First degree AV block or PR prolongation is a disease of the electrical conduction system of the heart in which the PR interval is lengthened. In first degree heart block, the disease is almost always at the level of the atrioventricular node (AV node). It has a prevalence in the normal (young adult) population of 0.65-1.1% and the incidence is 0.13 per 1000 persons.
First degree AV block may be due to conduction delay in the AV node, in the His-Purkinje system (made up by the bundle of His and the Purkinje fibers), or a combination of the two. The majority of cases are due to a dysfunction of the AV node; however, when first degree heart block coexists with a bundle-branch block, the cause is more likely to be a conduction delay in the His-Purkinje system.
The most common causes of first degree heart block are an AV nodal disease, enhanced vagal tone (for example in athletes), myocarditis, acute myocardial infarction (especially acute inferior MI), electrolyte disturbances and medication. The drugs that most commonly cause first degree heart block are those that increase the refractory time of the AV node, thereby slowing AV conduction. These include calcium channel blockers, beta-blockers, cardiac glycosides and anything that increases cholinergic activity such as cholinesterase inhibitors. Drugs which increase calcium concentration such as Digitalis, decrease AVN conduction time.
In normal individuals, the AV node slows the conduction of electrical impulse through the heart. This is manifest on a surface ECG as the PR interval. The normal PR interval is from 120 ms to 200 ms in length. This is measured from the initial deflection of the P wave to the beginning of the QRS complex.
In first degree heart block, the diseased AV node conducts the electrical activity slower. This is seen as a PR interval greater than 200 ms in length on the surface ECG. It is usually an incidental finding on a routine ECG.
First degree heart block does not require any particular investigations except for electrolyte and drug screens especially if an overdose is suspected.
The management includes identifying and correcting electrolyte imbalances and withholding any offending medications. This condition does not require admission unless there is an associated myocardial infarction. Even though it usually does not progress to higher forms of heart block, it may require outpatient follow up and monitoring of the ECG especially if there is a comorbid bundle branch block. If there is a need for treatment of an unrelated condition care should be taken not to introduce any medication that may slow AV conduction. If this is not feasible clinicians should be very cautious when introducing any drug that may slow conduction and regular monitoring of the ECG is indicated.
Isolated first degree heart block has no clinical consequences. There are no symptoms or signs associated with it, and there is no danger of progression to complete heart block. There is also no increased morbidity or mortality associated with it.
In a subset of individuals with the triad of first degree heart block, right bundle branch block, and either left anterior fascicular block or left posterior fascicular block (known as trifascicular block) may be at an increased risk of progression to complete heart block.