Oxygen lack and autonomic nervous control of the foetal circulation in the lamb

Tying the umbilical cord in the mature foetal lamb causes an abrupt increase of blood pressure and a large decrease in heart rate (Barcroft, 1946). As Barcroft was well aware, this procedure not only removes the normal channel of gaseous exchange from the foetus, and thus (in the absence of pulmonary ventilation) induces acute asphyxia, but also causes a profound circulatory disturbance, because more than half the output of the foetal heart normally passes through the placenta. Tying the cord, therefore, cannot give a reliable indication of the response of the foetus in utero to oxygen lack alone. In this paper the effect of oxygen lack has been investigated in lambs delivered by Caesarean section by supplying the ewe with a gas mixture of low oxygen content. The immediate effect was to cause a rise of foetal blood pressure and heart rate, provided that the foetal arterial 02 saturation did not fall too rapidly or too low. Very immature lambs did not react in this way.