[1] Eunice Kennedy Shriver Natl Inst Child Hlth & Hum, Div Epidemiol Stat & Prevent Res, NIH, Dept Hlth & Human Serv, Bethesda, MD USA
[2] Univ Penn, Sch Med, Philadelphia, PA 19104 USA
[3] Uniformed Serv Univ Hlth Sci, Dept Pediat, Bethesda, MD 20814 USA
[4] Univ Chile, Fac Med, Dept Pediat, San Borja Arriaran Clin Hosp, Santiago 7, Chile
[5] NHLBI, Div Cardiovasc Sci, NIH, Dept Hlth & Human Serv, Bethesda, MD 20892 USA
[6] Univ Chile, Fac Med, Inst Maternal & Child Res IDIMI, Santiago 7, Chile