Cardiology · Arrhythmias
The facts most likely to be tested
The hallmark ECG finding is a progressive prolongation of the PR interval until a dropped QRS complex occurs.
The RR interval progressively shortens prior to the non-conducted P wave due to the incremental decrease in the increment of PR prolongation.
The site of the conduction delay is typically located at the AV node, which is generally considered a benign and stable location.
Asymptomatic patients with Mobitz I require no specific treatment and should be managed with observation.
Symptomatic patients, such as those with syncope or hemodynamic instability, are treated with atropine or transcutaneous pacing.
Mobitz I is frequently associated with increased vagal tone, inferior wall myocardial infarction, or the use of AV nodal blocking agents like beta-blockers or calcium channel blockers.
The grouped beating pattern on an ECG strip is the classic visual representation of the Wenckebach phenomenon.
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A 68-year-old male presents to the emergency department for a routine follow-up of his hypertension. He denies chest pain, palpitations, or syncope. His current medications include metoprolol and lisinopril. An ECG is performed, revealing a progressive lengthening of the PR interval followed by a non-conducted P wave and a dropped QRS complex. The heart rate is 58 bpm and the patient is hemodynamically stable.
What is the most appropriate management for this patient?
Observation
The patient exhibits classic Mobitz I (Wenckebach) block, which is typically benign and located at the AV node; because the patient is asymptomatic and stable, no intervention is required.
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Etiology / Epidemiology
Common in athletes and patients with increased vagal tone or inferior wall MI.
Clinical Manifestations
Characterized by progressive PR interval lengthening followed by a dropped QRS complex.
Diagnosis
Diagnosed via 12-lead ECG showing the classic Wenckebach phenomenon.
Treatment
Usually asymptomatic and requires no treatment; if symptomatic, use atropine.
Prognosis
Generally benign; progression to higher-degree block is rare.
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Epidemiology & Etiology
Often seen in healthy individuals with high vagal tone or during sleep. Pathological causes include inferior wall myocardial infarction, myocarditis, or electrolyte disturbances. It is frequently associated with AV nodal ischemia.
Pertinent Anatomy
The block occurs at the level of the AV node. This location is typically supplied by the right coronary artery.
Pathophysiology
The AV node exhibits decremental conduction, where each successive impulse takes longer to traverse the node. Eventually, the refractory period is exceeded, resulting in a non-conducted P wave. The cycle then resets, creating the classic grouped beating pattern.
Clinical Manifestations
Patients are typically asymptomatic. If symptomatic, they may report dizziness or syncope. Red flags include hemodynamic instability or signs of hypoperfusion.
Diagnosis
The 12-lead ECG is the diagnostic standard. Look for progressive PR interval lengthening until a dropped QRS complex occurs. The R-R interval shortens prior to the pause.
Treatment
Observation is the standard for asymptomatic patients. If the patient is symptomatic, administer atropine. Avoid beta-blockers and calcium channel blockers that further suppress AV nodal conduction.
Prognosis
The prognosis is excellent as it is usually a transient condition. Permanent pacemaker placement is rarely indicated unless the patient remains symptomatic despite medical management.
Differential Diagnosis
Mobitz II: constant PR interval with sudden dropped beats
Third-degree block: complete AV dissociation
Sinus arrhythmia: respiratory-related heart rate variation
First-degree block: fixed PR interval prolongation
Non-conducted PAC: premature P wave with no QRS