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What is First-degree atrioventricular block and explain in detail?
First-degree atrioventricular block (AV block) is a disease of the electrical conduction system of the heart in which electrical impulses conduct from the cardiac atria to the ventricles through the atrioventricular node (AV node) more slowly than normal. First degree AV block does not generally cause any symptoms, but may progress to more severe forms of heart block such as second- and third-degree atrioventricular block. It is diagnosed using an electrocardiogram, and is defined as a PR interval greater than 200 milliseconds. First degree AV block affects 0.65-1.1% of the population with 0.13 new cases per 1000 persons each year.

Causes
The most common causes of first-degree heart block are AV nodal disease, enhanced vagal tone (for example in athletes), myocarditis, acute myocardial infarction (especially acute inferior MI), electrolyte disturbances and medication.  The medications 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.

Diagnosis
In normal individuals, the AV node slows the conduction of electrical impulses 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.In first-degree heart block, the diseased AV node conducts the electrical activity more slowly