Obstetrics and Gynæcology

THE authors report a study of 260 blood biochemistry estimations of maternal acid-base status during pregnancy, labour and the puerperium. The results are correl~ted with studies of alterations of respiratory dynamics durrng pregnancy with reference to tidal volumes, hyperventilation and respiratory alkalosis. Results Indicated that the pre.gnant woman has a respiratory alkalosis and a metabolic acldosls, with the pH tending to be in the higher range of normal. These changes were observed as early as the seventh week of pregnancy and became more pronounced as pregnancy advanced. There was a further increase in the degree of metabolic acidosis and respiratory alkalosis by the end of labour. During the puerperium hyperventilation and hypocapneea gradually disappear over several weeks, but the metabolic acid-base system resumes normal levels soon after delivery. The authors agree with other workers that the biochemical changes related to an increase in tidal volume and hyperventilation of pregnancy are the result of progesterone stimulation of the respiratory centre. This increased ventilation lowers the alveolar and arterial tension of carbon dioxide and increases oxygen tension, favouring transport of carbon dioxide from the fretal blood and increasing the oxygen supply to the fcetus. Among various complications of pregnancy studied In the series It was found that marked reduction in lung area can occur without preventing hyperventllation, except in cases of bronchiectasis and asthma. Studies on six patients with babies considered "small-fordates", two controlled diabetics and a patient with a missed abortion showed acid-base findings in accord with those of normal pregnancy. The condition of metabolic acidosis is considered to be the result of a reduction in the alkaline reserve of the blood and not of an Increase In free acid levels.