First degree AV block is defined as a constant PR interval greater than 0.20 seconds. Every P wave conducts to produce a QRS complex, but more slowly than normal. This is due to slowed conduction between the atrium and ventricles, most often in the AV node. Occasionally, the slowed conduction may be in the atria, His bundle, or bundle branch system. The most common underlying causes for slowed AV conduction include enhanced parasympathetic tone, drugs, or disease of these tissues. The following animation illustrates what we have just described.