In 1920 Parkinson et al,1 and in 1924 Cohn and Swift,2 observed an association of acute rheumatic fever and prolongation of the PR interval of the electrocardiogram. Ziegler3 has noted that the PR interval is prolonged in 2% of normal children. Because the PR interval varies with age and heart rate, Mirowski et al4 devised the PR index to provide a standard by which the duration of the PR interval of a subject may be judged. A value greater than 1.0 is considered evidence of delay in atrioventricular (AV) conduction.
In 1965, the revised Jones criteria5 listed prolongation of the PR interval as one of the minor criteria used in establishing the diagnosis of rheumatic fever. Other conduction abnormalities may be observed. These include second-degree heart block, complete heart block, AV dissociation, and junctional rhythm. Clarke and Keith6 detected AV conduction abnormalities in 84%