Question mentions - H/o smoker first thing that comes in mind is COPD & MC arrhythmia atrial fibrillation. We first see lead II on ECG RR approx. 1.5 and more towards left 2.5. HR=300/1.5=200/min Therefore, Patient suffers from tachycardia with variable RR interval. D/D Atrial fibrillation Multifocal atrial tachycardia P wave absent P wave present Variable RR interval Therefore ruled out P wave varying amplitual and answer is MAT. Trick to solve: First check if patient has tachycardia/bradycardia RR interval Narrow QRS suggests supraventricular tachycardia COPD is the reason for multifocal atrial tachycardia due to right ventricular strain causing RV enlargement causing right axis duration. Visible P wave variable amplitude RR interval changing Narrow QRS All point towards diagnosis of multifocal atrial tachycardia. Monomorphic VT is ruled out as it has broad QRS complex. Between MAT & AV nodal reentrant tachycardia the H/o smokers narrows down the diagnosis to MAT.