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Under physiological conditions, fetal blood pressure and fetal heart rate vary similarly to maternal blood pressure and maternal heart rate (Battaglia and Meschia 1986). The fetal circulation is regulated by the sympathetic and parasympathetic systems as well as by various hormones and vasoactive substances (Assalli et al. 1977; Mott and Walker 1983) including vasopressin, prostaglandins, and catecholamines. Umbilical blood flow is not subject to central nervous regulatory mechanisms but can be influenced by vasoactive substances such as angiotensin, prostaglandin, and bradykinin (Mott and Walker 1983). In the presence of physiologic conditions, however, these substances are of minor importance in phases with varying movement status. The fact that the pulsatility index of the umbilical artery shows less variation as a function of the movement status than that of the fetal vessels be attributable to the absence of central regulatory mechanisms. The variations in blood flow spectra with fetal respiration have been described by Marsal et al. (1984b).
KeywordsResistance Index Blood Flow Velocity Pulsatility Index Umbilical Artery Fetal Circulation
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