A new study ofnormal PR intervals was undertakenfor the purposes of analysing atrioventricular conduction. Abnormalities of conduction were identified in 84 per cent of So8 patients with acute rheumatic fever. The PR index was used as a simple, reliable method of showing these changes. Such conduction disturbances occur more commonly than carditis, arthritis, subcutaneous nodules, chorea, or erythema marginatum, the five major features of the disease. Streptococcal infection and acute glomerulonephritis were not associated with similar abnormalities of conduction. It appears that atrioventricular conduction disturbances that occur after a haemolytic streptococcal infection are relatively specific for rheumatic fever, either with or without carditis. It is suggested that reversible PR prolongation be used as a major criterion, when using Jones' criteria for the diagnosis of rheumaticfever, providing there is proof of a preceding streptococcal infection. A number of reports of PR prolongation in rheumatic fever studied by various methods have been published (Cohn and Swift, I924;
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