Mobitz type 1

The Mobitz type I (Wenkebach phenomenon) is characterised by the progressive lengthening of the PR interval with successive heart beats, culminating in a non-conducted atrial depolarisation. The next beat has a short PR interval and so the cycle continues.

The level of the block is generally at the level of the atrioventricular (AV) node, the QRS complexes are of normal duration and junctional automaticity, in general, is unimpaired.

In the healthy individual the Wenkebach phenomenon is often due to increased vagal tone and is abolished by exercise and atropine.

Note that can result from various other causes including (1):

• inferior myocardial ischaemia
• Lyme disease (cardiomyopathy)
• cardiac surgery)
• hyperkalemia
• medication toxicity e.g. beta-blockers and calcium channel blockers

Management

• seek expert advice
• treatment for a Mobitz type I (Wenckebach) is often not necessary (1)
• guidance suggests that for patients with Wenckebach, permanent pacing is only indicated if the AV block causes symptoms or if the conductions delay occurs below the bundle of His (2)

Reference:

1. Mangi MA, Jones WM, Mansour MK, et al. Second-Degree Atrioventricular Block. [Updated 2023 Aug 14]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan-.
2. Kusumoto FM et al. 2018 ACC/AHA/HRS Guideline on the Evaluation and Management of Patients With Bradycardia and Cardiac Conduction Delay: Executive Summary: A Report of the American College of Cardiology/American Heart Association Task Force on Clinical Practice Guidelines, and the Heart Rhythm Society. J Am Coll Cardiol. 2019 Aug 20;74(7):932-987.