Editor-In-Chief: C. Michael Gibson, M.S., M.D. 
Definitions of conduction abnormalities
Cardiac conduction through pathways not normally conducting cardiac impulses, particularly through ventricular tissue.
Transmission of a cardiac impulse in the normal direction, from the sinus node to the ventricles, particularly forward conduction through the atrioventricular node.
Incomplete penetration of a propagating impulse through the cardiac conducting system such that electrocardiograms reveal no evidence of transmission but the behavior of one or more subsequent impulses is somehow affected. A common example would be an interpolated PVC (a type of premature ventricular contraction) during normal sinus rhythm; the PVC does not cause an atrial contraction, because the retrograde impulse from the PVC does not completely penetrate the AV node. However, this AV node stimulation (which is not visible on EKG by itself, hence "concealed") can cause a delay in subsequent AV conduction by modifying the AV node's subsequent conduction characteristics. Hence, the P-R interval after the PVC is longer than the baseline P-R interval.
Another variation on this concept is seen in atrial flutter. As a result of the rapid atrial rate, some of the atrial activity fails to get through the AV node in an antegrade direction but can alter the rate at which a subsequent atrial impulse is conducted. In this circumstance, an alteration in the F-wave to QRS relationship is seen.
Concealed retrograde conduction
Retrograde conduction blocked in the atrioventricular node; it does not produce an extra P wave but leaves the node refractory to the next normal sinus beat.
transmission of a cardiac impulse backward in the ventricular to atrial direction, particularly conduction from the atrioventricular node into the atria.