When Non-Invasive Methods for Measuring Oxygen Saturation are Less Effective

Measuring oxygen concentration levels in the blood is an essential vital sign to track. It represents how well the body is oxygenating and can give a clue to the respiratory status of a patient.

Pulse oximetry is a popular method to obtain this information. It’s a cheap, non-invasive, and painless way to detect oxygen saturation. Its design and functionality are based on the rules and principles of spectrophotometry, which involves using a small device or probe that emits infrared lighting which penetrates the skin tissues. This will then pick up the percentage of hemoglobin that is saturated with oxygen. Hemoglobin is a protein that plays a crucial role in the transportation of oxygen. A healthy reading would be between 95% and 100%, but readings above 92% can be considered normal. These probes are typically placed on fingers or toes, but the earlobes and forehead are also top choices.

Another method is performing an arterial blood gas test, or ABG. This is certainly more expensive while at the same time requiring the invasive method of drawing out blood specifically from the artery using a syringe and needle. It has potential to be painful and does require more time and more steps, possibly involving a laboratory.

Although it appears that the best choice would be the pulse oximeter without question, there are valid reasons why drawing blood for an ABG is the preferred method. The biggest reason is accuracy. Because ABG uses the actual blood to test the levels of oxygen, it can also provide other critical readings like pH balances (or acidity) and CO2 levels which also play an important part of reflecting the ventilation and oxygenation status of the patient.

Many instances can make it possible for pulse oximetry to give inaccurate information as well. For starters, since it cannot differentiate between various forms of hemoglobin, it may reflect that a fire victim or a patient with carbon monoxide poisoning has healthy oxygen saturation levels. This happens because the oximeter is actually misinterpreting oxygen saturating the blood when carbon monoxide is actually present. What this can look like is an overestimated oxygen saturation level when in fact, the patient can be in severe respiratory distress. Another problem is that environmental factors such as certain lighting can affect the test. Similarity, there are a few patient factors that can alter the reading from a pulse oximeter. If a patient uses dark or brilliant fingernail polish, it will give false results. It is also common for patients with poor perfusion or blood circulation to give poor results. Body temperatures can play a big factor in inaccurate readings too.

If a physician needs accuracy first and foremost on a patient who is displaying signs of respiratory failure, he or she will likely seek the ABG for a better reflection of what the patient is doing. Otherwise, if the patient simply needs their oxygen saturation checked for surgery purposes, or other conditions that may be considered less emergent or severe, then a pulse oximeter may be sufficient.

Spencer Frandsen is a writer, father, and mountain man who lives in Salt Lake City.