2nd degree AV Block, Mobitz type 1 (Wenckebach phenomenon)
This is defined by progressive prolongation of the PR interval culminating in a non-conducted P wave. The PR interval is longest immediately before the dropped beat and the PR interval is shortest immediately after the dropped beat. The P-P interval remains relatively constant.
Mobitz type I AV block is usually due to reversible conduction block at the level of the AV node. The malfunctioning AV node cells tend to progressively fatigue until they fail to conduct an impulse. This is different to cells of the His-Purkinje system which tend to fail suddenly and unexpectedly i.e. producing a Mobitz II block.
Mobitz type I AV block is usually a benign rhythm, causing minimal haemodynamic disturbance and with low risk of progression to 3rd degree heart block. Asymptomatic patients do not require treatment. Symptomatic patients usually experience light-headedness and respond to atropine. Permanent pacing is rarely required.