RESUMEN
OBJECTIVES: We studied the responses of flow in the umbilical vein and of the circulation in the proximal segment of the middle cerebral artery (MCA) during partial external compression of the umbilical vein. METHODS: Partial occlusion of the umbilical vein was achieved by the pressure test, consisting of applying external pressure on the maternal abdominal wall for 1-2 s. RESULTS: During 60 pressure tests in 45 fetuses with nuchal cords, we observed complete cessation of the umbilical vein flow, while blood flow in the umbilical arteries was preserved, and proximal MCA flow was moderately or markedly decreased. Blood flow in other segments of the MCA was unchanged. This effect, which was more pronounced before the 32nd week of gestation, was also observed in association with spontaneous fetal movements. CONCLUSION: The pressure test decreases flow in the proximal MCA, and interrupts umbilical vein blood flow. Further analysis of this response suggested the presence of associated vasoconstriction of the proximal MCA.