What are the implications of a first-degree atrioventricular (AV) block?

From the Guidelines

First-degree atrioventricular (AV) block is generally a benign condition that requires no specific treatment for most patients, especially if they are asymptomatic, but may lead to symptoms of fatigue or exertional intolerance if the PR interval is long enough to cause loss of atrioventricular synchrony. This condition is characterized by delayed conduction through the AV node, resulting in a prolonged PR interval (greater than 200 milliseconds) on an electrocardiogram (ECG) 1. The implications of first-degree AV block vary and depend largely on the degree of atrioventricular block, the ventricular rate, and the frequency of its occurrence.

Key Considerations

• First-degree AV block is often an incidental finding during routine ECG and may be caused by increased vagal tone, medications (such as beta-blockers, calcium channel blockers, or digoxin), electrolyte abnormalities, or underlying heart disease 1.
• If the patient is taking medications that slow AV conduction, the physician may consider adjusting the dosage or switching medications if clinically appropriate.
• For symptomatic patients, which is rare in first-degree AV block, addressing the underlying cause is the primary approach.
• The condition itself has an excellent prognosis and rarely progresses to more severe forms of heart block, though patients with significantly prolonged PR intervals (greater than 300 milliseconds) may require closer monitoring 1.
• Lifestyle modifications are generally not necessary unless there is underlying heart disease.

Management Approach

• Regular cardiac follow-up with a primary care physician or cardiologist is recommended to monitor for progression to higher-degree AV blocks.
• Cardiac pacing is not recommended unless the PR interval fails to adapt to heart rate during exercise and is long enough (usually >300 ms) to cause symptoms because of inadequate LV filling, or an increase in wedge pressure, as the left atrial systole occurs close to or simultaneous with the previous LV systole 1.
• In such cases, small, uncontrolled studies have shown an improvement in patients’ symptoms 1.

From the Research

Implications of First-Degree Atrioventricular (AV) Block

The implications of first-degree AV block are multifaceted and can have significant effects on patients. Some key points to consider include:

• First-degree AV block is defined as a prolongation of the PR interval beyond the upper limit of what is considered normal (generally 0.20 s) 2.
• Historically, first-degree AV block was considered an entirely benign condition, but recent reports suggest that it may be a risk marker for more severe intermittent conduction disease and is not benign in all patients 3.
• Extreme forms of first-degree AV block (typically a PR interval exceeding 0.30 s) can cause symptoms due to inadequate timing of atrial and ventricular contractions, similar to the so-called pacemaker syndrome 2.
• Permanent pacemaker implantation is reasonable for first-degree AV block with symptoms similar to those of pacemaker syndrome or with hemodynamic compromise, but there is little evidence to suggest that pacemakers improve survival in patients with isolated first-degree AV block 2.
• Insertable cardiac monitors have revealed that some patients with first-degree AV block may progress to higher grade block or bradycardia requiring pacemaker implantation 3.

Clinical Manifestations and Indications for Pacing

Clinical manifestations of first-degree AV block can include:

• Symptoms similar to those of pacemaker syndrome, such as fatigue, shortness of breath, and dizziness 2.
• Hemodynamic compromise, which may require permanent pacemaker implantation 2.
• Marked first-degree AV block (PR > 0.30 s) can produce a clinical condition similar to that of the pacemaker syndrome, and may require conventional dual chamber pacing 4.
• Patients with suboptimally programmed pacemakers may develop functional atrial undersensing due to the P wave migrating into the postventricular atrial refractory period (PVARP) 4.

Outcomes and Prognosis

Outcomes for patients with first-degree AV block can vary depending on the severity of the condition and the presence of other underlying heart disease. Some key points to consider include:

• Patients with first-degree AV block may have a poorer outcome with cardiac resynchronization therapy (CRT) than patients with a normal PR interval 4.
• First-degree AV block during CRT predisposes to loss of ventricular resynchronization during biventricular pacing, which can lead to a poorer hemodynamic response 4.
• Asymptomatic first-degree AV block may rarely progress to transient Wenckebach (type 1 second degree) block, and in some cases, may require temporary pacing for elective general anesthesia 5.