A three dimensional electroanatomic mapping system (CARTO, Biosense Webster) can visualise the activation sequence during normal or abnormal rhythm, and has recently been used as a guide for catheter ablation for some tachyarrhythmias. Activation time at each site is measured from a bipolar (or a unipolar) electrogram and is determined as the interval between the QRS onset and the time minimum of the first derivative (V_min) of the QRS deflection. On the other …