Erythrocyte sodium lithium countertransport in normal and hypertensive pregnancy: relation to haemodynamic changes

Objective To establish the changes in erythrocyte sodium lithium countertransport (SLC) with advancing normal pregnancy and to determine if these changes were different in pregnancy induced hypertension (PIH). The changes in both groups were assessed in relation to haemodynamic changes. Design SLC, mean arterial pressure (MAP), cardiac output (CO) and total peripheral vascular resistance (TPVR) were determined serially during normal pregnancy and cross‐sectionally in PIH. Women were studied again 20 weeks after delivery where possible. Setting Routine antenatal clinic and antenatal ward of a regional reference centre. Subjects Fifty‐one normal primigravid women were studied serially and 41 primigravid women with PIH were studied at time of diagnosis. Results During normal pregnancy SLC (mmol Li/h/l cells) increased from a nonpregnant value of 0.24 ± 0.02 (mean ± SEM) to 0.32 ± 0.02 at 14 weeks, and 0.37 ± 0.02 at 20 weeks gestation. This was maintained until 38 weeks (0.40 ± 0.02). The increase until 20 weeks occurred at the time of greatest change in CO (5.10 ± 0.18 to 6.79 ± 0.20 l/min) and TPVR (1327 ± 58 to 969 ± 33 dyn/s/cm −5 ). The decrease in TPVR with a rise in SLC is opposite to the relation reported in essential hypertension so that a functional relation is unlikely. However, the changes within pregnancy were positively correlated (r = 0.43, P <0.01). In hypertensive pregnancies TPVR was elevated compared with normotensive pregnancies (1543 ± 100 vs 1090 ± 37) but the SLC was not different from that found in normotensive pregnancies (0.43 ± 0.02 vs 0.40 ± 0.02). Conclusions The changes in SLC activity suggest dynamic effects on erythrocyte membrane function during pregnancy. However, no differences could be found between normal and hypertensive pregnancy and SLC is unlikely to be of value as a marker of hypertensive risk during pregnancy.