Bioimpedance In Pregnancy and Labour: A Fluid Balance Concept Study.

Maternal cardiovascular physiology evolves during the course of pregnancy to accommodate the growing foetus. In order that the uterine artery can deliver 10% of the entire maternal cardiac output, there are considerable structural and physiological changes that take place. There is a 30% increase in global arterial compliance, predominantly due to vasodilation at the level of the renal arteries, which leads to a reduced total peripheral resistance. Stroke volume increases due to the plasma expansion and increasing total blood volume. All water compartments show an absolute and relative increase with advancing gestation and eventually water represents the largest component of weight gain in pregnancy. These changes have been measured and known to clinicians for decades. Yet, it is only in the recent past, that the possible clinical relevance of assessing total body water and extracellular body water has been enhanced by the cross over of hypertensive disorders of pregnancy with obesity epidemics.

Bio-electrical impedance analysis (BIA) measures whole body (or regional) impedance by means of an electric current transmitted at different frequencies. New techniques allow measurement of total body water with separation into extracellular and intracellular water. Current evidence suggests that BIA may provide useful information not only in different well-established patient groups (renal dialysis, malnutrition), but also in critically ill patients with burns, trauma and sepsis undergoing fluid resuscitation.

There are a limited number of studies assessing the use of BIA during pregnancy. In a longitudinal study of 126 pregnant women, Piuri and colleagues assessed changes of bio-impedance analysis among low-risk pregnant woman recruited in the first trimester and followed their progress. They found that women who developed hypertensive disorders of pregnancy (HDP) showed significantly different bioimpedance from control cases. Total Body Water (TBW)indices were highly significantly different from the first trimester. In pregnancies delivered of small for gestational age (SGA) newborns, these indices were the opposite of the values observed in patients with HDP-average for gestational age (AGA).TBW in these patients was significantly reduced compared with normal pregnancies. The authors concluded that the bioelectrical impedance is a fast, simple, noninvasive way to assess the TBW content in pregnancy and bio-impedance might help to identify, early in gestation, patients at risk of developing different clinical phenotypes of hypertensive disease of pregnancy and SGA fetuses.

In a prospective observational study of 3000 pregnant women, Kent and colleagues examined the use of multi frequency segmental bioelectric impedance analysis in the first trimester of pregnancy for assessment of maternal body composition as predictors of increased birth weight. They observed that on direct measurements of body composition, birth weight correlated positively with maternal fat-free mass and not adiposity.

Several factors may influence the progression of normal labour. It has been postulated that the routine administration of intravenous fluids to keep women adequately hydrated during labour may reduce the period of contraction and relaxation of the uterine muscle, and may ultimately reduce the duration of the labour. It has also been suggested that intravenous fluids may reduce caesarean section rates (CS) for prolonged labour. Conversely, excessive volumes of intravenous fluids may pose risks to both the mother and her new-born with different fluids being associated with different risks.