Pulse oximetry can under and over estimate oxygen saturation
About
- A life threatening cause of tissue hypoxia
- Due to Iron losing electrons (Oxidation) so Fe2+ -> Fe3+
- Fe3+ (ferric) iron cannot bind and transport oxygen.
- Caused by oxidizing agents
Aetiology
- The Hb of a red blood cell the haem group contains an iron molecule in the reduced or ferrous form (Fe2+). In this form, iron can combine with oxygen, by sharing an electron, to form oxyhaemoglobin. When haemoglobin is oxidised, it is converted to the ferric state (Fe3+) or methaemoglobin which lacks the electron needed to form a bond with oxygen.
- Therefore MetHb is incapable of oxygen transport. Because red blood cells are continuously exposed to various oxidant stresses, blood normally contains approximately 1% methaemoglobin levels. Methaemoglobin reductase (NADH dependant b 5 reductase) catalyses the reduction of Methaemoglobin to Haemoglobin
Precipitant
- Oxidizing agents: dapsone, phenacetin, sulfonamides, aniline dyes, lidocaine, nitrites and nitrates.
Clinical
- Cyanosed patient, SOB, Anxiety, seizures, coma
- Pulse oximetry can under and overestimate oxygen saturation
- 100% oxygen should be administered in significant cases
- Known or suspected G6PD deficiency is a contraindication to the use of methylene blue
Investigations
- CXR, FBC, U&E, Lactate
- ABG: Chocolate brown arterial blood that does not become red with exposure to air. ABG will reveal a normal arterial oxygen tension (Po2) and a metabolic acidosis proportional to the severity and duration of tissue hypoxia
Methaemoglobin levels
- < 1% Normal range
- 3-5% slight skin discolouration
- 15-20% cyanosis but no symptoms
- 25-50% breathless cyanosed headache
- 50% confusion is due to the oxidation of Fe2+ to Fe3+
Methylene blue is a MAOI and, when administered to a patient taking any other serotonergic drug, can lead to serotonin syndrome. Methylene blue should be used cautiously and judiciously in infants and patients with G6PD-deficiency, but is not contraindicated.
Management
- ABC, 100% oxygen, ITU
- Arterial blood gas analysis will typically reveal a normal arterial oxygen tension (pO2) and a metabolic acidosis proportional to the severity and duration of tissue hypoxia
- Aggressive medical therapy, including administration of methylene blue, should be considered for patients with overt signs of tissue hypoxia (not just cyanosis). Methylene blue is an exogenous electron donor and so reduces Fe3+ to Fe2+
- One hundred per cent oxygen should be administered in significant cases
- Known or suspected G6PD deficiency is a contraindication to the use of methylene blue
References