Objective: To characterise the haemodynamic, renal-electrolyte and hormonal parameters in normal near-term pregnancy.
Design: Observational prospective case-series study.
Setting and population: Eleven women with normal pregnancies at 35-39 weeks gestation.
Methods: Following baseline laboratory assessments and placement of a right-atrial catheter, serial measurements were obtained for 2 hours in the supine position (SP) followed by a change to the (LLP) and subsequent observations for 2 hours.
Main outcome measures: Blood pressure (BP), central venous pressure (CVP), atrial natriuretic peptide (ANP), plasma renin activity (PRA), plasma aldosterone (ALDO), diuresis, creatinine clearance, sodium and potassium excretion.
Results: In the SP, the subjects' BP remained stable while their CVP decreased. In the LLP, the subjects' systolic and diastolic BP consistently decreased by about 15 mmHg and their CVP increased within the first 60 minutes. ANP levels doubled in the subjects while they rested in the LLP, whereas the subjects' PRA and ALDO levels decreased by half compared with when they rested in the SP. In the LLP, the subjects' creatinine clearance significantly increased by 12% and their sodium excretion and diuresis increased by 38% and 59% respectively.
Conclusion: Rest in the LLP induces systemic and intra-renal haemodynamic and hormonal changes that may play a central physiological role in the renal excretory response to restore excessive sodium/water retention in late pregnancy.