2:1 AV block is a form of second degree AV nodal block and occurs when every other P wave is not conducted through the AV node to get to the ventricles and thus every other P wave is NOT followed by a QRS complex.
2:1 AV block can possibly be from either second degree type I AV nodal block (Wenckebach) or second degree type II AV nodal block. This distinction is crucial since the former is usually benign while the later requires implantation of a permanent pacemaker.
A general rule to remember is that if the PR interval of the conducted beat is prolonged AND the QRS complex is narrow, then it is most likely second degree type I AV nodal block (Wenckebach). Alternatively, if the PR interval is normal and the QRS duration is prolonged, then it is most likely second degree type II AV block and a pacemaker is probably warranted.
Remember that second degree type I AV nodal block is an issue in the AV node itself which is subject to sympathetic and parasympathetic tone while second degree type II AV block is "infranodal" conduction disease of the His-Purkinje system, therefore altering AV nodal conduction would have no effect.
In order to distinguish between the two potential rhythms when an ECG reveals 2:1 AV nodal block, a couple different maneuvers can be employed:
Carotid sinus massage or adenosine: This slows the sinus rate allowing the AV node more time to recover, thus reducing the block from 2:1 to 3:2 and unmasking any progressing prolonging PR intervals that would indicate second degree type I AV nodal block.
Atropine administration: This enhances AV nodal conduction and could eliminate second degree type I AV nodal block since it is due to slowed AV nodal conduction)
Exercise ECG testing (enhances AV nodal conduction and could eliminate second degree type I AV nodal block since it is due to slowed AV nodal conduction)