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| Pulse oximeter |
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What is oxygen saturation? What does it mean? How does it work? What is oxygen saturation? 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 does it mean? 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. How does it work? 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. Limitations Pulse oximeters rarely cause any harm directly, though apparently some older models could cause burns and there are reports of the probes causing pressure ulcers. However if their limitations are not borne in mind harm could be caused by someone having the wrong (or no) treatment. So it's important to know what the limitations are. One limitation is that pulse oximeters cannot operate reliably with a poor signal. This has been refered to as a 'safe' limitation, in that the pulse oximeter is not able to give an accurate reading but in some way indicates this fact. Obviously pulse oximeters need to be technically capable of indicating this and the person using it must be aware of this point. The other sort of limitation is more dangerous in that the pulse oximeter may appear to have a good signal and be displaying a saturation figure, but either the figure is inaccurate or gives a false sense of security. Poor signal Pulse oximeters need a strong regular pulse in the finger (or ear etc) that the probe is on. A common problem is that people can have cold hands and feet, and have only a very weak pulse. In this case a pulse oximeter may display a reading but it might not be accurate. Some pulse oximeters have a means of indicating how strong the signal is they are receiving and it is important to check this. A still weaker signal may mean the pulse oximeter is not able to work at all. An irregular signal can also cause problems for a pulse oximeter trying to determine oxygen saturation. This can be caused by an irregular heart beat or by the patient moving, shivering or fitting. Poor positioning of the probe can cause inaccurate readings due to various problems. This can be a particular problem with very small fingers and very large ones. Make sure the probe is well on the finger. Carbon dioxide A pulse oximeter can cause a false sense of security by giving a good saturation figure when someone's breathing is completely inadequate. This is especially true when a patient is getting supplementary oxygen. There are two main functions of breathing, one is getting oxygen out of the air and into the body, the other is getting carbon dioxide out of the body and into the air. It possible for someone to be getting enough oxygen into their body but not be getting rid of enough carbon dioxide. Oxygen saturation by itself does not tell the whole story about breathing - this is especially true if someone is being given oxygen. As a minimum it is also necessary to record the respiratory rate, and if they are having oxygen, how much they are having. Delays There will be a delay between an event such as a patient taking off an oxygen mask and the subsequently less oxygenated blood passing through the finger the probe is on. It has been reported that there will be a longer delay if the pulse oximeter probe is attached to a toe compared with a finger or ear. Another point is that pulse oximeters average signals over a period of time, this will cause a delay in giving a new (real) oxygen saturation. Carbon monoxide Carbon monoxide is a colourless, odourless gas that is produced in most fires. Breathing in carbon monoxide will lead to it becoming attached to haemoglobin in preference to oxygen, so it is only necessary to breath in a small amount of carbon monoxide to have a large amount of haemoglobin taken up by it and therefore not available to carry oxygen. For instance if 25% of someone's haemoglobin is taken up by carbon monoxide then only 75% is available to carry oxygen, and so their oxygen saturation could, at best, be only 75%. Pulse oximeters will display an oxygen saturation which is approximately equal to the percentage of haemoglobin combined with oxygen plus the percentage of haemoglobin combined with carbon monoxide 1. So if someone has 25% of their haemoglobin saturated with carbon monoxide and a true oxygen saturation of 70% a pulse oximeter will display an oxygen saturation of about 95%. This is obviously extremely dangerous and for this reason pulse oximeters should not be used with people who may have inhaled smoke, ie anyone who has been involved with any sort of fire, unless you are certain that they do not have any significant level of carbon monoxide in their blood. Methaemoglobin Methaemoglobin is an abnormal type of haemoglobin that does not bind oxygen well. Normally 1-2% of people's haemoglobin is methaemoglobin, a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems. A higher level of methaemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. For more information see: Moyle JTB (2002) Pulse Oximetry 2nd ed. BMJ books, London Skin pigmentation Previous research on pulse oximetry has found that skin pigmentation has no clinically significant effect 1,2. Recently however it has been found that pulse oximeters from three manufacturers overestimated oxygen saturation in individuals with darkly pigmented skin at saturations below 80% 3. The authors suggested that pulse oximeters should carry a warning about this. Above 80% the researchers concluded that the effect was probably of no clinical significance. Other issues Nail varnish - Some research has shown that dark nail varnish bias pulse oximeter readings, but not by a clinically significant amount 1,2. However there are limitations to this research such as a lack of trials at lower saturations, so in practice nail carnish should still be removed. Intravenous dyes (such as methylene blue, indigo carmine and indocyanine green) can cause inaccurate readings 3. External interference. There have been reports of interference with pulse oximeter readings, for example from a nerver stimulator 4 and from a fluorescent light 5. Bilirubin There is evidence that bilirubin does not affect the accuracy of pulse oximetry and pulse oximeters are still accurate in the presence of severe hyperbilirubinaemia 1. Anaemia In theory there is no reason anaemia should cause pulse oximeters to be inaccurate and experiments in dogs have shown accuracy reliable enough for "clinical purposes" (Moyle 2002:49)2 as long as the packed cell volume was larger than 15%. Clinical experience has shown good performance with haemoglobin as low as 2.3 g/dl 2. Sickle cell disease It has been shown that pulse oximetry can detect low oxygen saturation accurately in sickle cell disease 3, however other researchers have suggested caution in interpreting pulse oximetry values in sickle cell disease. |