Pulse oximetry measures oxygen in the blood

With pulse oximetry, the doctor can monitor the level of oxygen in the blood. The glowing red clip on the finger or ear is not only part of the routine during operations

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This finger clip helps monitor vital functions

© Corbis / Sciencephoto

What is a pulse oximeter? What oxygen levels do we need?

Lack of oxygen can be fatal. It usually arises when the person concerned cannot breathe enough, either due to illness or the effects of medication or drugs. Doctors cannot tell the lack of oxygen with the naked eye until the skin and mucous membranes turn blue (cyanosis). A sufficient supply of oxygen is vital for our cells. The brain in particular reacts quickly to an insufficient supply of oxygen.

A lack of oxygen can be discovered earlier if the so-called oxygen saturation of the blood is measured with technical aids. It indicates what percentage of the red blood pigment hemoglobin is loaded with oxygen. Normally it is around 93 to 99 percent. For a long time, this measurement was only possible by examining blood samples from an artery. Pulse oximetry has been used in routine clinical practice as a non-invasive procedure since the 1980s.

A pulse oximeter is a measuring device that measures the oxygen saturation of the blood and can therefore quickly give an indication of the oxygen supply to humans.

Because the pulse oximetry also records and displays the pulse rate, the doctor also has a clue about the circulatory function, i.e. the heartbeat and at least to some extent about the blood pressure - because the device would not be able to take a measurement without sufficient blood pressure.

When is pulse oximetry necessary?

Pulse oximetry is necessary whenever a patient is given medication that can affect their breathing and consciousness. This is common with any anesthesia or sedation. With puloximetry, the doctor can also easily see whether there is a lack of oxygen in acute lung diseases such as pneumonia or an asthma attack. Small, portable devices for the rescue service have also been around for a long time. In addition, pulse oximetry helps in the sleep laboratory, for example, in diagnosing obstructive sleep apnea syndrome.

There are special pulse oximeters for measurements in low saturation ranges below 70 percent. For example, they are used on children with certain heart defects. In the meantime, some mountaineers also use pulse oximeters. Because a drop in oxygen saturation can warn you of an impending altitude sickness. And even sports pilots sometimes use pulse oximeters when flying at high altitudes.

How does pulse oximetry work?

Pulse oximetry uses the fact that the blood pigment shows different colors depending on its condition: Saturated, oxygen-laden hemoglobin is bright red and primarily absorbs red light. Unsaturated hemoglobin appears dark red to bluish and primarily absorbs light in the infrared range.

There is a light source on one side of the pulse oximeter. It emits red light with a wavelength of 660 nanometers and infrared light with a wavelength of 940 nanometers. There is a photodetector on the other side of the pulse oximeter. This detector measures how much light arrives on the other side of the finger or earlobe. From these measured values, a computer determines how much and what light blood and tissue have absorbed at the location of the measurement.

In the finger or the earlobe between the light source and the photodetector there are various tissues as well as the blood in the veins and the blood in the arteries. They all absorb light. The only interesting thing is the amount of light that the blood in the arteries absorbs. The computer calculates it like this: The tissue and venous blood are always there. So you always take in a constant part of the light from the light source. This is called the background absorption. The heartbeat, on the other hand, pumps the blood in the arteries through the tissue in a pulsed manner. There is more arterial blood at the time of the pulse. Therefore, more light will be absorbed at this point. That is then the peak absorption. To determine the amount of light that the pulsating arterial blood absorbs, the computer subtracts the background absorption from the peak absorption. He also compares the absorption at 660 and 940 nanometers. From this measurement data, he then calculates the proportion of saturated and unsaturated hemoglobin in the arterial blood.

Modern mobile devices show saturation and pulse rate

© Fotolia / rdnzl

What are the limits or risks of pulse oximetry?

Pulse oximetry is non-invasive (non-intrusive) and easy to use. It can monitor the oxygen saturation in the blood continuously (continuously) and in real time without any risk to the patient. It reliably measures between 70 and 100 percent saturation.

However, the pulse oximetry fails if the blood flow is restricted at the location of the measurement. This can sometimes be the case with cold hands, but also with low blood pressure or even cardiovascular arrest. Movements of the hand also easily interfere with the signal transmission.

In addition, darkly painted or artificial fingernails as well as nail fungus can falsify the results. The pulse oximetry may also measure incorrectly in the event of poisoning with smoke gases (carbon monoxide, CO): Hemoglobin, the transport molecule for oxygen and carbon dioxide (CO2), becomes saturated with carbon monoxide when smoke gases or exhaust gases are inhaled, as carbon monoxide binds to hemoglobin very quickly. The "hemoglobin-carbon monoxide complex" has the same color properties as hemoglobin saturated with oxygen, which can lead to incorrect measurements. Good oxygen saturation readings are displayed, although oxygen saturation may be very poor. Certain drugs such as methylene blue or typical changes in hemoglobin such as methemoglobinemia can also falsify the measurement results. If such disorders are suspected, the doctor can check the oxygen saturation with an arterial blood gas analysis by taking blood from an artery.

The most common error messages and alarms during pulse oximetry are caused by technical problems, for example if the sensor has slipped or fallen off. For this reason, doctors or nurses first check whether the device is measuring correctly every time there is an alarm. This comparatively large number of "false alarms" can be exhausting and grueling, especially for patients or visitors to the intensive care unit. Overall, however, this monitoring method makes a significant contribution to patient safety and is an indispensable part of today's anesthesia and intensive care medicine.

Consulting expert: Dr. med. Julia Sadgorski, specialist in anesthesiology, Rotkreuzklinikum Munich

Swell:

  • Advanced Trauma Life Support (ATLS), 1st German edition, pulse oximetry, p. 17, p. 55, ed. ACS

Important NOTE:
This article contains general information only and should not be used for self-diagnosis or self-treatment. He can not substitute a visit at the doctor. Unfortunately, our experts cannot answer individual questions.

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