First-degree strioventricular (AV) block is the mildest form of atrioventricular block.
In this type of AV block, there is a conduction delay as the impulse travels from the atria to the ventricles. This delay results in a prolonged interval before the stimulus triggers ventricular depolarization (QRS complex).
In first-degree AV block, the conduction disturbance is typically localized at the level of the atrioventricular node and, more infrequently, within the His-Purkinje system (see cardiac conduction system).1
ECG Characteristics of First-Degree AV Block
First-degree AV block with a prolonged PR interval (0.52 s).
On the electrocardiogram (ECG), this condition is characterized by P waves followed by QRS complexes, but with a prolonged PR interval exceeding 0.20 seconds (one large square on the ECG grid).
In first-degree AV block, the stimulus is always conducted to the ventricles; therefore, despite the PR interval prolongation, every P wave is followed by a QRS complex (distinguishing it from other types of AV blocks).1 2
In some cases, the interval is so significantly prolonged that the P wave may be embedded within the preceding T wave or even appear before the preceding QRS complex.
Electrocardiographic Criteria for First-Degree AV Block
- Prolonged PR interval, >0.20 seconds (one large square).
- Every P wave is followed by a QRS complex (1:1 conduction).
- QRS complexes exhibit a normal morphology in the absence of other coexisting conduction abnormalities.
Clinical Presentation and Treatment of First-Degree AV Block
First-degree atrioventricular block is typically asymptomatic and does not result in significant alterations in cardiac function.1
In the absence of structural heart disease, the prognosis for first-degree AV block is generally excellent, and progression to high-grade heart block is infrequent.3
Upon diagnosing a first-degree AV block, it is essential to rule out drug-induced etiologies, particularly from antiarrhythmic agents (e.g., beta-blockers, digoxin, among others); if present, the risk-benefit ratio of continuing the medication should be assessed. This conduction delay may also manifest in the setting of hyperkalemia.
When the PR interval prolongation is marked (>0.3 s), atrioventricular dyssynchrony may occur. This can lead to symptoms, particularly during physical exertion, although current clinical evidence remains weak.3
In the vast majority of cases, first-degree AV block does not require pacemaker implantation.
However, patients with a severely prolonged PR interval who experience symptoms during exercise may require a DDD pacemaker to restore and ensure atrioventricular synchrony.
The presence of first-degree AV block concomitant with bifascicular block may indicate significant disease of the conduction system. In such cases, the onset of symptoms typically constitutes a criterion for permanent pacemaker implantation.
