First-degree AV block: ECG criteria and clinical characteristics
In this article you will learn about first-degree AV block, which may also be referred to as 1st degree AV block, AV block 1 or AV block I. Before reading this article, make sure that you have read the fundamentals of AV blocks. The term block is somewhat misleading in the case of first-degree AV block because there is no actual block, but merely an abnormal delay of the impulse conduction from the atria to the ventricles. This manifests as a prolonged PR interval on the ECG. The PR interval is ≥0.22 s in first-degree AV-block. All P-waves are followed by QRS complexes. First-degree AV block is rarely serious and may be left untreated in the majority of cases. The block is located in the atrioventricular node in most cases.
ECG criteria for first-degree AV block
- PR interval ≥0,22 s.
- All P-waves are followed by QRS complexes.
First-degree AV block with wide QRS complex
First-degree AV block with normal QRS complex (QRS duration <0.12 s) is localized in the AV node in 90% of the cases and the bundle of His in 10% of cases. If the ECG displays first-degree AV block (PR interval ≥0,22 s) along with wide QRS complexes (QRS duration ≥0,12 s) there is a high probability that the block is located bilaterally in the bundle branches. This type of first-degree AV block often progresses to third-degree (complete) AV block, and therefore necessitates an artificial pacemaker.
Blocks located proximal to the AV node (prenodal blocks)
First-degree AV block may actually be due to slowing of impulse conduction prior to the AV node. This is mostly due to fibrosis of atrial myocardium. The ECG shows, besides prolonged PR interval, wide P-waves with low amplitude. Prenodal block is, however, uncommon.
Prognosis of first-degree AV block
Isolated first-degree AV block with normal QRS complexes has very good prognosis and may even occur in otherwise healthy individuals. However, if the QRS complexes are wide there is a risk of distal block which may progress to more advanced block (second- or third-degree AV block). A pacemaker is often necessary in individuals with first-degree AV block and wide QRS complexes.
Treatment of first-degree AV block
Treatment of 1st degree AV block, 2nd degree AV block and 3rd degree AV block is discussed in a separate article.