A pulse oximeter is a portable, noninvasive computerized device that allows healthcare workers or patient caretakers to measure the amount of hemoglobin in the red blood cells saturated with oxygen, or O2. The device consists of a pulse oximetry probe that attaches to a patient's finger, toe, across the bridge of the nose or on the earlobe and the computerized unit that displays the subsequent reading, usually in numbers as a percentage. This reading is referred to as a 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 a clip-on style, somewhat resembling an old-fashioned wooden clothes pin with springs. The probe measures the percentage of oxygenated hemoglobin using both red and infrared types of light. These light beams are projected through thin, vascular tissue from one side of the probe to a photodetector on the opposite side of the probe's clip. The amount of light remaining after absorption by the oxygenated hemoglobin is then measured by the photodetector, converted to a comparable oxygen saturation percentage, and displayed by the computerized unit.
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Accurate oxygen saturation readings require correct placement of the pulse oximetry probe. It must be clipped with the light signal emitters directly across from the photodetector to an area of tissue with adequate blood perfusion, which means that nutrient rich blood from the arteries is being delivered to the capillaries. A digital pulse oximetry probe cannot provide valid readings if used on fingers with dark-colored nail polish or significant bruising under the nail bed. Movement of the extremities with an attached oximetry probe will often cause transient erroneous readings that may be disregarded. These readings are different from low oxygen saturation readings demonstrated with patient activity that exceeds oxygenation ability or those suffering from sleep apnea.
An oximeter unit's readings should not considered accurate with demonstration of hypoxic symptoms such as cyanosis, a rapid respiratory rate or difficulty breathing. The pulse oximetry probe is a valuable tool, but it does have limitations. Attempted use on a patient with known circulatory deficiencies or cardiac arrest will result in erroneous readings. Further, a pulse oximetry probe cannot differentiate between hemoglobin bound to oxygen versus hemoglobin bound to carbon monoxide. Thus, readings taken on a patient with suspected carbon monoxide poisoning will also be incorrect.
Lastly, a pulse oximetry probe does not provide measurement of the blood's carbon dioxide level, the 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 withdrawn from a patient's radial artery with a needle and a test tube of blood is sent to the laboratory before readings are available. Before the development of the portable oximeter and the pulse oximetry probe, this was the method used to obtain patient oxygen saturation levels.