G5i: Cardiovascular responses to anaemia

Definition

Anaemia = ‘lack of blood’

  • ↓ Hb content in arteries ∴O2 carrying capacity of blood

This affects:

CaO2

    • CaO2 = O2 bound to Hb + O2 dissolved

          = [[Hb] x SpO2 x 1.34] + (0.03 x PaO2)

    • Large proportion of O2 transported bound to Hb
    • ∴↓Hb = ↓O2 carrying capacity of blood

DO2

  • DO2 = CaO2 x CO
  • ∴↓CaO2 = ↓delivery of O2 to organs

Compensatory Mechanisms

  • Aim to maintain O2 supply:
    1. ↑CO
    2. Redistribution of CO
    3. ↑O2 extraction
    4. Late changes

1. ↑ CO

  • CO is proportional to [Hb] → half Hb = double CO
  • CO = HR x SV
  • ↑CO achieved by
    • ↓Viscosity
    • ↑Symp. stimulation

Viscosity

  • Loss of RBC = ↓viscosity of blood
  • HP equation of FLOW = ↓
  • ∴↓ viscosity = ↑flow
  • ↑flow = ↑VR → via F – S → ↑CO

Symp. stimulation

  • ↑anaerobic metabolic products (↑H+, lactate, K+)
  • ↓SVR
  • MAP = CO x SVR
  • Detected by High Pressure Baroreceptors
  • Vasomotor Centre Medulla : ↑symp. & ↓parasymp. outflow
    1. Heart → ↑HR, ↑FoC
    2. Resistance Vessels → VC → ↑SVR
    3. Capacitance vessels → VC → ↑VR
  • Overall ↑CO → restore MAP

2. Redistribution of CO

      • Redistribution to tissues with higher extraction ratios i.e. brain, heart
      • Kidney tolerates ↓RBF because has v. high perfusion in excess of metabolic requirements (for GFR)
      • ↓RBF → oliguria → minimise intravascular volume loss

3. Improved O2 Extraction (VO2)

FICK EQUATION: VO2 = Q (CaO2 – CvO2)

∴↓CvO2 will ↑VO2

  • ↑H+ → Hb proteins release their O2 (KA BOHR EFFECT)
  • ↓CvO2
  • BOHR EFFECT results in R) shift ODC

4. Late Changes

    • ↑2,3 DPG → R) shift ODC → ↑O2 offloading & improved O2 extraction
    • ↑EPO production to ↑Hb
    • Kidneys retain more Na+ & H2O → ↑ blood volume to ↑CO