Pulse Oximeters measure the percentage of arterial oxyhemoglobin (hemoglobin saturated with oxygen) and changes in blood volume in the skin, producing a photoplethysmograph. Measurements are taken from a probe that attaches to a finger. Other probes are available to take readings from more central locations but are reserved to institutional use for the most part.
As prices for pulse oximeters have come down, there are more and more in the hands of home oxygen users. Do people really know what these units are telling them?
Most people are unaware that ventilation (removing CO2 from the body) drives our breathing. Most of us do not breathe to add oxygen. There are some people (called CO2 retainers) whose breathing mechanism is driven by oxygen but more on them later. The pulse oximeter tells a person the level of oxygen but does not measure carbon dioxide levels. Oxygen can be added to improve oxyhemglobin but compromised lungs do not efficiently remove CO2. This means that during physical activity a person with unhealthy lungs is likely to experience a feeling of shortness of breath. This happens because the feeling is being generated by excess CO2 in the body. This is why we breathe harder and faster during exercise. The body creates more CO2 during exercise than at rest. This can confuse oxygen users who throw on their pulse oximeter only to read normal oxygen levels. Some may even turn up the oxygen volume, which for most isn’t dangerous. For the CO2 retainer turning up the oxygen volume can be deadly, though.
The CO2 retainers body has adapted to excess CO2 to the point that excess CO2 no longer drives their breathing. Their breathing becomes driven by oxygen levels. It is vital that these people do not add excess oxygen because it causes acidosis. Although unproven, the acidosis has lead to the belief that hyperoximia can suppress their breathing and in extreme cases they can cease breathing. When a person feels short of breath, their initial reaction is usually to want to turn up the oxygen volume. Known CO2 retainers should never turn their oxygen up beyond the prescribed amount even though shortness of breath in a CO2 retainer can be from low oxygen. For that matter, no oxygen user should exceed their prescription oxygen volume.
The pulse oximeter can be a valuable tool to spot check oxygen levels. Oxygen users can be comfortable knowing their oxyhemoglobin is between 90-94%. And if they experience shortness of breath while their oxygen levels are normal, they should be aware that it is probably from increased CO2.
Since pulse oximeters measure oxyhemoglobin in the blood of the skin, conditions that limit the pefusion of blood to the extremities will effect the accuracy of readings. Generally, if the pulse reading is accurate the oxygen measurement will be accurate. If you are questioning the reading, count your pulse rate and see if it matches the oximeter reading. If you have poor perfusion in your fingers (your fingers are always cold) warm the finger prior to taking a reading. You can hold your finger in your other hand, place it in warm water or wrap a warm towel around it. If that doesn’t help, you could consider an ear probe but that will significantly increase the cost of the pulse oximeter.