Novel aspects on osmolality and plasma sodium during pregnancy and labour

Abstract

This thesis was designed to study causes and effects of changes in plasma sodium and osmolality during pregnancy and labour. The physiological changes of early pregnancy include reduction in plasma sodium and osmolality, as well as increased ventilation with reduced arterial pCO2. These changes are usually considered due to progesterone. Some studies have however correlated these two events and found that reduction in osmolality and strong ion difference (SID, the difference between strong cations and anions) may predict respiratory changes in pregnant women.

We hypothesised that changes in osmolality should influence ventilation also in non-pregnant women and men, and we therefore studied the ventilatory response to lowered and increased osmolality in healthy volunteers, ten men and nine women. Ingestion of water and intravenous infusion of hypertonic saline 3% caused osmolality to decrease and increase, respectively. SID and base excess decreased in both conditions, and a respiratory compensation was observed during water loading, but although the metabolic acidosis was more pronounced during salt loading, no respiratory compensation was observed. The principal influence of osmolality on ventilation could be inhibitory when osmolality is increased, but this inhibitory effect could cease at decreased osmolality. The hyperventilation observed during pregnancy often exceeds the respiratory compensation observed in our study participants, but this remains to be explained.

We also studied the feto-maternal osmotic and electrolyte relationship, and contrary to the literature, we found that fetal plasma sodium and osmolality were higher than maternal levels. Our results indicated that the physiologic hyponatraemia of pregnancy caused both a compensatory respiratory alkalosis that favours removal of fetal CO2, as well as an osmotic gradient that favours water transport to the fetus. This unifying explanation of maternal physiologic adaptation and feto-maternal relationship differs from the conventional explanatory model that considers the respiratory alkalosis primary, and the metabolic acidosis compensatory.

In addition to the physiological lowering of plasma sodium in the pregnant woman, hyponatraemia may develop during labour. We included 287 women during labour in a prospective observational study, and found that hyponatraemia was common following long lasting labour. Sixteen women (26 %) of those who received more than 2.5 liters of fluid during labour developed hyponatraemia ≤130mmol/L. Multivariate analysis revealed that hyponatraemia was significantly correlated to total fluid volume, but not to oxytocin or epidural analgesia. All women consumed a similar hourly fluid volume of approximately 300 milliliter, but during long lasting labour, probably with an increased secretion of vasopressin, free water clearance was reduced, and hyponatraemia could develop.

Reduction in plasma sodium during labour also significantly correlated to instrumental delivery. We therefore studied the effect of hyponatraemia on human myometrium in an in vitro study. Hyponatraemia induced reversible changes with increased frequency of contractions and also of multiphasic contractions. These changes could indicate decreased myometrial contractility. In the second part of the study we found that initial multiphasic response to increasing doses of oxytocin was significantly more frequent in the hyponatraemic solution. These results indicate that hyponatraemia could be one of several factors leading to dystocia. Electrolyte free solutions should not be administered during labour, and a bedside registration of fluid consumption would be a simple preventive measure against hyponatraemia.