Second-Degree Atrioventricular Block (AV Block) Lecture Notes

Overview

• AV Conduction Block: Delay between atria and ventricles, indicated by PR interval prolongation on ECG.
• Classification:
  • First-degree
  • Second-degree (focus of this lecture)
  • Third-degree
• Types of Second-Degree Block:
  • Mobitz Type I (Wenckebach)
  • Mobitz Type II

Objectives

• Identify etiology of second-degree AV blocks
• Explain identification on electrocardiogram (ECG)
• Discuss management strategies
• Highlight interprofessional team roles

Introduction

• Electrical impulse travels from sinoatrial node through atria, AV node, and His-Purkinje to ventricles.
• Second-Degree Block: Impulse delay leading to PR interval prolongation.
• Symptoms: May be asymptomatic, but can include syncope and lightheadedness.
• Mobitz Type II: Potential progression to complete heart block, can lead to death.

Etiology

• Mobitz Type I (Wenckebach):
  • Normal variant in high vagal tone individuals.
  • Causes: inferior myocardial ischemia, medication toxicity, hyperkalemia, cardiomyopathy (e.g. Lyme disease), cardiac surgery.
• Mobitz Type II:
  • Rare without structural heart disease.
  • Associated with myocardial ischemia, fibrosis, or sclerosis.
• Other Causes: Infiltrative heart disease, rheumatic fever, malignancies, thyroid disorders, etc.
• Genetic Links: Mutations in SCN5A gene.

Epidemiology

• Lack of large studies on prevalence.
• Seen in athletes and patients with congenital heart disorders.

Pathophysiology

• Mobitz Type I:
  • Progressive prolongation of PR interval until an atrial impulse is blocked, resulting in "dropped beat."
• Mobitz Type II:
  • Constant PR interval, with occasional non-conducted P wave.
  • Associated with widened QRS complex.

History and Physical

• Inquire about cardiac history, procedures, and medications.
• Mobitz Type II Symptoms: Fatigue, dyspnea, chest pain, syncope.
• Physical exam may reveal bradycardia.

Evaluation

• ECG: Key for diagnosis.
• Further Tests: Cardiac biomarkers, chest radiography, electrolyte levels.
• Electrophysiologic testing for block level and pacemaker need.

Treatment / Management

• Mobitz Type I:
  • Often no treatment needed.
  • Management of hypotension and bradycardia with atropine or pacing.
• Mobitz Type II:
  • Immediate pacing required.
  • Often progresses to complete block; requires permanent pacemaker.

Differential Diagnosis

• Congenital heart block, sinoatrial exit block, myocardial infarction, etc.

Prognosis

• Mobitz Type I: Excellent, usually asymptomatic.
• Mobitz Type II: Varies, may require pacemaker to prevent complications.

Complications

• Complete heart block, syncope, dizziness, chest pain, death.

Postoperative and Rehab Care

• Lifelong follow-up for AV block patients.

Consultations

• Cardiologist consultation recommended.

Pearls and Other Issues

• Wenckebach Rhyme: "Longer, longer, longer, DROP, now you have a Wenckebach."
• Differentiating 2:1 conduction ratio can be challenging; manage as type II when in doubt.

Enhancing Healthcare Team Outcomes

• Interprofessional approach involving cardiologist, ICU nurse, internist.
• Ensure no aggravating medications.
• Educate patient on both types of heart block.

References

• Extensive list of references provided for deeper exploration of the topic.