Exit Block

Pacing is delivered at high output (e.g., 10 mA at 2 ms) from the poles of the catheter situated inside the pulmonary vein. A lack of subsequent activation in the left atrium confirms complete circumferential isolation. Operators must be highly vigilant regarding "far-field capture." Due to the massive current (10 mA), the pacing stimulus can travel beyond the isolated vein and directly capture the left atrial appendage or the superior vena cava, giving the false illusion that the vein is still connected. Lowering the pacing output (e.g., to 5 V / 1 ms) often resolves this far-field capture, correctly confirming true exit block.