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A pulse oximeter is a portable device that measures the amount of oxygen in red blood cells. It consists of a probe that attaches to the patient’s finger, toe, nose, or earlobe, and a computer unit that displays the reading. Accurate readings require proper placement of the probe, and it cannot distinguish between oxygen-bound and carbon monoxide-bound hemoglobin. It also does not measure blood carbon dioxide level or pH level, which requires an arterial blood gas test.
A pulse oximeter is a portable, non-invasive computerized device that allows health care professionals or caregivers to measure the amount of hemoglobin in red blood cells saturated with oxygen or O2. The device consists of a pulse oximetry probe that attaches to the patient’s finger, toe, across the bridge of the nose or earlobe, and the computer unit that displays the next reading, usually in numbers as a percentage. This reading is referred to as the patient’s “oxygen saturation level” or “O2 sat”. Normal values range from 95 to 99 percent in a healthy individual.
Most pulse oximetry probes are of the clip-on type, somewhat resembling an old wooden clothespin with springs. The probe measures the percentage of oxygenated hemoglobin using both types of red and infrared light. These beams of light are projected through thin vascular tissue from one side of the probe to a photodetector on the opposite side of the probe clip. The amount of light remaining after absorption by oxygenated hemoglobin is then measured by the photodetector, converted into a comparable oxygen saturation percentage, and displayed by the computer unit.
Accurate oxygen saturation readings require proper placement of the pulse oximetry probe. It must be locked with the light signal emitters directly in front of the photodetector to an area of tissue with adequate blood perfusion, which means that nutrient-rich blood from the arteries is delivered to the capillaries. A digital pulse oximetry probe may not provide valid readings when used on fingers with dark-colored nail polish or significant bruising under the nail bed. Movement of extremities with an oximetry probe attached often causes transient erroneous readings that can be ignored. These readings are different from low oxygen saturation readings demonstrated with patient activity exceeding the capacity for oxygenation or those suffering from sleep apnea.
Readings from an oximeter should not be considered accurate with demonstration of hypoxic symptoms such as cyanosis, rapid respiratory rate, or difficulty breathing. The pulse oximetry probe is a valuable tool, but it has limitations. Attempted use on a patient with known circulatory deficiencies or cardiac arrest will result in erroneous readings. Also, a pulse oximetry probe cannot distinguish between oxygen-bound hemoglobin and carbon monoxide-bound hemoglobin. Therefore, readings taken on a patient with suspected carbon monoxide poisoning will also be in error.
Finally, a pulse oximetry probe does not provide measurement of blood carbon dioxide level, acid-base (pH) level, and partial pressures of carbon dioxide and oxygen. These results are only available when an arterial blood gas (ABG) test is performed. Blood is drawn from a patient’s radial artery with a needle and a tube of blood is sent to the lab before readings are available. Prior to the development of the portable oximeter and pulse oximetry probe, this was the method used to obtain patient oxygen saturation levels.
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