Cardiovascular Changes of Pregnancy

Explain the physiological changes during pregnancy, and parturition

Physiological consequences of changes in posture during pregnancy

Pregnancy is a time of increased metabolic demand, which cardiovascular changes reflect. These changes include:

  • Increased intravascular volume Occurs via two mechanisms:
    • Increased oestrogen causes an increased plasma volume
      • This decreases capillary oncotic pressure, predisposing to oedema
        This may be exacerbated by the gravid uterus compressing the IVC, especially near-term
        • Increased EPO causes an increased red cell volume
  • Increased venous return Due to increased intravascular volume and MSFP.
    • The gravid uterus may compress the IVC and impair VR, hence pregant women are positioned with a left lateral tilt to displace the uterus off the IVC
  • Increased VR causes an increase in CO (with both an increase in HR and SV, as well a decrease in SVR)
  • Decreased SVR results in SBP, DBP and MAP dropping (despite the increase in CO)

Magnitude of Changes by Trimester

Parameter Direction First Trimester Second Trimester Third Trimester Notes
Plasma volume 35% 45% 50% Peaks between 32-36th week, decreases slightly thereafter
Blood volume 5% 15% 20% Increases less than plasma volume, resulting in the fall in haematocrit to 33%
HR 15% 18% 25% Increases progressively throughout
SV 20% 25% 30% Increases progressively throughout
CO 20% 40% 45% Increases throughout and dramatically in labour

Changes During Labour

  • Uterine contraction boluses ~300ml of blood into the maternal circulation
    Causes an increase in CO by up to 30% during the active phase and 45% during ejection.
    • Associated with corresponding increase in SBP and DBP by 10-20mmHg
  • Post-partum CO is up to 80% of pre-labour values due to autotransfusion, and returns to normal within 2 weeks of delivery

References

  1. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
Last updated 2017-09-12

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