Special Circumstances

Fetal Hb (HbF)

HbF causes a left shift of the oxy Hb curve, as its P₅₀ is 2.5 kPa vs a P₅₀ of 4.0 kPa for adult Hb. This results from the reduced interaction with 2,3-DPG (adult Hb binds O₂ more avidly in the absence of 2,3-DPG). Additionally HbF exists at a higher concentration of 180 g/L to augment oxygenation.

Methaemoglobin (MHb)

MHb is the oxidized Fe³⁺ form of Hb and lacks the electron needed to permit O₂ binding, thus is incapable of oxygen transport. Due to continuous exposure of RBCs to various oxidant stresses, blood normally contains approximately = 1% MHb levels.

Learning bite

When approximately 50 g/L of Hb is at least 85-90% saturated i.e. deoxygenated, cyanosis occurs giving a dark blue colour to the skin. Consequently patients who are moderately to severely anaemic may be hypoxaemic without being cyanotic.

MHb level is kept in check by the enzyme MHb reductase (cytochrome b5 reductase) which returns the Fe³⁺ to the Fe²⁺ state. This enzyme may be congenitally absent or oxidizing drugs and chemicals may overwhelm its function.

Substances with the potential to cause methaemoglobinaemia include:

• Sulphonamide antibiotics (eg. Trimethoprim, Dapsone)
• Local anaesthetics (eg. Lignocaine/Prilocaine)
• Metoclopramide
• Aniline (found in many industrial chemicals)
• Chlorates, Bromates & Nitrates (found in fertilisers, pesticides)

Treatment of methaemoglobinaemia is with methylene blue.