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Basic Information ⬇

AUTHOR: Danylo Zorin, MD

Definition

First-degree atrioventricular (AV) block is defined as a PR interval exceeding 200 msec with preserved 1:1 AV conduction relationship. AV block is actually a misnomer because every atrial impulse is conducted to ventricles, and some refer to this finding as first-degree AV delay.1,2

Synonyms

First-degree heart block

Atrioventricular conduction delay

First-degree AV delay

ICD-10CM CODE
I44.0Atrioventricular block, first degree; heart block, first degree
Epidemiology & Demographics
Incidence

Based on studies conducted in healthy volunteers, the incidence varies between 5 and 10 in 1000.

Prevalence

Overall prevalence between 1% and 2% has been reported. It increases with age from ∼1% in young (20- to 30-yr-old) adults to ∼3% to 4% by age 60. In a cohort over age 60, prevalence is ∼6%.3

Predominant Sex & Age

More common in older male patients.

Risk Factors

Age (more common with advanced age), male sex (men on average have longer PR intervals), race (African Americans tend to have longer PR intervals), family history, and comorbid conditions (e.g., hypertension, diabetes) are major risk factors.

Genetics

Genome-wide association studies identified genes (e.g., SCN5A and more recently SCN10A, encoding voltage-gated sodium channel) that can determine PR interval duration.4

Physical Findings & Clinical Presentation

  • Clinical presentation of a first-degree AV block depends on the duration of PR interval. Insignificant prolongation has no hemodynamic consequences and hence is asymptomatic. Progressive prolongation causes decrease in intensity of the S1 heart sound and sometimes its complete disappearance. This is due to premature partial or complete closure of atrioventricular valves caused by reverse diastolic ventriculoatrial pressure gradient, especially with decreased ventricular compliance.
  • In some patients, diastolic mitral regurgitation can be demonstrated. Further lengthening of AV conduction time (particularly beyond 300 msec, termed marked first-degree AV block) causes “pseudopacemaker syndrome” when atria contract against closed atrioventricular valves due to encroachment of atrial systole on the previous ventricular systole. This leads to increased pressure in both atria, inferior vena cava, superior vena cava, and pulmonary veins, and loss of “atrial kick” with ensuing drop in end-diastolic volume, stroke volume, and cardiac output. Typical symptoms of the pseudopacemaker syndrome include malaise, poor exercise tolerance, dyspnea, orthopnea, cough, atypical chest discomfort, sensation of throat fullness, and occasionally presyncope or syncope.
  • Physical examination may reveal hypotension, rales, increased jugular venous pressure with cannon A waves, peripheral edema, and murmurs of tricuspid and/or mitral regurgitation.1,5
Etiology

Causes of AV block are numerous. They can be broadly divided into congenital and acquired. Acquired causes can in turn be broken down into infectious, inflammatory, degenerative, ischemic, vagotonic, metabolic, and iatrogenic. Congenital causes include conditions where a heart block is a part of a syndrome (such as myotonic dystrophy type 1, Emery-Dreifuss, limb-girdle type 1B) or a primary abnormality (as in familial Lev-Lenègre disease due to mutation of the SCN5A gene). A number of infections (Lyme disease, Chagas disease, toxoplasmosis), metabolic disorders (hyperkalemia, hypothyroidism), inflammatory conditions (systemic lupus erythematosus, rheumatoid arthritis, sarcoidosis), therapeutic interventions (beta-blockers, nondihydropyridine calcium channel blockers, valve surgeries, digoxin, etc.), toxins (CO, cyanide, mercury), degenerative processes (senile idiopathic fibrosis of the conduction system, aortic valve calcification), ischemia (acute myocardial infarction, chronic stable coronary artery disease), and increased vagal tone (in athletes or patients with sleep apnea) can be responsible for acquired AV block.1

Diagnosis ⬆ ⬇

Differential Diagnosis

Diagnosis of first-degree AV block is typically straightforward (see Definition) and made on a 12-lead electrocardiogram (ECG) or rhythm strip (Fig. E1). In rare cases of extreme AV delay, the P wave may be buried in preceding T wave or QRS complex, causing apparent AV dissociation.

Figure E1 76-Yr-Old Man with Marked First-Degree AV Conduction Delay on 12-Lead ECG (PR Interval = 393 Msec)

AV, Atrioventricular; ECG, Electrocardiogram.

Workup

The goal of workup is to establish the level of AV delay, its etiology, and its relationship to symptoms (if present). Workup is also indicated in asymptomatic patients who are at risk for progression to higher degrees of AV block (e.g., when certain hereditary conduction system disorders are suspected). It includes full history, physical examination, laboratory tests, and imaging studies.1,5

Laboratory Tests

Selection of a particular test(s) is dictated by preliminary diagnosis(es), considered most likely based on the history and physical findings. Examples include thyroid function studies, basic metabolic panel, Lyme serology, endocardial biopsy, and genetic testing.

Imaging Studies

  • Can be divided into three broad categories:
    1. Studies that help establish relationship between AV delay and symptoms (outpatient electrocardiographic monitoring, treadmill stress test)
    2. Studies that help diagnose underlying cause of AV conduction delay (cardiac magnetic resonance imaging, echocardiogram, cardiac computed tomography, left-sided heart catheterization)
    3. Studies that help determine a level of AV conduction delay (surface ECG and/or intracardiac electrogram recording during various pharmacologic and physiologic maneuvers). However, an electrophysiology study is not generally performed in the evaluation of AV conduction delay.

Treatment ⬆ ⬇

Treatment (Fig. E2) is indicated in patients with symptoms that correlate with the first-degree AV block or in patients without symptoms, but who are at risk for progression to higher degrees of AV block. The management of first-degree AV block is summarized in Table E1.

Figure E2 First-Degree Atrioventricular (AV) Block

AVR, Aortic Valve Replacement; MI, Myocardial Infarction; PM, Pacemaker.

!!flowchart!!

From Olshansky B et al: Arrhythmia essentials, ed 2, Philadelphia, 2017, Elsevier.

TABLE E1 First-Degree Atrioventricular Block Management

SettingTherapy
Asymptomatic
  • Usually benign and requires no therapy.
  • If due to drugs, no need to discontinue.
Symptomatic, PR very long (300-400 msec)
  • Because such symptoms as light-headedness and dizziness are nonspecific and common, especially in older persons, it is essential to exclude other causes before ascribing symptoms to first-degree AVB.
  • A rare indication for a DDD pacemaker. Shortening the PR interval to 150-200 msec may improve weakness, fatigue, and shortness of breath by restoring optimal AV synchrony. Consider His bundle pacing if the conduction prolongation is at the level of the AVN.
  • If patient is hospitalized and with refractory low cardiac output, consider temporary AV sequential pacing to improve hemodynamics and symptoms and to determine the benefit of permanent pacing.
MI
  • No therapy usually required (possible exception if the PR is very long).
  • May be due to drugs to treat MI (e.g., β-adrenergic blockers, calcium channel blockers, digoxin). If asymptomatic, no medication changes are usually required. If symptomatic or PR very long, consider reducing dosage.
  • Consider atrial infarction or elevated atrial pressures as the cause and treat appropriately.
Preoperative
  • No specific therapy.
  • No need for temporary prophylactic pacing as it does not presage higher degrees of AVB.
  • If due to a drug, decrease the dose or stop using the drug if it is not essential.
Postoperative
  • If PR is markedly prolonged (300-400 msec) and hemodynamic compromise is suspected, dual-chamber pacing (at PV or AV interval of 150-200 msec) may improve hemodynamics.
  • This may be instituted if temporary pacing wires are present after cardiac surgery.
  • If first-degree AVB appears for the first time after aortic valve surgery, it may indicate damage to the His-Purkinje system (usually associated with left bundle branch block in these cases).
  • Prolonged PR intervals can cause hemodynamic problems after cardiac surgery since the timing of the “atrial kick” may be more important during this time in which diastolic dysfunction is possible.
Endocarditis
  • Carefully evaluate for the presence of aortic insufficiency; consider TEE.
  • Acute development of first-degree AVB, especially if it occurs with a new bundle branch block (transient or persistent), is highly suspicious for the presence of a valve ring abscess.
  • Requires surgical treatment.
  • Consider TEE.
  • Transfer patient to a unit with cardiac telemetry (if not already there), as progression to higher levels of AVB can occur.
Rheumatic fever
  • First-degree AVB is a sign of rheumatic carditis and may presage the development of higher levels of AVB, including complete heart block.
  • May be an indication for steroids.
Infiltrative and restrictive cardiomyopathies
  • If PR is markedly prolonged (>300 msec), dual-chamber pacing (at PV or AV interval of 150-200 msec) may improve hemodynamics and symptoms.
  • Ventricles with severe diastolic dysfunction are especially dependent on atrial kick to maximize cardiac output.
  • If prolongation is at the level of the AV node, consider His bundle pacing.

AV, Atrioventricular; AVB, atrioventricular block; AVN, atrioventricular node; DDD, dual-chamber; MI, myocardial infarction; TEE, transesophageal echocardiography.

From Olshansky B et al: Arrhythmia essentials, ed 2, Philadelphia, 2017, Elsevier.

Nonpharmacologic Therapy

In asymptomatic patients without high risk of progression to advanced AV block, treatment includes observation, reassurance, and education. Cardiac pacing is a nonpharmacologic treatment modality for appropriately selected patients (see below).

Acute General Rx

Patients with symptoms due to reversible causes of AV conduction delay should receive appropriate treatment (e.g., correction of electrolyte abnormalities, antimicrobial therapy of infections associated with AV block, withdrawal of offending medications) of underlying conditions responsible for AV block.1,5

Chronic Rx

Permanent cardiac pacing may be indicated in patients with symptoms that correlate with the first-degree AV block and persist after correction of reversible causes (class IIA). Patients without symptoms but at risk for progression to higher degrees of AV block also benefit from permanent cardiac pacing (Lamin A/C mutations with PR >240 msec and left bundle branch block class IIA), certain neurodegenerative disorders (including myotonic dystrophy type 1) with PR >240 msec and QRS >120 msec, or fascicular block (class IIB).1

Disposition

AV conduction delay has been shown to be associated with increased risk of all-cause mortality, heart failure, and atrial fibrillation.2

Referral

Patients who are candidates for cardiac pacing should be referred to a cardiologist or cardiac electrophysiologist.

Pearls & Considerations ⬆ ⬇

Comments

  • First-degree AV block can be caused by slowing of electrical impulse conduction at any of the following levels: Atria, AV node (most often), His-Purkinje system.
  • Typically it is a benign condition (if caused by increased vagal tone) and does not require treatment except in patients with marked PR prolongation (≥300 msec) and associated symptoms (pseudopacemaker syndrome) or in asymptomatic individuals at high risk of progression to higher degrees of AV block.
Prevention

No specific preventive measures exist other than those recommended for prevention of underlying conditions.

Related Content ⬆

  1. Kusumoto FM et al: 2018 ACC/AHA/HRS Guideline on the evaluation and management of patients with bradycardia and cardiac conduction delay: a report of the American College of Cardiology/American Heart Association Task Force on clinical practice guidelines and the Heart Rhythm Society. Circulation 140(8):e382-e482, 2019.
  2. Cheng S. : Long-term outcomes in individuals with a prolonged PR interval or first-degree atrioventricular blockJAMA. ;301(24):2571-2577, 2009.
  3. Joyce C.K. : Variant intronic enhancer controls SCN10A-short expression and heart conductionCirculation. ;144:229-242, 2021.
  4. Shan R. : Prevalence and risk factors of atrioventricular block among 15 million Chinese health examination participants in 2018: a nation-wide cross-sectional studyBMC Cardiovasc Disord. ;21, 2021.doi:10.1186/s12872-021-02105-3
  5. Barold S.S. : First-degree atrioventricular block. Clinical manifestations, indications for pacing, pacemaker management and consequences during cardiac resynchronizationJ Interv Card Electrophysiol. ;17:139-152, 2006.