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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 at 4 is obtained.
The R value at 4 intersects the inappropriate original R curve at 5
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.