Anaemia

Master answer

Anaemia

• condition (acute or chronic) a/w deficiency of RBC or Hb in blood
• Low haemoglobin
  • Males <140g/L
  • Females <120g/L
• Eg. due to haemolysis, chronic disease, Fe-deficiency, Vitamin B12/folate deficiency, haemorrhage, etc.

Normal O₂ supply to tissues

Approx 1l/min

Consequence of anaemia

• ↓ O2 content of blood (CaO2), and thus ↓ O2 delivery to tissues (DO2)
• Risk of tissue hypoxia – Increase products of anaerobic metabolism
  • CO₂, Lactate
  • Decrease in pH as a consequence

Physiological Responses /
Compensatory Mechanisms that maintain Tissue Oxygenation

Effects on oxyhaemoglobin dissociation

• Increased PaCO₂, Decreased pH and Decreased O₂ cause right shift in Hb O₂ dissociation curve → ↑ Tissue O₂ extraction

• For given PO₂, Hb O₂ affinity is reduced → increased offloading of O₂ to tissues
• Decreased pH → Production of 2,3-DPG via Leubering-Rapaport Pathway and the enzyme BPG mutase
  • Net loss of 1 ATP per 2,3-DPG generated
  • Further right shift in the Oxyhaemoglobin dissociation curve

Cardiac Output Increases

• Blood flow and resistance governed by Poisuille-Hagen Equation
  • Hb significant factor contributing to blood viscosity
    • Anaemia → Decreased viscosity → Increased cardiac output
    • Decreased viscosity → decreased resistance to venous return → increased preload → increased cardiac output
• Stimulation of aortic chemoreceptors by decreased pH and increased CO₂
  • Increased signalling via vagus nerve NTS vasomotor sensory area → increase sympathetic output by anterolateral upper medulla → increased HR and contractility → Increased CO

Redistribution of blood flow

• To tissues where adequate O₂ delivery is critical
• esp heart and brain

Respiratory effects

• Increased CO₂ and decreased pH stimulates central and peripheral chemoceptors
  • Medullary respiratory centre increases minute ventilation
  • Increased respiratory clearance of CO₂
    • Alveolar CO₂ ∝ arterial CO₂ → increased PAO₂ and hence increased PaO₂

Renal compensation

• Decreased O₂ delivery to renal interstitium → Hypoxia Inducible Factor (HIF) → Erythropoietin → Stimulates erythropoiesis via EPO-R JAK/STAT signalling.

#Extra:
Techniques to maintain tissue O2 balance in a patient with anaemia:

• ↑ tissue O2 supply (or DO2) by:
  • ↑ C.O. → volume load to significantly ↑ C.O. as per Frank-Starling mechanism (preferably with PRBC to replace Hb also, but OK with colloids/crystalloids)
  • ↑ [Hb] → replace Hb with PRBC transfusion, and support ongoing haemopoiesis with haemotinic factors (Eg. Fe, vitamin B12, folate)
  • ↑ SpO2 and PaO2 → supplemental O2 (FiO2 100%)
• ↓ tissue O2 demand by:
  • Sedation, paralysis and artificial ventilation → ↓ muscle MRO2 a/w activity and respiration
  • Maintain normal core body temperature → avoid hypothermia and shivering 2° to hypothermia, which ↑ MRO2
  • Minimal use of inotropes to maintain C.O. → avoid ↑ cardiac MRO2

CICMWrecks 2016