The atropine test is most commonly used as an aid in the clinical diagnosis of sick sinus node syndrome. In patients with significant sinus bradycardia, if the sinus rate does not increase to 90 beats/min after drug administration, it is suggestive of sick sinus node syndrome and often requires pacemaker therapy; however, one needs to be alert that not all patients with a positive atropine test necessarily have sick sinus node syndrome. The atropine test can also be used to determine whether the prolonged P-R interval is due to vagal hyperfunction or to organic heart disease, and can be used in the differential diagnosis of sinus bradycardia and second-degree type 1 sinus block, as well as the differential between second-degree type 1 and second-degree type 2 atrioventricular block.