the bayley scales of infant development yield scores on two indices-the psychomotor development index (pdi) and the mental development index (mdi)- which can be used to assess a child's level of functioning in each of these areas at approximately one year of age. among normal healthy infants, both indices have a mean value of 100. as part of a study assessing the development and neurologic status of children who have undergone reparative heart surgery during the first three months of life, the bayley scales were administered to a sample of one-year-old infants born with congenital heart disease