Pulse oximetry uses a noninvasive, photometric method to determine the oxygen saturation of arterial blood by attaching a clip containing infrared light sources and a receiver to the patient’s skin. This clip determines the light absorption of the blood based on the fluoroscopy rate and, when converted to blood oxygen saturation, takes advantage of the fact that blood with different oxygen content has different brightness and, as a result, absorbs light to different degrees. Although the measurement does not pose any risks or side effects to the patient, it is often subject to measurement errors, such as those that can result from poorly attached clips or painted fingernails.
What is pulse oximetry?
Pulse oximetry determines the oxygen saturation of arterial blood in relation to the pulse. Pulse oximetry determines the oxygen saturation of arterial blood in relation to the pulse. The measurement method is a non-invasive, photometric and percutaneous procedure that determines the degree of light absorption or remittance under fluoroscopy of the skin. The oxygen content of arterial blood refers to the loading of hemoglobin with oxygen. Depending on the oxygen loading, hemoglobin absorbs light in different ways, so that a conclusion about the oxygen content of hemoglobin can be drawn from the qualities of light absorption. The determined data of light absorption are thus converted into percentage oxygen content during pulse oximetry. The physician finally compares the oxygen content calculated in this way with the reference values and may make a diagnosis on the basis of this comparison. Values of 90 percent or less must usually be treated with medication. Values of 85 percent are already alarming for the physician.
Function, effect and goals
For the intensive care unit, rescue service and anesthesia, pulse oximetry is standard. Outside of hospitals, mountaineers and sport pilots at high altitudes sometimes use a pulse oximeter for self-monitoring, protecting themselves from altitude sickness. The procedure also plays an increased role in the home care of premature infants and, in some cases, nursing cases. During each pulse oximetry procedure, a saturation transducer in the form of a clip or adhesive sensor is attached to an easily accessible part of the body. Usually, the physician attaches the clip to the patient’s toe or earlobe. On one side, the clip carries terminating light sources in an infrared range. On the other side, it is equipped with a photosensor that plays the role of a receiver. Since oxygen-saturated hemoglobin has a different brightness than oxygen-free hemoglobin, fluoroscopy results in a different absorption rate, which is measured by the clip’s photosensor. At the same time, the clip detects the pulse in the capillary vessels in order not to measure in tissue, but exclusively in the arterial area. In addition to light absorption according to the Beer-Lambert-Bouguer law in the 660 nm range, the sensor also measures absorption in the 940 nm range. For taring purposes, measurements are also made once without the radiation from the measuring light sources. A monitoring monitor compares the measured values with a reference table to determine the percentage oxygen saturation of the blood. Values between 97 and 100 percent are considered healthy. A special pulse oximetry procedure is cerebral pulse oximetry, which measures through the skull instead of on the skin. In this method, the transmitter and receiver are attached to the forehead. The method can help doctors detect a lack of oxygen in the brain, which can be life-threatening in some circumstances. In the brain, a saturation of 60 to 70 percent is considered the norm, although older people may have lower saturations without disease value. However, 50 percent is considered the absolute lower limit for cerebral pulse oximetry. Blood oxygen measurement in regions close to the brain plays a role especially during surgery on vessels supplying the brain. If blood oxygen drops alarmingly during such an operation, the physician may have to interrupt the surgery to protect the patient.
Risks, side effects, and hazards
As a noninvasive procedure, pulse oximetry does not pose any risks or side effects to the patient. However, many sources of error may be present during measurement.If peripheral blood flow is poor due to shock or cold, for example, this can significantly distort the data obtained. In addition, intoxications are one of the most frequent sources of error in pulse oximetry. In the case of carbon monoxide intoxication, for example, the pulse oximeter detects that the hemoglobin carries charge. This can result in normal values for the oxygen content, although the hemoglobin actually transports carbon monoxide instead of oxygen. Today, however, modern pulse oximeters are capable of determining the CO-saturated portion of the hemoglobin as well, thus eliminating these measurement errors. Even with modern devices, however, painted fingernails can falsify the test results, since nail polish absorbs light. Only for purple and red polishes this does not apply in most cases, so that no serious measurement errors can be expected with polished nails of this color. With acrylic nails, on the other hand, incorrect values are always to be expected. A final source of error is infrared heat lamps, which usually cause falsely low values. During high-altitude flights or in the mountains, uneven terrain can also falsify the measurement data under certain circumstances. In addition, since slipping or poorly fastened clips can give erroneous results, the greatest care should be taken in fastening the probe.
