Pulse oximetry

Pulse oximeter as a finger clip, S _p O ₂ on the left, pulse rate on the right

The pulse oximetry or pulse oximetry is a method for non-invasive determination of arterial oxygen saturation by measuring the light absorption or the light reflectance on transillumination of the skin ( percutaneous ). The pulse oximeter is a spectrophotometer specially optimized for this application . In addition, the devices used are also used to monitor the pulse frequency at the same time .

history

The optical measuring principle of blood oxygen saturation was first described in 1935 by Karl Matthes in Leipzig on the human earlobe. This was followed in the 1940s by Glenn Allan Millikan , who developed the first optical oximeter. At that time, however, the technical prerequisites for practical implementation were not yet in place, for example only comparatively voluminous photocells were available as optical receivers , which is why practical further development for simple clinical applicability did not take place at first. It was not until 1972 that the Japanese bio-engineer Takuo Aoyagi succeeded in realizing it . In a parallel in-house development, this method was first used in Germany in 1976 under the name of photoplethysmography in psychophysiological research in a migraine research project, designed and applied by Christian-Peter Bernhardt, published in June 1978 at the University of Hamburg .

At the end of the 1990s, the Clinic for Anaesthesiology at the Medical University of Lübeck became Europe's leading reference center for the development and validation of pulse oximetry devices.

Measuring principle

Absorption ( extinction coefficient ) as a function of the wavelength of oxygenated hemoglobin (HbO2, in red) and deoxygenated hemoglobin (Hb, in blue). NIR = near infrared; Oxygenated blood absorbs less red and therefore appears red.

The optical measuring principle is based on the fact that hemoglobin loaded with O ₂ , so-called oxygenated hemoglobin (HbO ₂ ), has a significantly different absorption curve at optical wavelengths than deoxygenated hemoglobin (Hb) - i.e. hemoglobin whose transport sites for O _{2 are} still free . The absorption curve for both variants is shown in different colors in the diagram opposite. It can be seen that deoxygenated hemoglobin at a wavelength of approx. 680 nm - this corresponds to visible red light - has a significantly higher absorption than oxygenated hemoglobin. At longer wavelengths of approx. 800 nm upwards, this relationship is reversed : At approx. 900 nm - this wavelength corresponds to invisible near infrared radiation - the absorption of HbO _{2 is} higher and that of Hb is lower.

Simple, commercially available pulse oximeters measure the wavelength-dependent absorption at two different and unchanging wavelengths: Two monochromatic light-emitting diodes (LEDs) with different wavelengths are used in the measuring device to generate light . A red light-emitting diode at 660 nm and an infrared LED with 905 nm to 920 nm, which operate alternately. A photodiode is used as the receiver , the sensitivity of which covers the entire wavelength range. Alternatively, instead of the two light-emitting diodes, it is also possible to use two laser diodes which, in contrast to the light-emitting diodes, have a narrower optical bandwidth. In the case of laser diodes, the two wavelengths used are 750 nm and 850 nm for manufacturing reasons of the laser diodes and with the otherwise identical measuring principle.

Since the optical measurement can be disturbed by external light such as artificial light or daylight, the optical unit is located in a clip which largely shields external light. This clip is attached to an easily accessible part of the body, preferably to a finger , to a toe , to an earlobe or, in the case of premature infants, to the ball of the foot or to a wrist. The sensor shines through the relevant body part: The two light-emitting diodes are located on one side as a light source, and on the opposite side the photodiode as an optical receiver. The light emitted by the light-emitting diodes is modulated at a low frequency to improve interference suppression .

Since various factors affect the optical absorption, for example different types of tissue such as the skin also have an influence, an absolute value measurement is not practical. A relative measurement of the absorption rates takes place, which is set via the pulsatile blood flow in the measuring device. As a side effect, this also enables the pulse rate to be displayed. The heartbeat causes the arterial blood vessels to pulsate and thus the path of light through the arterial blood and thus the absolute absorption rates at a certain wavelength also change. The ratio of the maximum value of the light intensity to the minimum value can then be calculated for each heartbeat for each wavelength. If these values ​​are put in relation to one another, the result is independent of the light absorption of the surrounding tissue and is only determined by the ratio of oxygenated to deoxygenated hemoglobin in the arterial blood.

The oxygen saturation determined in this way is referred to as functional or partial oxygen saturation and S _p O ₂ . In order to clearly differentiate the measured value of this non-invasive, photometric determination from the arterial oxygen saturation determined on a blood sample taken by means of blood gas analysis , the p in this designation indicates the pulse oxymetric measurement method. The following relationship applies:

${\displaystyle S_{\mathrm {p} }O_{\mathrm {2} }={\frac {HbO_{\mathrm {2} }}{HbO_{\mathrm {2} }+Hb}}}$

with HbO ₂ for the concentration of oxygenated hemoglobin and Hb for deoxygenated hemoglobin.

Based on a comparison of the measurement result with a reference table, the evaluation electronics in the pulse oximeter determine what percentage of oxygenated hemoglobin is present. Usual values ​​for arterial oxygen saturation in healthy individuals are between 96 and 100%. In addition, the determined pulse rate is displayed. Depending on the device, supplemented by various additional functions, such as storage functions for measured values.

CO-oximeter

CO-oximeter with detached finger clip

Since other molecules such as toxic carbon monoxide (CO) can be bound to hemoglobin in addition to oxygen , incorrect measurements can occur with simple pulse oximeters that only work with two wavelengths and carbon monoxide poisoning cannot be distinguished from sufficient oxygen saturation. Advanced pulse oximeters, also known as CO-oximeters, can also optically distinguish the bond with carbon monoxide from the bond with oxygen. Using the same measuring principle, several absorption values ​​are determined at four to seven different wavelengths and related to one another.

The extended fractional oxygen saturation determined with CO-oximeters is:

${\displaystyle S_{\mathrm {p} }O_{\mathrm {2} }={\frac {HbO_{\mathrm {2} }}{HbO_{\mathrm {2} }+Hb+COHb+MetHb}}}$

with the percentage of the hemoglobin covered with carbon monoxide in the blood, COHb and MetHb for methemoglobin , a dysfunctional form of hemoglobin that can still absorb oxygen in the lungs but can no longer release it in the tissue.

Practical application of pulse oximetry

Pulse oximetry with plethysmogram (3rd curve from above, second measured value from above)

In emergency services and intensive care units and in anesthesia is the pulse oximetry portion of the standard monitoring of patients . In the case of premature births, a surveillance monitor is often used for further domestic monitoring , which shows the respiratory rate, oxygen saturation and pulse.

Pulse oximetry screening for critical congenital heart defects has been mandatory for all newborns in Germany since January 28, 2017. At the age of 24 to 48 hours after birth, a value is measured on a foot of the newborn and, if the values ​​are abnormal, further clarification is initiated.

When monitoring premature infants in neonatal intensive care medicine, dual pulse oximetry (right / left) is often used in order to record the difference between preductal and postductal oxygen saturation over time in the case of a diagnosed persistent ductus arteriosus .

In sleep medicine , pulse oximetry is an important measurement method for detecting sleep apnea .

Pulse oximetry is used in sport aviation for flights to high altitudes in order to be able to prevent hypoxia through self- monitoring .

Pulse oximeters are being used more and more often when climbing at high altitudes in order to receive early warning of an impending altitude sickness .

Pulse oximeters or wearables with integrated pulse oximeters are also used more frequently in the private sector as part of the quantified self movement .

In the wake of the COVID-19 pandemic , emergency physician Richard Levitan recommended the use of pulse oximeters in the New York Times to monitor infected people in their homes so that they can determine in good time when inpatient treatment is indicated. As a justification, Levitan stated that the lungs of the patients continued to evacuate carbon dioxide for a while, even when the disease was severe, in spite of the decreasing oxygen content, so that the patients did not experience any shortness of breath, although treatment in the hospital was already necessary.

Cerebral oximetry

Special devices are able to measure the oxygen saturation not only through the skin but also through the skull bone. With so-called cerebral oximetry, the light transmitter and light receiver cannot be placed in a line one behind the other. The transmitter and receiver are attached to the forehead a few centimeters apart. Small amounts of infrared light pass through the skull and brain and are scattered there to a depth of up to 2.5 cm. Due to the scattering , the light is distributed in all directions and thus also reaches the recipients on the skin. The two receivers measure the saturation at a certain distance from each other. In this way, the two slightly different measured values ​​with a known scattering angle can be used to estimate the oxygen saturation of the blood in the brain near the skull. In young, healthy patients who breathe normal air, the saturation in this capillary-like area is approx. 60-70%. This baseline value can also be lower in elderly or sick patients. If there is a lack of oxygen in the brain, e.g. B. due to an insufficient supply of blood, this value drops. According to estimates, 50% should be regarded as the absolute lower limit at which brain damage can occur.

Cerebral oximetry is used for operations on the vessels supplying the brain, e.g. B. the carotid artery . During these operations, the blood supply to the brain must be interrupted in a controlled manner on one side. By measuring the cerebral oxygen saturation, it is possible to estimate how long the brain can survive with the restricted blood supply. If the saturation drops, it may be necessary to interrupt the operation on the artery and, for example, insert a temporary shunt to restore the blood supply.

Measurement error

• With lacquered fingernails (blue, green and black; not red and purple lacquer, however, since red and infrared light are better permeable), light is absorbed too strongly by the dye and only reaches the photodiode in a weakened manner, so that measurements are no longer possible can be. Usual pulse oximeters indicate this as an error.
• Artificial fingernails made of acrylic also lead to measurement errors, depending on the pulse oximeter.
• In patients with reduced peripheral capillary blood flow, for example in the event of a shock and hypothermia , it can happen that incorrect values ​​are displayed or that pulse oximetry is not possible because the pulse detection required for relative measurement value generation does not work.
• With movements and mechanical shock , e.g. B. in the event of a vibration or a drive over uneven terrain in a vehicle, errors occur due to changes which, under certain circumstances, generate patterns for the optical measuring system similar to those caused by the pulse. Ideally, the pulse oximetry should be carried out in the resting position.
• Strong infrared heat lamps attached close to the measuring device, methaemoglobin values in the range 0.4–8.4% in normoxia, onychomycoses and certain substances such as methylene blue cause incorrectly low S _p O ₂ values.
• The weaker venous pulsation, which is disruptive to the measuring principle, causes S _p O ₂ values that are too low .

literature

• K. Matthes: Investigations into the oxygen saturation of the human arterial blood. In: Naunyn-Schmiedeberg's archive for experimental pathology and pharmacology. Volume 179, Issue 6, 1935, pp. 698-711.
• JA Pologe: Pulse Oximetry: Technical Aspects of Machine Design. In: Internat. Anesthesia Clin. 25 (3), 1987, pp. 137-153.
• JG Webster: Design of Pulse Oximeters. Taylor & Francis, 1997, ISBN 0-7503-0467-7 .
• John W. Severinghaus, Yoshiyuki Honda: History Of Blood Gas Analyzes. VII. Pulse Oximetry. In: Journal of Clinical Monitoring. 3 (2), Apr 1987, pp. 135-138.
• C.-P. Bernhardt: Construction of a photoplethysmograph with infrared sensor and its application in psychophysiological research. Thesis . University of Hamburg, 1978.

Web links

Commons : Pulse Oximeter  - Collection of images, videos and audio files

Individual evidence

1. ↑ Karl Matthes: Investigations into the oxygen saturation of the human arterial blood . In: Naunyn-Schmiedeberg's Archives of Pharmacology . 179, No. 6, 1935, pp. 698-711. doi : 10.1007 / BF01862691 .
2. ^ Glenn Allan Millikan: The oximeter: an instrument for measuring continuously oxygen saturation of arterial blood in man . In: Review of Scientific Instruments . 13, No. 10, 1942, pp. 434-444. bibcode : 1942RScI ... 13..434M . doi : 10.1063 / 1.1769941 .
3. ^ Meinolfus Strätling, A. Schneeweiß, Peter Schmucker: Medical University of Lübeck: Clinic for Anesthesiology. In: Jürgen Schüttler (Ed.): 50 Years of the German Society for Anaesthesiology and Intensive Care Medicine: Tradition and Innovation. Springer, Berlin / Heidelberg / New York 2003, ISBN 3-540-00057-7 , pp. 479-486, here: p. 483.
4. ↑ ^{a b c} Basics of pulse oximetry. (PDF) Nellcor Purita Bennett Inc., company publication, 1997, accessed April 14, 2017 (clinical training documents).  
5. ↑ Sonnia Maria Lopez Silva, Maria Luisa Dotor Castilla, Juan Pedro Silveira Martin: Near-infrared transmittance pulse oximetry with laser diodes. (PDF) Journal of Biomedical Optics, July 2003, accessed April 28, 2017 .  
6. ↑ M. Coulange, A. Barthelemy, F. Hug, AL Thierry, L. De Haro: Reliability of new pulse CO-oximeter in victims of carbon monoxide poisoning. In: Undersea Hyperb Med. 35 (2), Mar-Apr 2008, pp. 107-111. PMID 18500075
7. ↑ Appendix 6 - Parents' information on pulse oximetry screening - Federal Joint Committee. Retrieved April 6, 2017 . 
8. ↑ Richard Levitan: "The Infection That's Silently Killing Coronavirus Patients" nytimes.com of April 20, 2020
9. ↑ J. Schön, H. Paarmann, M. Heringlake: Cerebrale Oxymetrie. In: The anesthesiologist. 61, 2012, pp. 934-940, doi: 10.1007 / s00101-012-2066-5 .
10. ^ ^{A b} Michael Heck, Michael Fresenius: Repetitorium Anästhesiologie. 5th edition. Springer, Berlin 2007, ISBN 978-3-540-46575-1 .
11. ↑ J. Hinkelbein, HV Genzwuerker, R. Sogl, F. Fiedler: Effect of nail polish on oxygen saturation determined by pulse oximetry in critically ill patients. In: Resuscitation. 72 (1), 2007, pp. 82-91.
12. ↑ J. Hinkelbein, H. Koehler, HV Genzwuerker, F. Fiedler: Artificial acrylic finger nails may alter pulse oximetry measurement. In: Resuscitation. 74 (1), 2007, pp. 75-82.
13. ^ HM Sami, BS Kleinman, VA Lonchyna: Central venous pulsations associated with a falsely low oxygen saturation measured by pulse oximetry. In: J Clin Monit. 7, 1991, pp. 309-312.

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