First-degree AV block (PR prolongation)
is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds. In first-degree AV block, the impulse conducting from atria to ventricles through the AV node is delayed and travels slower than normal. It has aprevalence in the normal (young adult) population of 0.65-1.1% and the incidence is 0.13 per 1000 persons. Causes: 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. Digitalis is a sodium/potassium ATPase inhibitor and also prolongs AV conduction. * In first-degree heart block, the diseased AV node conducts the electrical activity more slowly. 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.