The development of the HDP knowledge base has been the result of a long-term collaboration with Drs. Shapur Naimi and M. G. Criscitiello of the Tufts-New England Medical Center. However, they are not responsible for the pedagogical changes made for this paper. This research was supported by National Institutes of Health Grant No. R01 HL33041 from the National Heart, Lung, and Blood Institute.