Cardiology · Arrhythmias
The facts most likely to be tested
Third degree AV block is defined by complete AV dissociation where atrial and ventricular activities are entirely independent.
The ECG hallmark is P waves and QRS complexes that march out at their own independent rates with no constant PR interval relationship.
Patients frequently present with syncope, dizziness, or hemodynamic instability due to a profound bradycardia and inadequate cardiac output.
The ventricular escape rhythm is typically wide complex if the block is located at the His-Purkinje system level, indicating a more unstable rhythm.
Atropine is generally ineffective for third degree AV block because the site of block is usually distal to the AV node.
The definitive treatment for symptomatic third degree AV block is the immediate placement of a permanent pacemaker.
Transcutaneous pacing serves as the necessary bridge therapy for patients who are hemodynamically unstable while awaiting permanent intervention.
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A 78-year-old male is brought to the emergency department after a witnessed syncopal episode at home. He reports a two-day history of lightheadedness and generalized fatigue. On physical examination, his pulse is 34 beats per minute and irregular, and his blood pressure is 88/50 mmHg. An ECG reveals P waves occurring at a rate of 80/min and QRS complexes occurring at a rate of 34/min, with no relationship between the two. The QRS complexes are wide.
What is the most appropriate next step in the management of this patient?
Transcutaneous pacing
The patient is hemodynamically unstable with complete heart block; therefore, immediate transcutaneous pacing is required as a bridge to permanent pacemaker placement.
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Etiology / Epidemiology
Caused by inferior MI (AV nodal ischemia) or fibrotic degeneration of the conduction system. Common in elderly patients with structural heart disease.
Clinical Manifestations
Presents with syncope, cannon a-waves, and bradycardia. Patients often exhibit hemodynamic instability.
Diagnosis
Confirmed via ECG showing AV dissociation where P-waves and QRS complexes are completely independent.
Treatment
Immediate transcutaneous pacing followed by permanent pacemaker implantation. Avoid beta-blockers.
Prognosis
High risk of asystole and sudden cardiac death. Requires permanent pacemaker for definitive management.
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Epidemiology & Etiology
Most commonly seen in elderly patients due to Lenègre-Lev disease or Lev disease (idiopathic fibrosis). Acute causes include inferior wall MI (due to RCA occlusion) and Lyme carditis. Iatrogenic causes include post-surgical complications following valve replacement.
Pertinent Anatomy
The block occurs at the AV node (narrow QRS) or the His-Purkinje system (wide QRS). Damage to the bundle of His or bilateral bundle branches results in total electrical isolation of the ventricles from the atria.
Pathophysiology
Complete failure of electrical conduction between atria and ventricles. The ventricles rely on a subsidiary escape rhythm which is typically slow and unreliable. This results in a fixed, low heart rate that cannot increase with exertion, leading to decreased cardiac output.
Clinical Manifestations
Patients present with syncope, dizziness, or Stokes-Adams attacks. Physical exam reveals cannon a-waves due to atrial contraction against a closed tricuspid valve. Red flags include hypotension, altered mental status, and pulmonary edema.
Diagnosis
The ECG is the gold standard, demonstrating AV dissociation where the atrial rate is faster than the ventricular rate. P-waves and QRS complexes show no relationship, with a constant PP interval and constant RR interval.
Treatment
Initial stabilization requires atropine (often ineffective in high-grade blocks) followed by transcutaneous pacing. If unstable, proceed to transvenous pacing. The definitive treatment is a permanent pacemaker for all symptomatic patients. Avoid AV nodal blocking agents like beta-blockers or calcium channel blockers.
Prognosis
High risk of ventricular standstill and sudden cardiac death. Patients require permanent pacemaker placement to prevent recurrent syncope and mortality. Long-term monitoring is essential to ensure lead integrity.
Differential Diagnosis
Second-degree Type II: intermittent rather than complete conduction failure
Second-degree Type I: progressive PR prolongation before a dropped beat
Sinus bradycardia: P-waves are always associated with QRS complexes
Junctional rhythm: narrow QRS with retrograde P-waves
Ventricular tachycardia: wide QRS complexes with AV dissociation