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Department of Obstetrics and Gynaecology, Royal Free and University College Medical School, London WC1E 6HX, United Kingdom
To study the regulation of the ductus venosus
(DV) inlet in vivo, we measured the effect of vasoactive substances and
hypoxemia on its diameter in nine fetal sheep in utero at 0.9 gestation under ketamine-diazepam anesthesia. Catheters were inserted into an
umbilical vein and a fetal common carotid artery, and a flowmeter was
placed around the umbilical veins. Ultrasound measurements of the
diameter of the fetal DV during normoxic baseline conditions [fetal
arterial PO2 (PaO2) 24 mmHg] were compared with measurements during infusion of sodium
nitroprusside (SNP; 1.3, 2.6, and 6.5 µg · kg
1 · min
1) or the
1-adrenergic agonist phenylephrine (6.5 µg · kg
1 · min
1) into the
umbilical vein or during hypoxemia (fetal PaO2 reduced to 10 mmHg). SNP increased the DV inlet diameter by 23%, but
phenylephrine had no effect. Hypoxemia caused a 61% increase of the
inlet diameter and a distension of the entire vessel. We conclude that
the DV inlet is tonically constricted, because nitric oxide dilates it but an
1-adrenergic agonist does not potentiate
constriction. Hypoxemia causes a marked distension of the entire DV.
circulation; hypoxia; nitroprusside; vein
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