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NAVHS operation. In the upper panel a single beat AV conduction search (AV delay extension to 300 ms) is applied and intrinsic conduction time measured (upper right): (PR interval 220 ms + QRV interval 57 ms = 277 ms). In the lower panel Negative AV hysteresis (Neg-HYS) is commenced after the first 3 beats of intrinsic rhythm. The AV delay is then shortened by the 80 ms delta, from 277 to 197 ms, resulting in fusion and QRS narrowing from 153 to 106 ms).
