Circulatory responses to maternal hyperoxaemia and hypoxaemia assessed non-invasively in fetal sheep at 0.3-0.5 gestation in acute experiments

Objectives To determine fetal haemodynamic responses to hyperoxaemia and hy poxaemia in early pregnancy. Design Repeated measurements in acute experiments. Setting Experimental physiology laboratory. Methods Non-invasive Doppler ultrasound of the umbilical vein, ductus venos us, umbilical and common carotid arteries of 12 fetal lambs (0.27-0.56 gest ation) during maternal hyperoxaemia and hypoxaemia under ketamine anaesthes ia. The effect of gestational age, hyperoxaemia, and hypoxaemia were assess ed based on analysis of variance for dependent measurements and P less than or equal to 0.05 was considered significant. Differences between groups we re considered significant if the 95% confidence interval did not include ze ro. Results Gestational age had a significant effect on the blood velocity in t he umbilical vein and ductus venosus. There were no circulatory changes dur ing hyperoxaemia, but a simultaneous increase of pCO(2) was an important co nfounder. However, hypoxaemia caused significantly reduced heart rate, redu ced maximum and weighted mean blood velocity, and augmented pulsation in th e umbilical vein. Hypoxaemia also caused reduced Velocities in the ductus v enosus (peak velocity during systole and minimum during diastole, and time- averaged velocity) and augmented pulsation of the flow velocity. Additional ly, the pulsatility of blood flow increased in the umbilical artery and was reduced in the common carotid artery. Conclusions Maternal hypoxaemia in early pregnancy causes similar fetal cir culatory responses to those in late pregnancy: bradycardia, reduced venous flow velocities, augmented pulsatility in veins and a redistributional flow velocity pattern of the umbilical and common carotid arteries.