An oximeter is a device that measures the amount of oxygen in your arterial blood. Most oximeters clip onto a finger and send two different wavelengths of infrared (950 nm) and near infrared light (650 nm) through that finger. Your blood absorbs different wavelengths of this light depending on how much oxygen is in it and by measuring how much light at each wavelength is transmitted and how much is absorbed an oximeter can estimate the oxygen level.
Your finger has both arteries and veins however, and there is less oxygen in venous blood than there is in arterial blood. An oximeter is able to differentiate between arterial and venous blood because the amount of the light that is transmitted through your finger varies with your pulse. Less light is transmitted when the amount of blood in your finger increases and this is also when there is more arterial blood. The oximeter uses this fact to subtract the low oxygen reading it gets when there is more venous blood in the light signal and at the same time it can also measure your heart rate. This is why this type of oximeter are called pulse oximeters.
Why should you measure the oxygen levels in your blood?
Breathing is all about getting oxygen into your bloodstream and carbon dioxide out. This is called gas exchange and lung disorders that are as different as COPD and Pulmonary Fibrosis can affect the ability of your lung to perform this function. There are a number of reasons why it’s much easier for our lungs to get carbon dioxide out of our bodies than it is to get oxygen into them but when we feel short of breath this is usually because of the amount of carbon dioxide in our bodies is high and not because our oxygen levels are low. This is why we need assistance in order to know what our oxygen levels are.
If you have a gas exchange problem then you may have a normal oxygen level while sitting quietly but as soon as you stand up and start exerting your muscles the need for oxygen increases dramatically. The human body has a very limited ability to store oxygen and if you have a gas exhange problem you will quickly use up these reserves and then be unable to replenish them. This is why you may need supplemental oxygen if you have a problem with gas exchange.
The amount of oxygen that muscles need will also vary according to how much work you are performing and this is why climbing stairs requires a lot more oxygen than does walking on level ground. For this reason if you use supplemental oxygen you should monitor your oxygen levels regularly and learn to adjust the amount of oxygen you are receiving according to what you are doing.
What’s a normal oxygen level?
An oximeter measures oxygen levels as the percent of oxygen saturation. Hemoglobin carries almost all of the oxygen that is present in blood and the percent of oxygen saturation refers to how much oxygen is being carried by the hemoglobin. Oxygen saturation can never be above 100% and a normal oxygen saturation is usually considered to be 95% or above.
Oxygen saturation levels below 90% are usually considered to be abnormal and when blood oxygen is this low it can cause a significant increase in stress on the heart, lungs and cardiovascular system. Notably, Medicare will pay for supplemental oxygen if your resting oxygen saturation is 88% or if it drops to 87% or less with exertion and most insurance companies follow Medicare’s lead in this.
There are reasons however, why an oxygen saturation that is 90% or above may be considered adequate. One of these is that giving too much supplemental oxygen can cause some individuals to hypoventilate and retain too much carbon dioxide in their blood. Another reason is that there are limits to how much supplemental oxygen can be provided by a portable system of any kind. Any prescription for supplemental oxygen has to balance a variety of factors and as long as it is able to keep oxygen saturation above 90% this is likely acceptable.
Should you have your own pulse oximeter?
If you are using supplemental oxygen then you should probably have a pulse oximeter.
If you don’t have supplemental oxygen but you do have a lung disorder that affects the ability of your lung to exchange gases (which includes COPD, Pulmonary Fibrosis and Sarcoidosis), then a pulse oximeter may be useful in order to monitor your condition and can alert you and your physician to a need for supplemental oxygen not seen in routine office visits.
Although individuals with Asthma and Cystic Fibrosis don’t necessarily have a gas exchange problem airway obstruction can prevent adequate lung ventilation and for this reason a pulse oximeter may be useful, particularly for individuals with the more severe forms of these disorders or during an exacerbation.
How do you get a pulse oximeter?
Although medical insurance will pay for supplemental oxygen it is unlikely to pay for an oximeter. The costs of these devices however, have decreased markedly during the last decade. Although this is not a commonly stocked item in most pharmacies they are readily available for ordering on the internet (prices on Ebay and Amazon start in the $10-$20 range and go up from there. Hospital quality oximeters start in the $300-$400 range and go up from there).
Other things to consider:
Pulse oximeters are affected by circulation and poor peripheral circulation is probably the most common cause of falsely low readings. If you have cold hands or know that you have poor peripheral circulation then you will need to take the time to warm your hands completely before attempting to measure your oxygen saturation.
Pulse oximeters cannot measure carbon monoxide and in fact will give a falsely elevated oxygen saturation reading when carbon monoxide is present. Cigarette smokers routinely have elevated carbon monoxide levels of 3% to 5% in their blood, and levels as high as 10% are not impossible. For this reason the oxygen saturation readings made on smokers with a pulse oximeter are often higher than they really are and cannot be relied upon.
Pulse oximeters can also be affected by motion and by fingernail polish.
PFT Patient by Richard Johnston is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License.