Second-degree atrioventricular (AV) block refers to delayed electrical conduction from the atria to the ventricles that results in periodically nonconducted atrial beats. This can occur in a regular pattern (eg, in a 2:1 or 3:2 ratio of atrial beats to ventricular beats).

Second-degree AV block has 2 subtypes, which are referred to as Mobitz type I ("Wenckebach") and Mobitz type II. Type I block is characterized by a progressively increasing PR interval with consecutive beats culminating in a nonconducted beat, after which the PR interval returns to baseline and the pattern repeats. Mobitz type II has a constant PR interval (whether normal or prolonged) with periodic nonconducted P waves.

Potential underlying causes for second-degree AV block include myocardial ischemia, amyloidosis, sarcoidosis, endocarditis, Lyme carditis, hyperkalemia, AV blocking medications (beta blockers, nondihydropyridine calcium channel blockers, digoxin, and some anti-arrhythmic drugs), increased vagal tone, iatrogenic injury during cardiac surgery, percutaneous coronary intervention, and transcatheter aortic valve replacement.

Clinical presentation is variable. Asymptomatic presentation is common with Mobitz type I. Mobitz type II is more often associated with palpitations, fatigue, light-headedness, presyncope, or syncope. Patients with Mobitz type II block may also progress to complete heart block, sometimes intermittently, and have an increased mortality risk.

Related topics: first-degree atrioventricular block, complete atrioventricular block