachycardia.
AV re-entry tachycardia, includes an accessory pathway, eg WPW.