Pulse Oximetry

What is it?
A microprocessor calculating % of saturated oxygen in each pulse of arterial blood flowing past a sensor.
Calculates heart rate at same time.

How does it work?
A low-intensity light is beamed from an LED to a photodiode.
Two beams (one red, one infrared) pass through blood and body tissue, partially being absorbed.
- absorption differs for saturated and unsaturated blood.
- the microprocessor measures this difference and reports it as % saturation.

Is it reliable?
Not when there is poor peripheral perfusion.
- eg vasoconstriction, hypotension, upstream BP cuff, hypothermia
Not when there is severe anaemia.
High carboxyhaemoglobin, mehaemoglobin and circulating dye may interfere.
Movement, bright light, dirty skin or other devices (eg diathermy) may interfere.

How to use one?
Attach
Compare HR with known / ECG monitor.
If sats are low or poorly read, look for a physiological cause first.

How do sats compare with PaO2?
- remember it does not equate with PaO2.
- rough guide:
PaO2
Sats
90mmHg
100%
60mmHg
90%
30mmHg
60%
27mmHg
50%
Note
This is a non-linear relation; small PaO2 changes cause large saturation changes.

Note
Variables shift the relationship between PaO2 and saturation:
- ie right shift --. greater delivery of O2 to tissues
Left (SaO2:PO2 greater) Right (SaO2:PO2 lower)
Incr pH Decr pH
Decr Temp
Incr Temp
Decr PaCO2
Incr PaCo2
Decr 2,3 DPG
Incr 2,3 DPG