Explain Mobitz Type 1 block.

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"Mobitz Type I" AND "atrioventricular block"

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Mobitz type I second degree AV block Merck Manual Wenckebach

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ECG Mobitz type I Wenckebach progressive PR prolongation dropped QRS

A 12-lead electrocardiogram (ECG) displayed on a standard grid (25 mm/s, 10 mm/mV) demonstrating second-degree atrioventricular (AV) block, specifically Mobitz Type I (Wenckebach phenomenon). The tracing includes leads I, II, III, aVR, aVL, aVF, and V1-V6, with a long rhythm strip of Lead II at the bottom. The hallmark diagnostic feature is the progressive prolongation of the PR interval in successive beats until a P wave is non-conducted (a 'dropped' QRS complex). Following the dropped beat, the PR interval resets to its shortest duration, and the cycle repeats. This finding is critical in cardiology for identifying supra-Hisian conduction delays, often associated with increased vagal tone or, in specific clinical contexts like early disseminated Lyme disease, inflammatory cardiac involvement (Lyme carditis). The ECG also shows normal QRS morphology and axis, consistent with conduction delay occurring at the level of the AV node.

Mobitz type I block, also called Wenckebach phenomenon, is a form of second-degree atrioventricular (AV) block.

What happens?

The AV node progressively delays conduction of atrial impulses to the ventricles until one impulse fails to conduct.

On ECG, this produces the repeating pattern:

PR interval gets progressively longer
A P wave occurs without a following QRS complex: a dropped ventricular beat
The next conducted beat has the shortest PR interval
The sequence repeats

Example:
PR 0.18 s → 0.22 s → 0.28 s → P wave with no QRS → PR resets to 0.18 s.

Mobitz type I ECG showing progressively prolonged PR intervals followed by a nonconducted P wave

Where is the block?

Usually at the AV node. The PR interval largely reflects AV nodal delay, so progressively worsening nodal conduction produces the characteristic ECG pattern. Goldman-Cecil Medicine, p. 559.

Common settings and causes

It can be a normal, transient finding, particularly with high vagal tone:

Sleep
Young healthy people and endurance athletes
Increased vagal stimulation
AV nodal blocking medicines: beta-blockers, verapamil/diltiazem, digoxin
Inferior myocardial ischemia/infarction
Myocarditis, including Lyme carditis

Clinical importance

Mobitz I is often benign, especially if the QRS complex is narrow and the person has no symptoms. It may cause no symptoms, or may cause fatigue, dizziness, presyncope, or syncope if the ventricular rate becomes too slow. A Merck Manual review notes that it often requires no treatment unless symptomatic bradycardia occurs.

Management in principle

Look for and correct a reversible cause, especially AV nodal blocking drugs or ischemia.
Observe if asymptomatic and stable.
If symptomatic bradycardia occurs, urgent clinical assessment is needed; atropine may be used in an acute setting.
Permanent pacing is usually not needed for asymptomatic AV nodal Mobitz I, but may be considered if block is symptomatic, persistent, or suspected to be below the AV node.

Distinguish it from Mobitz II

Feature	Mobitz I	Mobitz II
PR interval	Progressively lengthens	Constant before dropped beat
Usual site	AV node	His-Purkinje system
QRS	Usually narrow	Often broad
Risk of complete heart block	Lower	Higher
Pacemaker requirement	Usually no	Often yes

A simple mnemonic: “Longer, longer, longer, drop, then reset” = Mobitz I.

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