ABSTRACT
OBJECTIVE: To measure the renal resistive index (RI, an estimate of renal vascular resistance, used to assess upper tract obstructive uropathy) from the 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 routinely. During the first semester of life significant haemodynamic changes during the physiological development of the kidney cause considerable variability 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 pregnancy (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 and 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 the third month. The variability in RI continuously declined with age, becoming less important. The normal ranges for groups 1-4 were 0.67-0.88, 0.57-0.90, 0.60-0.84 and 0.65-0.75, respectively. There were no statistically significant differences between the left and right kidneys. CONCLUSION: In the first semester of life there is more than one landmark value of RI depending on the month of age of the infant. This should be considered when assessing upper tract obstructive uropathies after birth and the RI should be compared with the normal ranges reported herein.
