Second-Degree AV Block Type I (Wenckebach)

Second-degree atrioventricular (AV) block Type I, also called Mobitz Type I or Wenckebach block, is an intermittent failure of conduction through the AV node. The atria continue to depolarize, but the AV node progressively delays conduction until one atrial impulse fails to reach the ventricles. That produces a dropped QRS complex. The AV node has decremental conduction: with repeated impulses arriving close together, conduction becomes progressively slower. Once one impulse is blocked, the node recovers, so the next conducted beat has a shorter PR interval and the sequence begins again. The pattern is therefore not random. It is a repeating cycle of increasing delay, one missed ventricular response, and reset. The usual clinical effect is a slower ventricular rate and reduced cardiac output. Many patients remain well, particularly when the block is related to sleep or high vagal tone. Symptoms become more likely when the ventricular rate is slow, the patient has limited cardiac reserve, or the block occurs during acute cardiac disease.