A study of maternal acid-base state during labour.

Arterialized venous blood was used to investigate maternal acid-base state during the first and second stages of normal labour. During labour the range of values obtained for acid-base variables is wide but patterns are discernible. At the beginning of labour Pco, was lower than it is in non-pregnant subjects, and the pH was higher despite a substantial base deficit. At the end of the first stage the Pco2 declined, the base deficit increased, and blood pH rose slightly. There were further changes in the second stage but these were quite small. The use of various drugs and of epidural analgesia in the first stage had litde effect on Pco., though the use of premixed nitrous oxide and oxygen in the Entonox apparatus resulted in a slightly increased degree of hyperventilation.