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Figure 4 Left atrial electroanatomic maps acquired during tachycardia in Patient 7, who had a left atriotomy. (A) Voltage map in the right lateral projection. An extensive region of low voltage is present in the posteroseptal wall adjacent to the right pulmonary veins. (B) Surface leads I, aVF and V1 and intracardiac recordings from the His bundle (HB) and coronary sinus (CS) during atrial tachycardia. Coronary sinus activation is earliest at the posterior mitral annulus (CS pole 5 to 6). (C and D) Isochronal maps in the right lateral and anteroposterior projections. Re-entry occurs around the posteroseptal lesion and right pulmonary veins, with a zone of slow conduction and low amplitude electrograms in the roof of the left atrium. The right superior pulmonary vein (RSPV), left superior pulmonary vein (LSPV) and left inferior pulmonary vein (LIPV) are identified. The wavefront bifurcates at the posterior aspect of the mitral annulus, which explains early CS activation in CS pole 5 to 6. MV = mitral valve.