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Figure 4 Activation mapping of ventricular tachycardia obtained during a transthoracic epicardial ablation procedure. A midiastolic signal found in the epicardial catheter distal bipole (Epi-d) precedes the onset of the QRS complex by 100 ms (upper panel). Application at this site interrupted VT after 1.4 s which could not be reinduced. Abbreviations as in Figure 3. b and 1 = distal par of electrodes; a = proximal par of electrodes.