Monitors

Pulse oximeters provide an easy way of partly assessing someones breathing by measuring the oxygen saturation of arterial blood.

They're often used, but not everyone expected to use them has had adequate training. The aim of the pulse oximetry section of this site is that if someone has read through it and understood the contents then they should be able to safely record an oxygen saturation as part of respiratory observations in most situations.

UNDERSTANDING PULSE OXIMETERS

What is oxygen saturation? what does it mean? and how do pulse oximeters measure it? and also at the limitations of pulse oximeters.

Some basic knowledge of anatomy and physiology is assumed - for instance the idea that blood carries oxygen from the lungs to the rest of the body through arteries and the blood then returns through veins.

It is not a complete reference to every aspect of pulse oximetry, in particular please note my experience is with adults and this web site refers to pulse oximetry with adults.

Some pages have a 'more depth' link on them, these links provide more information for learners who are interested in that point.

Pulse oximetry

Oxygen is carried in the blood attached to haemoglobin molecules. Oxygen saturation is a measure of how much oxygen the blood is carrying as a percentage of the maximum it could carry.

One haemoglobin molecule can carry a maximum of four molecules of oxygen, if a haemoglobin molecule is carrying three molecules of oxygen then it is carrying 3/4 or 75% of the maximum amount of oxygen it could carry.

One hundred haemoglobin molecules could together carry a maximum of 400 (100 x 4) oxygen molecules, if these 100 haemoglobin molecules were carrying 380 oxygen molecules they would be carrying (380 / 400) x 100 = 95% of the maximum number of oxygen molecules that could carry and so together would be 95% saturated.

Oxygen saturation is also refered to as SpO2.

What is oxygen saturation?

People always want to know what their oxygen saturation "should be". I hate giving figures (what should your blood pressure be?) but a fit, healthy young person will probably have an oxygen saturation of 95 - 99%. This will vary with age, degree of fitness, current altitude, oxygen therapy etc etc.

Pulse oximeters can either be used to take a 'one-off' reading from someone or can be left on for period of time. A single one-off reading often isn't much use, trends over a period of time give more information.

It is important to remember that pulse oximetry is only one way of monitoring breathing. It is also necessary, as a minimum, to record respiratory rate and if pulse oximetry is used the amount of oxygen they are receiving must be recorded. As with all clinical assessments the 'whole picture' must be looked at.

What does it mean?

The colour of blood varies depending on how much oxygen it contains. A pulse oximeter shines two beams of light through a finger (or earlobe etc.), one beam is red light (which you can see when a pulse oximeter is used), one is infrared light (which you don't see).

These two beams of light can let the pulse oximeter detect what colour the arterial blood is and it can then work out the oxygen saturation. However there are lots of other bits of a finger which will absorb light (such as venous blood, bone, skin, muscle etc.), so to work out the colour of the arterial blood a pulse oximeter looks for the slight change in the overall colour caused by a beat of the heart pushing arterial blood into the finger.

This change in colour is very small so pulse oximeters work best when there is a good strong pulse in the finger (etc.) the probe is on. If the signal is too low the measured oxygen saturation may not be reliable and lower than this the pulse oximeter will not be able to work.

How does it work?