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What is an Oximeter and how does it work?
 

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Pulse oximeters are wonders of technology which have gotten better and better over the last 15 years. They measure the interference by blood cells with the transmission of light at various wavelengths. The presence or absence of oxygen on the red cells changes the interference the cells cause. From that interference, they can determine the degree to which hemoglobin on the red blood cell is loaded with oxygen ("oxygen saturation"). But some things must be remembered about oximeters and the readings they provide.

1. Oxygen saturation is not the same oxygen delivery. The best example of the difference is an extreme situation where a person has very little hemoglobin or very few red blood cells. The few that are available could be fully loaded (100% saturation) but the total amount of oxygen getting to the body would still be dangerously low. The heart rate would increase to try to deliver more blood, and thus more oxygen, to the body, but there is a limit to the impact this can effectively have.

2. Oximeters are susceptible to error because of anything that would interfere with the ASSUMED flow of blood and absorption of light by the skin and tissues. So, low blood flow in the finger where the pulse oximeter is attached can cause erroneous readings. 
So can dark skin pigmentation, very cold fingers (decreases blood flow), finger nail polish. Like any technology, pulse oximeters are neither foolproof nor 100% reliable. 
The ability of the person taking the readings to recognize any interfering conditions, and determining if the readings make sense WHEN COMPARED TO THE PATIENT'S CONDITION is critical.

3. Oxygen saturation is not a "one size fits all" measurement. Some people are uncomfortable and SOB with a saturation of 91%, others are fine with a saturation of 88% or less. Some bodies are able to adapt better (increase red blood cells and hemoglobin to carry more oxygen even though none of the cells are fully loaded, for example) than others.
4. The relationship between oxygen saturation and oxygen in the blood is not linear, but an "s-shaped" curve, and this curve can be reshaped by blood acidity, body temperature and other factors, so a particular saturation does not always mean the same level of oxygen in the blood.

All that said, do I think pulse oximeters are good things? Yes, but only when properly understood, and correlated to the particular person. I certainly don't advocate treating oneself based upon them alone. A little knowledge CAN be a dangerous thing.

As far as oxygenation problems versus ventilation problems, yes they are two different things and can present separately from each other.

When one says you have a ventilation problem, that means your lungs are having difficulty removing carbon dioxide (CO2) from your blood and body. Medically, ventilation is defined relative to CO2 elimination.

When one has an oxygenation problem, that means your lungs are having difficulty getting oxygen (O2) into your blood and body. Oxygenation is defined relative to O2 delivery and blood O2 levels. CO2 can move out of the blood to the lungs over 20 times easier than O2 can move from the lungs into the blood.


So, If you have an infection with lots of secretions that line the air sacs of the lung (alveoli), these secretions can make it hard for oxygen to get into the blood even though the carbon dioxide can get out of the blood easily (because they can move through the membrane between them more than 20 times easier). The problem may not be secretions but swelling and/or increased thickness of the membrane between the blood and the air sacs in the lungs. So your oxygen level could drop even though your carbon dioxide level is not abnormally (for you) high. You thus have an oxygenation problem without a ventilation problem.

On the other hand, you could have an infection or other problem that has been making breathing hard for days or weeks. Your chest and breathing muscles are dog tired, and they begin to loose the ability to struggle as hard. The work of the struggle produces more CO2 and the lungs can't physically get rid of it as effectively. Your CO2 levels begin to rise. Your lungs struggle and keep your O2 level up, but wear themselves down by doing so. CO2 is up, O2 is OK, you have a ventilation problem (CO2),not an oxygenation problem.

True, exclusively ventilation or oxygenation problems are not too common in emphysema/chronic lung patients, because at a certain level of impairment they go hand in hand. If CO2 continues to rise, it can actually "crowd out" O2 in the lung, causing an Oxygenation problem. But they can occur singly. The nature of a crisis can be one or the other.

The above scenarios are overly simplified for the purpose of brevity. The interplay between O2 and CO2 and the other body systems that come into play are very complex and can't be conveyed in a brief forum. But hopefully this helps you see the basic difference and where the physician was coming from.

Of course, none of this should be taken as medical advise or be said to replace appropriate care and visits with your physician. Only your doctor can tell you what all of these things means for YOUR particular case.

If I have totally confused you all, I apologize.
Larry Conway, RRT Tupelo, MS lconway@dixie-net.com 


Additional Information

Pulse oximeters do a number of different things to 'process' the incoming signal before 'reporting' it out on your LCD. The reported measurement is 'updated' frequently, usually on an interval of no more than a few seconds, maximum. Ronda Bradley may know more of this sort of details.

When considering the measurements from the oximeter, look at the 'trend', rather than any particular reported measurement. Ways to tell if the signal is reliable include, (as Doug has already pointed out) looking for (on the Nonin) a 'green' signal quality indicator light. If it is yellow or red, it is not reliable. Look for a pulse signal (on most oximeters) 
that is regular, or at least matches your rate and rhythm as you are able to ascertain it by feeling your own pulse, or having someone check it for you (while observing the pulse signal light for simultaneous pulse signals).

Barring the possibility that Neil was observing oximetry data that was the result of interference making it unreliable, what he reports is not a bit out of line with what might be expected, if he has any significant degree of lung disease. I encounter similar findings MOST of the time in the folks I work with in my rehab. program. Step-exercisers are a VERY high 'load' form of exercise and as such, result in a lot of oxygen utilization by the muscles. The heart rate increasing to 145/min. would not be unusual in view of that exercise. And, depending upon the state of one's heart-health also would not necessarily be harmful. Neil, you may repeat your test, to confirm reproducibility of your findings.

If indeed one finds that their oxygen drops to a level like the 87 % that Neil finds in himself, then you would be advised to do one of two things.
Either lighten the 'load' of the exercise - or pick and exercise with a lighter load - to avoid allowing your saturation to drop below 90 - 92 %.
The other possibility would be to use oxygen sufficient to keep your oxygen saturation > 
92 % during the exercise. It is not good, though, to exercise, especially vigorously, in the face of a saturation of 87 %. It puts too much strain on the heart and over a period of time of doing it, will hasten the onset of 'cor-pulmonale', or 'right-sided' heart failure (characterized by congestive heart failure and worsened hypoxia).

Being an advocate of aggressive management of COPD, I support the approach of tackling the highest exercise load you can 'sustain' ( to encourage development of both strength and especially endurance, which is time-dependent) and using all the oxygen you need to keep a desirable saturation level. I prefer 92 % or greater, or what ever level above that one finds they need to achieve the comfort necessary to complete the exercise period - - - keeping in mind that when you exceed 94 % you don't achieve any increasing advantage. (In other words, there is less 'relative' difference between 94 % and 99 % than there is between 94 % and 90 %, so there is no need to try to reach any higher than 94 % because it doesn't represent a significant increase in available oxygen.)

If you observe 'sudden' changes in the measurements reported by your oximeters, the most likely cause will be signal interference (artifact).
But in most cases, you should see a coinciding change in the signal quality indicator light to yellow or red. If not, that still doesn't mean that the measurement isn't erroneous. One of the technical problems of using oximeters during exercise is that you measure through your finger. Under many exercise circumstances, you must also use your hand to grip a support bar, or hand rail. This injects both movement artifact and changes in the blood circulating to your fingers - - which negatively influence accuracy of measurements. If you observe sudden changes in the measurements, such as Neil describes, then release your grip and try to hold your arm as still as possible to achieve a stable signal. This can take up to 15 or more seconds, at times, so be patient.

On the other hand, if you have a stable signal and reason to believe it is reliable, then check your pulse, of have someone check if for you. It is not uncommon for some people to develop heart rhythm disturbances, or 'arrhythmias' during exercise, and you would want to rule out that possibility.

The other point I forgot to make earlier was that contrary to what was suggested by a couple of folks, you don't need to hold your hand at heart level to achieve an accurate measurement. Pulse oximeters are not affected by orientation to body position as they work only on extraction of 'localized' measurements. On the other hand, if you hold your hand out-stretched ABOVE your head, you might decrease the circulation and therefore the signal strength, such that you make it difficult or impossible to obtain a measurement, or at least an accurate one. (So, don't do this!)

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