Objectives: Reduction in plasma volume (Pvol) of women with pregnancy-induced hypertension (PIH; preeclampsia) has both physiological and clinical implications. This study was undertaken to determine the following variables in women with PIH: (1) the incidence of reduced Pvol; (2) the distribution of total extracellular fluid volume (ECFV); (3) the relationship between Pvol and birth weight; and (4) whether any readily available clinical or laboratory parameters predict the presence of reduced Pvol.
Setting: Teaching hospital obstetric unit and antenatal clinic.
Participants: Forty-nine primigravidae with PIH (28 mild, 21 severe), 54 normotensive primigravidae and 25 non-pregnant controls.
Design: Pvol was measured using Evans Blue dye and ECFV as the mannitol space. These measures were compared amongst groups, and also within groups for those with PIH, according to the severity of their disorder and the presence of proteinuria or oedema. Blood pressure, haematocrit, uric acid and serum albumin were also evaluated as predictive indices of reduced Pvol in women with PIH.
Results: Pvol, ECFV and the Pvol:ECFV ratio all increased during normal pregnancy. Pvol in women with PIH was reduced compared with normal pregnancy and correlated significantly with birth weight. Total ECFV was unchanged in women with PIH, but their Pvol:ECFV ratio was significantly reduced compared with normal pregnancy. Although there was a significant correlation between Pvol and haematocrit in women with PIH, haematocrit was a poor predictor for reduced Pvol. Diastolic blood pressure greater than 100 mmHg, persistent proteinuria and severe PIH were the only reliable positive predictors of a reduced Pvol.
Conclusions: Pvol is related to birth weight, but is reduced in only approximately half of women with PIH. This reduced Pvol is the result of maldistribution, not loss, of total ECFV, and can be predicted by high diastolic blood pressure, proteinuria or other clinical signs of severity, but not by haematocrit.