The renal-resistive index from the last 3 months of pregnancy to 6 months old

Objective To measure the renal resistive index (RI, an estimate of renal va scular resistance, used to assess upper tract obstructive uropathy) from th e last trimester of pregnancy to the sixth month of life in a large series of healthy subjects, and thus to identify normal values that can be used ro utinely. During the first semester of life significant haemodynamic changes during the physiological development of the kidney cause considerable vari ability in RI, which is thus considered less reliable in this period. Subjects and methods From September 1998 to October 1999, 93 subjects (186 renal units, RU) were enrolled: 32 were fetuses in the last trimester of pr egnancy (group 1. 64 RU) and 61 were children (122 RU), 30 aged 0-1 month ( group 2. 60 RU), 20 aged 1-3 months (group 3, 40 RU) and 11 aged 3-6 months (group 4, 22 RU). All subjects underwent colour Doppler ultrasonography an d the RI of the renal artery was measured for each kidney. Results The RI was very high in group 1 but decreased noticeably during the first 6 months of life, reaching values similar to those in adults after t he third month. The variability in RI continuously declined with age, becom ing less important. The normal ranges for groups 1-4 were 0.67-0.88, 0.57-0 .90, 06.0-0.84 and 0.65-0.75, respectively, There were no statistically sig nificant differences between the left and right kidneys. Conclusion In the first semester of life there is more than one landmark va lue of RI depending on the month of age of the infant. This should be consi dered when assessing upper tract obstructive uropathies after birth and the R1 should be compared with the normal ranges reported herein.