First egree 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.