From the Guidelines
First-degree atrioventricular (AV) block in a symptomatic patient typically requires careful evaluation and management, though the block itself is rarely the direct cause of symptoms. Management begins with identifying and treating the underlying cause, such as discontinuing medications that prolong AV conduction (beta-blockers, calcium channel blockers, digoxin, or certain antiarrhythmics) 1. If the patient has symptoms like dizziness, syncope, or exercise intolerance, and these are determined to be related to the conduction delay, a cardiology consultation is warranted. Some key points to consider in the management of first-degree AV block include:
- Laboratory tests should be performed to check for electrolyte abnormalities, particularly potassium and magnesium levels, which should be corrected if abnormal 1.
- Thyroid function tests may also be appropriate as hypothyroidism can cause conduction delays.
- In rare cases where the PR interval is extremely prolonged (>300 ms) and clearly linked to symptoms, permanent pacemaker implantation might be considered, though this is uncommon for first-degree AV block alone 1. The rationale for this approach is that first-degree AV block represents a delay in conduction through the AV node rather than a complete block, and it typically doesn't progress to higher-degree blocks 1. Most patients with first-degree AV block have a benign course, but symptomatic patients require monitoring to ensure the condition doesn't worsen or indicate a more serious underlying cardiac issue. Key considerations in deciding on the management approach include the presence of symptoms, the duration of the PR interval, and the potential for underlying cardiac conditions that may be contributing to the AV block 1.
From the Research
Symptoms of First-Degree AV Block
- Symptoms of first-degree AV block can be similar to those of the pacemaker syndrome, especially in cases with marked first-degree AV block (PR ≥ 0.30 s) 2.
- Patients may experience symptoms during mild or moderate exercise due to the inability of the PR interval to adapt appropriately.
- Common symptoms are not explicitly stated in the provided studies, but it can be inferred that they may include fatigue, shortness of breath, and decreased exercise tolerance.
Management of First-Degree AV Block
- The management of first-degree AV block in symptomatic patients may involve conventional dual-chamber pacing, which has been shown to improve symptoms in patients with normal left ventricular function 2.
- In patients with left ventricular systolic dysfunction and heart failure, the use of a biventricular DDD device may be considered to avoid the risks associated with right ventricular pacing.
- Proper programming of pacemakers is crucial to avoid functional atrial undersensing and endless loop tachycardia.
Effects of Medications on AV Nodal Conduction
- Slow channel blockers, such as verapamil and diltiazem, can prolong AV nodal conduction and refractoriness, but rarely cause more than first-degree AV block 3.
- Beta blockers can also prolong the AH interval and AV nodal refractory periods, but rarely result in more than first-degree AV block.
- Combination therapy with a slow channel blocker and a beta blocker can rarely cause second-degree AV block.