Core physiology: When the pulse oximeter and blood gas disagree!

Question.

A normally well 17yr old woman is noted to have marked peripheral cyanosis. Pulse oximetry shows oxygen saturations of 79% though a follow up arterial blood gas records a normal Pa02. Which of the following conditions might explain the discrepancy between clinical signs, pulse oximetry and arterial PaO2?

    a.Carbon monoxide poisoning
    b.Methaemoglobinaemia

    c.Red nail polish

    d.Cyanide poisoning

    e.Peripheral vasoconstriction

 
Answer.

The answer is a. Methaemoglobinaemia

Explanation.

Pulse oximetry measures the ratio of oxygenated to de-oxygenated haemoglobin in arterial blood using their differential absorption of red and infrared light. It reports the result as a percentage oxygen saturation of a patient’s blood.

Pulse oximetry only measures oxygenated and deoxygenated haemoglobin and may give falsely high readings in the following circumstances:

  • Methaemoglobinaemia (MetHb). MetHb contains an oxidised form of haemoglobin, ferric (Fe3+) Hb which cannot bind oxygen. In the presence of MetHb, therefore, the average haemoglobin oxygen saturation is reduced causing cyanosis and low saturations as measured by pulse oximetry. Arterial blood gas analysis does not take into account the presence of methaemoglobin and so shows high PaO2 levels, reflecting the near full saturation of normal ferrous (Fe2+) Hb, even in the presence of cyanosis.
  • Carbon monoxide has a much higher affinity for haemoglobin than oxygen, causing a cherry red appearance of the skin and falsely high pulse oximetry readings.
  • Cyanide interferes with the dissociation of oxygen from haemoglobin in tissues, and while the high pulse oximeter readings in cyanide poisoning reflect the true state of haemoglobin oxygen saturation, they do not reflect at all the profound hypoxia occurring at the tissue level.

Falsely low pulse oximeter readings may arise from, motion artefact, venous congestion, tachycardia, poor tissue perfusion and opaque nail polish (of any colour!).

 

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