<P> First - degree atrioventricular block (AV block), or PR prolongation, is a disease of the electrical conduction system of the heart in which the PR interval is lengthened beyond 0.20 seconds . </P> <P> In first - degree AV block, the impulse conducting from atria to ventricles through the atrioventricular node (AV node) is delayed and travels slower than normal . It has a prevalence in the normal (young adult) population of 0.65 - 1.1% and the incidence is 0.13 per 1000 persons . </P> <P> 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 . </P> <P> In normal individuals, the AV node slows the conduction of electrical impulse through the heart . This is manifest on a surface electrocardiogram (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 . </P>

Drugs that increase the refractory time of the avn
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