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The present popularity of catheter ablation for atrial fibrillation has generated interest in the pathophysiology of arrhythmias originating from the left atrium. An improved understanding of ECG manifestations of reentrant left atrial arrhythmias has obvious clinical implications. An ideal goal would be to achieve diagnostic precision similar to the ‘sawtooth’ ECG morphology of typical atrial flutter.Recent studies have shown that nonreentrant arrhythmias originating from the left atrium are characterized by specific ECG features of the P wave, the most consistent being a positive or a dominantly positive deflection in V1. Similarly, bystander activation of the right atrium generates a dominantly positive deflection in V1 even during left atrial reentry. Although continuous reentrant activation should generate continuous electrical activity on surface ECG, this may not be the case if part of the reentry circuit generates feeble electrical forces, for example, a thin strand of myocardium. As a result, discrete P waves separated by a 12-lead synchronous isoelectric interval may be produced. Their morphology can be used to infer the location of the exit from this slow-conducting isthmus and facilitate ablation.The progressive accumulation of accurate activation mapping data of the full spectrum of left atrial reentry will certainly allow the development of clinically useful ECG algorithms.