PMC:2675008 / 21538-24775 JSONTXT

{"project":"2_test","denotations":[{"id":"19175600-16565125-7908387","span":{"begin":271,"end":273},"obj":"16565125"},{"id":"19175600-7858729-7908388","span":{"begin":797,"end":799},"obj":"7858729"},{"id":"19175600-14071843-7908389","span":{"begin":963,"end":965},"obj":"14071843"},{"id":"19175600-8614488-7908390","span":{"begin":1185,"end":1187},"obj":"8614488"},{"id":"19175600-16885666-7908391","span":{"begin":1560,"end":1562},"obj":"16885666"},{"id":"19175600-16565125-7908392","span":{"begin":2449,"end":2451},"obj":"16565125"},{"id":"19175600-8614488-7908393","span":{"begin":2452,"end":2454},"obj":"8614488"},{"id":"19175600-18448591-7908394","span":{"begin":2455,"end":2457},"obj":"18448591"},{"id":"19175600-2860030-7908395","span":{"begin":2652,"end":2654},"obj":"2860030"},{"id":"19175600-12200908-7908396","span":{"begin":3235,"end":3237},"obj":"12200908"}],"text":"Sodium and the corresponding anions are the main osmoles in extracellular fluid, and sodium concentration mainly reflects body water content. Reabsorption of water in the collecting ducts of the kidney is controlled by vasopressin, in turn regulated by plasma osmolality.15 Vasopressin is secreted in response to increased osmolality, causing water retention that will restore osmolality. Reduction of plasma osmolality by approximately 10 mOsm/kg occurs in early pregnancy mainly as a result of a decrease in plasma sodium by 3–5 mmol/l. By 12 weeks of gestation, this adaptation has stabilised, and plasma osmolality remains low until after delivery. Release of vasopressin in response to adequate physiological stimuli corresponds to the nonpregnant state, but a lower osmolality is maintained.16 The maximum capacity of excreting a water load at rest is reported as approximately 900 ml per hour in healthy women but is reduced by one-third in late pregnancy.17 Pain, stress, and fear are nonosmotic stimuli for vasopressin secretion. Hypovolaemia is, however, the most potent stimulus for vasopressin secretion and may cause water retention even in the presence of hypoosmolality.18 Oxytocin, a short peptide with a structure similar to vasopressin, may also cause water retention by stimulation of the specific V2 vasopressin receptors in the kidney. This receptor is not downregulated during prolonged stimulation as is the case with the oxytocin receptor. However, the infusion rate of oxytocin must exceed 20 mU/minute to cause antidiuresis in humans.19 Hyponatraemia during labour caused by oxytocin administered in electrolyte-free solutions is well recognised, but despite this, hypotonic solutions are often used as a vehicle for oxytocin.20 In our study, fluid volumes needed to administer oxytocin were modest, whereas remaining fluid administration was unrestricted (Table 1). Multivariate logistic regression showed that total fluid volume correlated significantly with hyponatraemia (P \u003c 0.001), whereas oxytocin administration did not significantly correlate with hyponatraemia (P =0.072) (Table 3). Women in fluid groups 2 and 3 received oxytocin at rates necessary for oxytocin to express antidiuretic effect but probably during too short a period of time for significant antidiuretic effect to develop. The administration of hypotonic fluids in conditions with increased vasopressin activity can cause dilutional hyponatraemia.15,18,21 Typically, this may occur postoperatively, but during labour stimuli for vasopressin release are abundant. Labour itself is, however, not a situation causing inappropriate vasopressin secretion.22 The women in our study received fluid volumes well below their predicted maximum capacity of renal excretion at rest; therefore, the development of hyponatraemia indicates increased vasopressin activity during labour. Our results indicate that fluid volume is the major determinant of hyponatraemia, but the antidiuretic effects of endogenous vasopressin and oxytocin administration increase the susceptibility of women to develop hyponatraemia during labour. Fetal vasopressin levels are high during birth, thus also exposing the overhydrated fetus to the risks of hyponatraemia.12"}