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
|