G4iii: Uteroplacental Blood Flow

  • Uteroplacental BF = delivers O2 & nutrients to foetus
  • Abnormalities result in threatened foetal viability & gestational pathology

Anatomy

  • Main supply → UTERINE A.
  • Smaller supply → OVARIAN A.
Anatomy
Anatomy1
  • Uterine BF:
    • 50mL/min (before pregnancy) → 700mL/min (at term)
  • 3 phases of ↑:
    1. FIRST PHASE: ↑2° ↑O & P hormones
    2. SECOND PHASE: from growth & remodelling of uteroplacental vasculature to support placental development
    3. THIRD PHASE: ↑uterine a. VD to meet demands of rapidly growing foetus
  • ↑BF to uterine a. results from preferential ↑flow to Internal Iliac a. but ↓flow to External Iliac a. → KA ‘STEAL PHENOMENON’

Blood Flow Regulation

  • Pre-pregnancy → subject to pressure autoregulation
  • Pregnancy → Pressure Dependent BF
    • Uteroplacental circulation is widely VD low-resistance system whose perfusion is pressure-dependent
    • No ability for autoregulation

Uterine Perfusion Pressure = Uterine Arterial P – Uterine Venous P

↓Uterine BF due to:

  • ↓Perfusion P

↓Uterine Arterial P

  • Supine → aortocaval compression
  • ↓blood volume
  • Drug induced inc neuroaxial anaesthesia

↑Uterine Venous P

  • Vena cava compression
  • Uterine contractions
  • Sustained skeletal m. contraction (seizures, Valsalva)
  • ↑Vascular R
    • Endogenous + exogenous VC → catechols, vasopressin, AII

Vascular ∆ in Pregnancy

  • Pregnancy = ↓MAP to maintain Uterine BF
    • ↑Uterine Perf P
    • ↓Uterine R
  • ↑Vascular volume due to ↑O & P, which stimulate RAS
    • ↑effective circulation
    • Eventual ↑EPO will restore HCt
  • ↓Vasc R due to vascular remodelling & development of dilated placental circulation
    • ↑vessel diameter & length
R

∴↑radius = ↓R

  • Uterine A diameter is doubled by 21 weeks gestation
  • Steroid Hormones → O & P also came Uterine A VD & concomitant ↑Uterine BF
  • ↓response to vasoconstrictors inc. NA, Adrenaline & AII especially in uterine circulation of mother