• Wednesday April 8,2020

pulse oximetry

In pulse oximetry, the oxygen saturation of the arterial blood is determined in a non-invasive, photometric manner by attaching a clip with infrared light sources and a receiver on the skin of the patient.

This clip determines the light absorption of the blood using the fluoroscopic rate and, when converted to blood oxygen saturation, makes use of the fact that blood with different oxygen content has different brightness and absorbs light differently as a result. Although the measurement involves no risks or side effects for the patient, it is often subject to measuring errors, such as those resulting from poorly attached clips or lacquered fingernails.

What is the pulse oximetry?

Pulse oximetry determines the oxygen saturation of the arterial blood in relation to the pulse.

Pulse oximetry determines the oxygen saturation of the arterial blood in relation to the pulse. The method of measurement is a non-invasive, photometric and percutaneous procedure, which detects the degree of light absorption or photodermination by screening the skin. The oxygen content of the arterial blood refers to the loading of hemoglobin with oxygen.

Depending on the oxygen load, the hemoglobin absorbs light in different ways, so that conclusions about the oxygen content of the hemoglobin can be drawn on the qualities of light absorption. The determined data of the light absorption are converted in the pulse oximetry so in percentage oxygen content. The physician finally compares the calculated oxygen content with the reference values ​​and may make a diagnosis based on this comparison. Values ​​of 90 percent or less usually require medication. Values ​​of 85 percent are already alarming for the physician.

Function, effect & goals

For intensive care, emergency services and anesthesia, pulse oximetry is standard. Outside hospitals, mountain climbers and sports pilots sometimes use a pulse oximeter to control themselves at high altitudes and protect themselves from altitude sickness. The procedure also plays an increased role in the home care of premature births and in some cases also in nursing care cases.

In each pulse oximetry, a saturator in the form of a clip or adhesive sensor is attached to an easily accessible location on the body. Mostly the doctor attaches the clip to the toe or earlobe of the patient. On the one hand, the clip carries ending light sources in an infrared range. On the other hand, it is equipped with a photosensor, which assumes the role of a receiver. Since oxygen-saturated hemoglobin has a different brightness than oxygen-free, the fluoroscopy results in a different absorption rate, which is measured by the photosensor of the clip. At the same time, the clip detects the pulse in the capillaries in order not to measure in tissue but only in the arterial area.

In addition to the light absorption according to the Beer-Lambert-Bouguer law in the 660 nm range, the sensor measures the absorption in the 940 nm range. For the purpose of taring, measurements are also made without the radiation of the measuring light sources. A monitoring monitor compares the measured values ​​with a reference table and thus determines the percentage of oxygen saturation of the blood. Values ​​between 97 and 100 percent are considered healthy. A special method of pulse oximetry is cerebral pulse oximetry, which measures through the skull rather than on the skin. In this method, the transmitter and receiver are mounted on the forehead. The method can help the doctor to detect a lack of oxygen in the brain, which can sometimes take life-threatening proportions.

In the brain, a saturation of 60 to 70 percent is considered the norm, with older people also having lower saturations without disease. However, in cerebral pulse oximetry, 50 percent is considered an absolute lower limit. The blood oxygen measurement in brain-near regions plays a role, in particular, in surgery on vessels supplying the brain. If the blood oxygen drops alarmingly during such an operation, the doctor may need to interrupt the surgery to protect the patient.

Risks, side effects & dangers

As a non-invasive procedure, pulse oximetry is not associated with any risks or side effects for the patient. However, many sources of error can be present during the measurement. For example, if the peripheral circulation is poor due to shock or cold, it can significantly falsify the data obtained.

In addition, intoxications are among the most common sources of error in a pulse oximetry. For example, in the case of carbon monoxide poisoning, the pulse oximeter detects that the hemoglobin carries charge. Thus, although the hemoglobin actually transports carbon monoxide instead of oxygen, the oxygen content may be normal. Today, however, modern pulse oximeters are capable of detecting the CO-saturated content of hemoglobin and thus rule out these measurement errors. Even with modern devices, however, lacquered fingernails can falsify the test results since nail lacquers absorb light.

Only for purple and red paints, this does not apply in most cases, so that no serious measurement errors can be expected with painted nails of this color. In the case of acrylic nails, wrong values ​​are always to be expected. One last source of error is infrared heat lamps, which usually cause falsely low values. In high altitude or in the mountains, even uneven terrain may falsify the measurement data. In addition, since slipping or badly attached clips may give false results, the attachment of the probe should be done with the utmost conscientiousness.


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