The Committee believes that of these 5 underlying principles, the first 4 are valid and have not changed during the past 30 years. Numerous publications have questioned the validity of the fifth principle and suggested revision of the guidelines, primarily for reasons as shown in Table 2. Another reason that led the Committee to revise the 1997 document was that over the past 50 years, the AHA guide- lines on prevention of IE became overly complicated, making it difficult for patients and healthcare providers to interpret or remember specific details, and they contained ambiguities and some inconsistencies in the recommendations. The deci- sion to substantially revise the 1997 document was not taken lightly. The present revised document was not based on the results of a single study but rather on the collective body of evidence published in numerous studies over the past 2 decades. The Committee sought to construct the present recommendations such that they would be in the best interest Table 1. Summary of 9 Iterations of AHA-Recommended Antibiotic Regimens From 1955 to 1997 for Dental/Respiratory Tract Procedures* Year (Reference) Primary Regimens for Dental Procedures 1955 (6) Aqueous penicillin 600 000 U and procaine penicillin 600 000 U in oil containing 2% aluminum monostearate administered IM 30 minutes before the operative procedure 1957 (7) For 2 days before surgery, penicillin 200 000 to 250 000 U by mouth 4 times per day. On day of surgery, penicillin 200 000 to 250 000 U by mouth 4 times per day and aqueous penicillin 600 000 U with procaine penicillin 600 000 U IM 30 to 60 minutes before surgery. For 2 days after, 200 000 to 250 000 U by mouth 4 times per day. 