Sat27November0556PM 19

Which of the following options will give rise to falsely raised pulse oximeter (SpO2) readings?

(Please select 1 option)

Sickle cell disease

Anaemia

Prilocaine toxicity

Brown-red nail polish on finger

Heavy smoker Correct

Explanation

Standard pulse oximeters depend on spectral analysis non-invasively to measure oxygen saturation. The two physical principles of measurement are spectrophotometry and optical plethysmography.

Two wavelengths of light are emitted by diodes; these are 660 nm (red) and 940 nm (near infra-red). Oxyhaemoglobin and deoxyhaemoglobin have different absorbing capacities for these two wavelengths of light and it is these differences that are used to calculate oxygen saturation.

Heavy smokers may achieve carboxy haemoglobin (COHb) levels as high as 15-20%. COHb absorbs very little light at 940 nm, while at 660 nm its extinction coefficient is very similar to oxyhaemoglobin. Thus the presence of significant COHb will resemble the curve of oxyhaemoglobin in the red range, with no effect on the infrared, and "look like" oxyhaemoglobin, causing the pulse oximeter to over read. For every 1% of circulating carboxyhaemoglobin, the pulse oximeter over reads by 1%.

The metabolite of prilocaine o-toluidine can cause methaemoglobinaemia (MetHb). This can cause a pulse oximeter to under read, typically at 85% even if the true reading is 100%. If presence of these abnormal haemoglobins is suspected then the true oxygen saturation can be measured using a laboratory multiwavelength co-oximeter.

Anaemia with normal haemoglobins should not affect the SpO2.

Whilst sickle cell disease can cause hypoxia and a shift of the oxygen dissociation curve to the right, the general consensus is that pulse oximetry has acceptable accuracy for reliable clinical diagnosis of serious gas exchange abnormalities.

Pulse oximeters under read with brown-red nail polish.

Answer Statistics

1

3%

2

5%

3

36%

4

17%

5

41%

Times answered: 259