A one to one ratio of P to QRS.

A PR interval of greater than 200 ms (one large box)

May be without clinical significance if it represents an isolated condition. Some causes include:

Intrinsic AV nodal disease
Enhanced vagal tone
Acute MI, particularly acute inferior MI
Myocarditis
Electrolyte disturbances
Drugs (CCB, Digoxin, beta-Blockers)