6 - 10 Physiology SBAs for the Primary FRCA
Question 6
A 55 year old man in the emergency department has oxygen saturations of 100% on room air, however on blood gas analysis his arterial SaO2 is 85%.
Which of the following is the most likely cause ?
- Red nail polish
- Patent blue dye
- Methaemoglobinaemia
- Carboxyhaemoglobin
- Indocyanine green dye
Answer
- Carboxyhaemoglobin
Pulse oximetry is seriously useful and generally very reliable with numbers above 70%, however there are occasions when it can over, or underestimate, the patient's oxygen saturations.
The sats probe will give a falsely high reading in the presence of carboxyhaemoglobin.
The sats probe will give a falsely low reading in the presence of:
- Patent blue dye
- Indocyanine green dye
- Nail polish
- Methaemoglobinaemia
The reason we chose 85% for this question is as a reminder that methaemoglobinaemia will often give falsely low saturation readings of 85%, not arterial PaO2
Check out our full post on pulse oximetry here