In PV isolation, exit block is defined as which?

• A. Pacing from the left atrium fails to conduct into the pulmonary vein.
• B. There is no conduction from the left atrium into the pulmonary vein.
• C. There is absence of conduction from the pulmonary vein to the left atrium when pacing from within the PV.
• D. The pulmonary vein shows complete bidirectional block across the ostium.

Answer: (C)

In PV isolation, the test called exit block checks whether the pulmonary vein can drive the left atrium. If you pace from inside the vein and there is no activation seen in the left atrium, that means the vein cannot conduct impulses to the atrium—exit block is present. This specific direction—PV to LA conduction during PV pacing—is what defines exit block.

This is different from entrance block, which would be testing conduction from the left atrium into the pulmonary vein (pacing from the atrium and looking for PV activation). Bidirectional block would mean both PV to LA and LA to PV conduction are blocked, which is a more stringent assessment than exit block alone.

So the correct concept is the absence of conduction from the pulmonary vein to the left atrium when pacing from within the PV, indicating exit block.