2° AV Block – Mobitz I (Wenckebach) EKG

2° atrioventricular block – Mobitz I (often referred to as the Wenckebach phenomenon) is characterized by the progressive lengthening of the PR interval with each successive heartbeat until a beat is completely blocked (a P wave not followed by a QRS complex).

Mobitz I generally involves progressive fatigue of AV nodal cells until they fail to conduct an impulse (the dropped beat). This block can be caused by enhanced vagal tone, ischemia, medication effects (beta or calcium channel blockers), post-cardiac surgery, etc.

The treatment usually focuses on addressing the underlying problems (e.g., medication adjustments), but if Mobitz I is causing significant bradycardia or symptoms, a pacemaker is often the solution.

In the pictured EKG, the short red PR interval is followed by a longer blue PR interval and, ultimately, a dropped beat. Due to the cyclical pattern of prolongation and block, Mobitz I often has a grouped beating pattern. After a dropped beat, the PR interval resets to a shorter duration, and the elongation cycle begins anew.

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