Oxygen is transported from the lungs to the tissues of the body by hemoglobin. A pulse oximeter is a device that attaches to the skin and uses light wavelengths to measure the percentage of hemoglobin molecules carrying oxygen. Pulse oximetry may be used as an indirect marker of blood oxygen levels, but some important disadvantages need be considered.
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Limited Range of Precision
A wide variety of pulse oximeter models are available from several different manufacturers. A comprehensive 1999 review of pulse oximeters published in “Critical Care” noted that, on average, oximeters are accurate within 2 percent for oxygen saturations of 90 percent or more. Oximeters are far less reliable, however, once the oxygen saturation falls below 80 percent.
Ineffective in Poor Perfusion States
Pulse oximetry works by measuring the amount of oxygenated hemoglobin in the blood. It does not, however, detect whether or not enough blood is actually moving through the tissues. In states of low perfusion, such as shock, hypothermia and vasoconstriction, the oximeter probe may read a normal hemoglobin saturation level even though there is not enough blood moving through the area.
Ineffective in Anemia
When hemoglobin levels in the blood are low due to anemia, or loss of red blood cells, the pulse oximeter is not an accurate measurement of blood oxygen levels. The existing hemoglobin molecules may all be filled with oxygen, giving a high oximeter reading, but there are not enough hemoglobin molecules to carry the needed oxygen to the tissues.
Carboxyhemoglobin and Methemoglobin Interference
Carboxyhemoglobin is hemoglobin bound to carbon monoxide, a dangerous gas produced by things like fires and malfunctioning furnaces. It binds to hemoglobin irreversibly and prevents oxygen from binding. Methemoglobin is another type of altered hemoglobin that occurs with certain medications and medical conditions. According to MedlinePlus, both of these conditions cause a decrease in the total amount of oxygen carried by hemoglobin, but neither are detected on pulse oximetry, because oximeters only measure plain unbound hemoglobin and oxygenated hemoglobin, and cannot tell how many hemoglobin molecules have been converted to carboxyhemoglobin or methemoglobin.
Because the pulse oximeter works by sensing light wavelengths, anything that interferes with the light spectrum can prevent proper functioning of the sensor. Pulse oximeters are commonly placed on fingers or toes, and things like false nails or nail polish can interfere with the light sensor. These problems can be overcome by attaching the sensor to an earlobe or, on children and smaller persons, around the wrist. Intravenous dyes used in medical imaging studies can also interfere with the light wavelengths.
Similar to interference from colors applied to the skin or fingernails, skin pigmentation may also interfere with a pulse oximeter. A 2007 study in “Anesthesia and Analgesia” confirmed previous reports that a pulse oximeter is more likely to overestimate the oxygen concentration at low saturations in individuals with darker skin.
When a patient vigorously moves the extremity that the oximeter sensor it attached to, it can interfere with the wavelength signals and cause the oximeter to sound the low oxygen alarm. According to a 2005 study in the “American Journal of Critical Care,” all tested pulse oximeters suffered from this problem despite efforts by manufacturers to limit the impact of motion.