Pulse Oximeter Sensor Accuracy
Pulse oximeter calibration is achieved
during the design phase by carrying out clinical breath down trials with a
sensor of a particular specification. The SATs values of volunteers are dropped
by using controlled mixtures of oxygen and nitrogen. The R value results from
the pulse oximeter are compared with blood samples analysed with a co-oximeter.
The results of the breath down trials are commonly referred to as the "R curve".
This data is stored in the monitor in software form.
The R curve is a
fixed curve for a particular combination of red and infra red wavelengths and
enables values of R to be read off as SATs values.
Using a sensor with 664.5nm / 907 nm produces a R value at 1
The R value at 1
intersects the R curve at 2
A corresponding SATs value is read off at 3
If another sensor with different and erroneous wavelengths of, for example,
657nm / 907nm is used simultaneously on the same patient - a different R value
The R value at 4
intersects the inappropriate original R curve
A corresponding but incorrectly low SATs value
is read off at 6
Similarly if erroneous
wavelengths of 670nm / 907nm were used the value of R would be lower and an
incorrect high SATs value would be obtained.