What characterizes an exit block during pulmonary vein pacing?

An exit block during pulmonary vein pacing is characterized by the absence of a signal on the coronary sinus (CS). In this scenario, pacing occurs within the pulmonary veins, and if there is an exit block, the electrical impulses generated cannot propagate beyond the pulmonary vein into the surrounding atrial tissue. This results in no signals being detected in the coronary sinus, which is typically where we would expect to see some electrical activity if the impulses were able to exit the pulmonary vein and reach the atrium.

This characteristic helps to differentiate exit block from other phenomena that may involve normal conduction pathways, where signals would still be present in the CS if conduction were functioning as expected. The absence of a signal indicates a problem with the impulse transmission, highlighting the diagnosis of an exit block.