Mobitz type I, also called Wenckebach phenomenon, is a second degree atrioventricular (AV) block characterized by progressive lengthening of the PR interval until a beat is dropped. This pattern typically reflects transient AV node slowing rather than fixed conduction disease.
Below is a structured overview of key clinical and physiologic attributes to help clinicians and learners quickly compare features of Mobitz type I.
| Parameter | Mobitz Type I (Wenckebach) | Mobitz Type II | Third Degree (Complete) Block |
|---|---|---|---|
| PR interval behavior | Progressively lengthens until a dropped beat | Constant PR interval before dropped beat | No consistent relationship between P waves and QRS |
| Block location | Usually at the AV node | Often infra-Hisian | At or below the AV node, or surgical interruption |
| Symptom severity | Often asymptomatic or mild lightheadedness | More likely to cause syncope | Fatigue, near syncope, or heart failure symptoms |
| Likelihood of progression | Low risk of progression to third degree | Higher risk of complete heart block | Permanent; may require pacing |
Electrophysiology and Mechanism of Mobitz Type I
At the cellular level, Mobitz type I arises from decremental conduction within the AV node. Repetitive collisions between impulses and partially refractory tissue produce the classic Wenckebach sequence.
Recovery of excitability occurs progressively, so each succeeding conducted beat encounters a longer delay. When the nodal refractory period finally exceeds the arriving impulse, one P wave fails to conduct, resetting the cycle.
This phenomenon is often physiologic and enhanced by increased vagal tone, drugs, or acute conditions such as inferior myocardial infarction or myocarditis.
Clinical Presentation and Physical Examination
Patients with Mobitz type I may be entirely asymptomatic, with the block discovered incidentally on a routine ECG. Others report mild lightheadedness or a sensation of skipped beats, especially when the pause is prolonged.
Physical examination findings depend on the ratio of conducted to nonconducted P waves. Auscultation may reveal a transiently absent or diminished heart sound during the blocked beat, with no associated cannon A waves in the neck veins.
Diagnostic Evaluation and ECG Criteria
Diagnosis hinges on ECG documentation of the characteristic Wenckebach pattern. Typical criteria include progressive PR interval prolongation culminating in a nonconducted P wave, with the cycle repeating thereafter.
Additional measures such as telemetry or Holter monitoring can unmask intermittent Wenckebach, particularly when symptoms are provoked by activity or medication. Imaging and labs are directed at identifying reversible precipitants rather than confirming the block itself.
Management, Treatment, and Prognosis
In stable patients without adverse features, treatment focuses on reversible contributors, such as optimizing medications, correcting electrolyte abnormalities, and addressing underlying cardiopathy.
Temporary pacing is reserved for cases with significant symptoms, hemodynamic compromise, or high grade conduction disease suggestive of progression toward third degree block. Permanent pacing is seldom required for isolated Mobitz type I.
FAQ
Reader questions
Can Mobitz type I occur during a heart attack, and how does that affect management?
Yes, it commonly occurs with inferior myocardial infarction due to increased vagal tone and transient ischemia; management emphasizes atropine, pacing if symptomatic, and revascularization rather than long term pacing.
How does Mobitz type I differ from Mobitz type II on ECG?
Mobitz type I shows progressive PR lengthening with a dropped beat, whereas Mobitz type II has a constant PR interval before sudden nonconduction, often indicating infra-Hisian disease and higher risk of complete block.
Does Mobitz type I always need a pacemaker?
Not usually; isolated Wenckebach is frequently benign and managed by addressing reversible causes, medications, and monitoring, with pacing reserved for symptomatic patients or unstable rhythms.
Can medications like beta blockers or calcium channel blockers worsen Mobitz type I?
Yes, AV nodal blocking agents can exaggerate decremental conduction and provoke or deepen Wenckebach periods, particularly in susceptible individuals or when combined with vagal influences.