Mobitz I AV block, or Wenckebach block, is characterized by progressive prolongation of the PR interval causing progressive R-R interval shortening until a P wave fails to conduct to the ventricle.

Most patients are asymptomatic.
Patients may experience dizziness, lightheadedness, or syncope, but these are uncommon.
Patients may have symptoms of myocardial ischemia or myocarditis.
Patients may have a history of organic heart disease.

Patients may have an irregular heart rate, and often accompanying bradycardia, but the examination is otherwise generally unremarkable.
Symptomatic patients may have signs of hypoperfusion, including hypotension and altered mental status.
Patients with concomitant myocardial infarction (MI) may exhibit signs related to acute MI.

Mobitz I block can be caused by acute inferior MI, states of vagal stimulation or enhanced vagal tone, or toxicity relating to digitalis, beta-blockers, or calcium channel blockers.