Atrioventricular Nodal-His Block

KEY FEATURES

ESSENTIALS OF DIAGNOSIS

• First degree: prolonged PR interval > 0.20 second

• Second degree, type I (Mobitz): progressive increase in PR interval, then failure of atrioventricular (AV) conduction and absent QRS complex

• Second degree, type II (Mobitz): abrupt failure of AV conduction without prior increase in PR intervals

• High grade: AV conduction ratio > 3:1

• Complete or third degree: independent atrial and ventricular rhythms, with failure of AV conduction despite temporal opportunity for it to occur

GENERAL CONSIDERATIONS

• Common causes: degenerative process, ischemia, calcific aortic valve disease, AV node ablative procedures, medications, infections (aortic valve endocarditis), aortic valve surgery, and infiltrative diseases like amyloidosis

• Escape rhythm originating from cells of the atrionodal area has a faster depolarization rate (45–60/min) and responds to autonomic modulation

• Escape rhythm from the nodal-His area has a slower rate (about 40 bpm) and generally does not respond to autonomic influence

CLINICAL PRESENTATION

SYMPTOMS AND SIGNS

• Depends on degree of AV block but can be as extreme as syncope, lightheadedness, confusion

• Effort intolerance and exercise-related shortness of breath

• Rarely, bradycardia-mediated prolongation of QT interval may precipitate polymorphic ventricular tachycardia and cardiac arrest

PHYSICAL EXAM FINDINGS

• Clinical signs of complete AV block: cannon a waves in the jugular venous pulse and variable S₁ intensity

• Significant increase of systolic and pulse blood pressure due to large stroke volume secondary to bradycardia

• Rales in the chest and palpable liver if venous pressures are elevated due to bradycardia

DIFFERENTIAL DIAGNOSIS

• Causes of Mobitz I and first-degree AV block: increased vagal tone; drugs that prolong AV conduction such as beta blockers, digoxin, and calcium channel blockers

• Causes of Mobitz II and third-degree AV block: degenerative conduction system disease (Lev’s disease and Lenègre’s syndrome)

• Acute myocardial infarction: inferior myocardial infarction causes complete heart block at the AV node; anterior myocardial infarction causes heart block distal to it

• Other causes: acute myocarditis (viral, Lyme disease), digoxin toxicity, congenital heart blocks

DIAGNOSTIC EVALUATION

LABORATORY TESTS

• Electrolyte measurement, particularly for hyperkalemia

• Thyroid-stimulating hormone

ELECTROCARDIOGRAPHY

• ECG with rhythm strip to document rhythm

• Ambulatory cardiac monitoring if ECG unremarkable

• Event recorder to record infrequent events

• Implantable loop recorder if symptoms are rare but disabling

IMAGING STUDIES

• Echocardiogram in those with features of heart failure, aortic valve disease, and suspected infiltrative diseases

• Noninvasive ischemic evaluation, if appropriate

DIAGNOSTIC PROCEDURES

• Electrophysiologic study only if diagnostic uncertainty exists and symptoms are severe

TREATMENT

CARDIOLOGY REFERRAL

• Symptoms related to bradycardia

• Unexplained ...