CVS – Responses

Master answer

• Pregnancy is a time of increased metabolic demand, which cardiovascular changes reflect.
• Changes begin from week 8 and ↑ to plateau at 32 weeks → return to normal 2-8 weeks post delivery
• Changes depend on stage of pregnancy
• Hormonal changes: ↑ circulating concentrations of oestrogen, progesterone, hCG
• ↑ metabolic demand esp. during labour: ~↑60% O2 consumption/ CO2 production during labour
• Mechanical effects from gravid uterus

Characteristics

• Mechanical Effects:
  • Thoracic changes:
    • Anatomical compression of chest
    • Diaphragm pushed upwards by 4cm
    • ↑ AP + transverse diameter of chest wall (2-3cm)
  • placental circulation: ↓pressure, ↓resistance AV shunt
  • Aortocaval compression
    • Collateral blood flow via collateral paravertebral epidural veins
• Hormonal Changes:
  • ↑ circulating concentrations of oestrogen, progesterone, hCG
  • Oestrogen stimulation of RAAS
    • Increased plasma volume (40% or 1~1.5L positive)
  • Erythropoietin secretion
    • Increased erythropoiesis and red blood cell volume (20%)

Changes in CVS

• Anaemia of pregnancy
  • Disproportionate plasma volume expansion relative to erythropoiesis
• Increased cardiac output (40%)
  • Increased uterine blood flow (750ml/min)
  • Increased renal blood flow
• Increased HR (25% by second trimester)
• Increased SV (25% in first trimester)
• Decreased peripheral vascular resistance (30%)
  • Progesterone
  • Prostaglandins
  • Down-regulation of α receptors
• Decreased plasma oncotic pressure (15%) → peripheral oedema

During labour

• Contraction → 300ml return to central maternal circulation
• CO increases 15%, 30% and 45% in latent, active and expulsive phases of labour respectively
• Immediately after labour CO 80% pre-labour levels due to autotransfusion due to uterine involution
• Return to non-pregnant levels 2 weeks after delivery

Sakurai / Kerr / JC 2020

Answer by JC, Kerr, Sakurai