In patients with normal sinus rhythm, the vagally mediated effect of digitalis frequently cause sinus bradycardia and lengthening of the PR interval to more than 0.20 seconds, the upper limit of normal. This is referred to as first degree AV block. It is a common effect of digitalis and may be a manifestation of digitalis excess and a precursor of higher grades of AV block. In the example shown here, the rate is 45/min and the PR interval is 0.30 seconds.