Second-degree atrioventricular (AV) block Type I, also known as Mobitz Type I or Wenckebach block, is a conduction disturbance in which there is progressive prolongation of the PR interval on successive beats until an atrial impulse fails to conduct to the ventricles (a dropped QRS complex). After the dropped beat, the cycle resets with a shorter PR interval and the progressive prolongation begins again. The block typically occurs at the level of the AV node itself (supranodal), which has inherent decremental conduction properties meaning that the speed of impulse conduction decreases as the rate of stimulation increases. The AV node receives its blood supply primarily from the AV nodal artery, which arises from the right coronary artery (RCA) in 85-90% of patients. This is why Wenckebach block commonly occurs with inferior myocardial infarction (which involves RCA occlusion). Other causes include increased vagal tone (athletes, sleep), medications that slow AV conduction (beta-blockers, calcium channel blockers, digoxin), myocarditis, and post-cardiac surgery. Wenckebach block is generally considered a BENIGN rhythm that rarely progresses to complete heart block. Patients are usually asymptomatic or have...