First degree AV block is defined as a prolonged conduction of atrial impulses into the ventricles without loss of a single impulse. It can be due to an abnormality in the atria, the AV node (most common) or the His-Purkinje system. When it occurs, there is some type of delay of electrical impulses getting from the atria to the conduction system of the ventricles. It manifests on the EKG as a PR duration greater than 200 msec or 1 large EKG box.
See examples of first degree AV block