LEARNING TOPIC
Blood pressure regulation in pregnancy

  updated 13 May 1999 

Keywords
  • systemic vascular resistance
  • cardiac output
  • blood volume
  • nitric oxide
  • serum creatinine
   

Significant cardiovascular changes and associated changes in the autocrine and endocrine systems leading to alteration in renal function are all important adaptive changes in primate (and therefore human) pregnancy.

The major alterations begin early in the first trimester, reach their peak in the second trimester and remain constant till delivery. The first demonstrable change is a reduction in systemic vascular resistance with a consequent reduction in arterial blood pressure. This is due to initial widespread vasodilation involving all body regions, and later by the establishment of a low resistance-high volume circuit in the pregnant uterus (to allow constant placental blood supply). This is followed by activation of the renin-angiotensin aldosterone axis and an increase in extracellular fluid (30 – 40%) volume and an increase in plasma volume (6-7%) in response to the relative underfilling of this widely dilated circulation. At the same time there is an increase in cardiac output due to changes in cardiac afterload and increased total circulating volume.

These early changes are incompletely understood but the influence of humoral agents such as vasodilator prostaglandins, oestrogens and alterations in the endothelin/nitric oxide axis with an increase in nitric oxide synthesis are contributory. The vasodilation is unique to pregnancy as the secondary increase in renin activation and therefore salt and water retention are associated with an increased resistance of the circulation to usual constrictor effects (e.g. Angiotnesin II is less vasoconstricting in pregnancy). The decrease in blood pressure is associated with an in increase in glomerular filtration due to an increase in renal plasma flow, therefore the vasodilation does not spare the renal circulation as in other low pressure states.

The changes in cardiac output, and the associated local hormonal changes also contribute to the significant changes in renal function with a 50% increase in glomerular filtration rate. Resultant changes in renal tubular function contribute to a net sodium retention over the course of a pregnancy and are due to tubuloglomerular feedback, the response to local vasoactive hormones, and other endocrine effects in pregnancy. Therefore there is significant retention of salt and water throughout pregnancy associated with these changes.

These alterations in cardiovascular and renal function are important adaptive changes to support a normal pregnancy. A significant reduction in cardiac performance or renal function results in increased morbidity and mortality of pregnancy.

Recommended and Optional Readings and Resources

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Author/s:   Prof J. Horvath, Dept of Renal Medicine, Royal Prince Alfred Hospital