What is first-degree atrioventricular (AV) block?

What is First-Degree Heart Block?

First-degree atrioventricular (AV) block is a delay in electrical conduction from the atria to the ventricles, defined by a PR interval exceeding 0.20 seconds (200 ms) on electrocardiogram, where every atrial impulse is conducted to the ventricles but with abnormal prolongation. 1

Fundamental Characteristics

• First-degree AV block represents a conduction delay rather than a true "block" since all atrial impulses ultimately reach the ventricles, leading some experts to prefer the term "first-degree AV delay." 2

• The delay typically occurs within the AV node when the QRS complex is narrow (<120 ms), but may involve the His-Purkinje system when the QRS is wide (≥120 ms). 3, 2

• His-bundle electrography is required to precisely localize the site of delay in patients with wide QRS complexes. 3

Electrocardiographic Definition

• The diagnosis requires a PR interval greater than 200 ms with 1:1 AV conduction (every P wave followed by a QRS complex). 1

• The PR interval remains constant from beat to beat, distinguishing it from second-degree Mobitz Type I (Wenckebach) block where the PR progressively lengthens. 3

Clinical Significance Stratified by PR Duration

PR Interval 200–300 ms

• Generally asymptomatic and considered benign in most patients. 1, 4
• No treatment or pacemaker implantation is indicated for asymptomatic patients in this range. 1, 5

PR Interval ≥300 ms (Profound First-Degree AV Block)

• Can produce symptoms resembling "pacemaker syndrome" including fatigue, exercise intolerance, dizziness, and dyspnea due to loss of optimal AV synchrony. 1, 2, 4

• The mechanism involves atrial contraction occurring too close to the preceding ventricular systole, resulting in inadequate ventricular filling, elevated pulmonary capillary wedge pressure, and reduced cardiac output. 1

• Permanent pacemaker implantation is reasonable (Class IIa recommendation) when this degree of PR prolongation causes demonstrable hemodynamic compromise or pacemaker syndrome-like symptoms. 1, 5

Common Etiologies

• Medications are the most frequent reversible cause, particularly AV-nodal blocking agents (beta-blockers, non-dihydropyridine calcium channel blockers, digoxin) and antiarrhythmic drugs. 5, 6

• Structural heart disease including ischemic heart disease (especially inferior myocardial infarction), calcific valvular disease, and cardiomyopathies. 5, 6

• Infectious and infiltrative diseases such as Lyme disease, myocarditis, sarcoidosis, and amyloidosis. 5, 6

• Electrolyte disturbances, particularly hyperkalemia and hypomagnesemia. 5, 6

• Congenital heart disease including repaired tetralogy of Fallot, ventricular septal defects, and congenitally corrected transposition of the great arteries. 5

• Physiologic variation in highly trained athletes and during sleep. 1

Important Clinical Pitfalls

• Do not assume first-degree AV block is always benign: Continuous monitoring studies reveal that approximately 40% of patients with apparent isolated first-degree AV block have intermittent higher-grade block on extended monitoring. 5

• Intra-atrial conduction delay can masquerade as first-degree AV block: In patients with atrial fibrillation or atrial flutter, up to 63% of cases with prolonged PR intervals are due to delayed right intra-atrial conduction rather than AV nodal delay, which has implications for medication selection. 7

• Exercise testing is essential when PR ≥300 ms: The PR interval should shorten appropriately with exertion in benign cases; failure to shorten or progression to second-degree block during exercise indicates His-Purkinje disease requiring further evaluation. 5

• Neuromuscular diseases confer unpredictable risk: Patients with myotonic dystrophy, Kearns-Sayre syndrome, Emery-Dreifuss muscular dystrophy, or peroneal muscular atrophy may experience sudden progression to high-grade AV block even with only first-degree block at baseline. 1, 5

Prognostic Considerations

• Isolated first-degree AV block with PR <300 ms generally has an excellent prognosis in the absence of structural heart disease. 5, 4

• Recent observational data challenge the "benign" label: In patients with stable coronary artery disease, first-degree AV block is independently associated with increased risk of heart failure hospitalization (HR 2.33) and cardiovascular mortality (HR 2.33), even after adjustment for multiple confounders. 8

• The adverse prognostic association may reflect more advanced underlying cardiac disease, left ventricular systolic and diastolic dysfunction, or suboptimal AV synchrony during stress. 8

When Permanent Pacing is NOT Indicated (Class III)

• Asymptomatic patients with PR interval <300 ms and no structural heart disease. 1, 5

• First-degree AV block due to reversible causes (medications, electrolyte abnormalities, acute myocardial infarction) that resolve with treatment. 1, 5

• Isolated first-degree AV block without symptoms, as there is little evidence that pacing improves survival. 1, 4