[Blood flow in the umbilical vein and artery in pregnancy]

Abstract

Transplacental gas exchange and supply of nutritive substances to the foetus is guaranteed by an increase of umbilical blood flow (Qnv). It was investigated, whether the umbilical blood flow (Qnv) is influenced by physiological conditions during pregnancy and if changes occur in the umbilical blood flow velocity wave forms. In this study, 31 pregnant women between the 24th to 40th week of gestation were examined with a pulsed wave duplex scanner ADR Kranzbühler 8150. The mean flow velocity (Vnv) of the umbilical vein and the diameter of the vessel were parameters, which were included in the calculation of the umbilical blood flow. The flow profile of the umbilical artery allowed determination of the maximum systolic frequency (Fmax) and the end-diastolic frequency (Fmin). Based on these data, three indices--RI, PI, S/D ratio were calculated. The blood flow of the umbilical vein (Qnv) shows a linear increase up to the 36th week of gestation and remains at this level thereafter. The increase of umbilical blood flow (Qnv) is mainly caused by an increase of the diameter of the umbilical vein (81%, 26th-40th week of gestation) and to a lesser degree by an increase of the mean flow velocity (Vnv) (18%, 26th-40th week of gestation). The diameter of the vein is highly correlated both with the foetal birth weight (r = 0.60) and the umbilical blood flow (Qnv) (r = 0.73). Throughout pregnancy, flow velocity waveforms showed significant changes of the maximum systolic frequency (Fmax) from 973 Hz (SD 128 Hz; 26th week of gestation) to 1130 Hz (SD 152 Hz; 40th week of gestation) and an increase of the end-diastolic frequency (Fmin) from 246 Hz (SD 58 Hz; 26th week of gestation) to 423 Hz (SD 91 Hz; 40th week of gestation). The higher increase of the end-diastolic frequency (Fmin) results in a decrease of the resistance index (RI), pulsatility index (PI) and SD-ratio. None of the investigated cases showed an end-diastolic frequency (Fmin) of less than 200 Hz. These results reveal, that determination of the blood flow of the umbilical vein mainly depends on measuring the diameter of the vessel rather than on measurements of the blood flow velocity (Qnv) of the vein. With the presently available equipment, accurate measurements of the diameter are very difficult. The increase of blood flow of the umbilical vein is not proportional to the foetal growth; moreover, it remains constant from the 36th week of gestation.(ABSTRACT TRUNCATED AT 400 WORDS)