The minimum number of cardiac cycles necessary for calculation of renal blood flow velocity indices in pregnant and non-pregnant women.

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In this study Doppler ultrasound was used to measure the blood flow velocity in the main and segmental renal arteries in 16 healthy non-pregnant women. There were no differences in blood flow indices between the right and left kidneys as well as between the main and segmental arteries. The variability of the indices in the main and segmental renal arteries in healthy non-pregnant and in the segmental renal artery in normal pregnant women (32-37 weeks of gestation) was investigated. The within-patient error standard deviations of ten cycles and two, and three to nine averaged cycles were compared. In non-pregnant women, values for the error standard deviation comparable with those obtained from ten cycles for the systolic/diastolic ratio and pulsatility index from both main and segmental arteries were obtained by averaging from five to six consecutive cycles, while the error standard deviation for the resistance index stabilized when averaging only two to four cycles. In pregnant women, the error standard deviations for both the systolic/diastolic ratio and pulsatility index reached quite stable values after three cycle had been averaged as did the error standard deviation for the resistance index. We speculate that this difference between non-pregnant and pregnant women is due to more favorable conditions for renal visualization and consequently higher quality Doppler signals during pregnancy. A knowledge about the number of cardiac cycles required for an accurate calculation of renal blood flow velocity indices may prevent time-consuming calculations and inaccurate conclusions.