Pulse oximetry is a medical technical procedure that is used for continuous non-invasive measurement of oxygen saturation (SpO2) of arterial blood and pulse rate. It is a standard procedure used every day and is part of the basic monitoring (baseline monitoring) in the clinic. Pulse oximetry is mainly used in anesthesia (medical specialty that includes perioperative pain management and anesthesia medicine). However, it is also regularly used in many other medical disciplines. The measurement principle is based on the light absorption of hemoglobin (red blood pigment) in circulating erythrocytes (red blood cells), which allows conclusions to be drawn about arterial oxygen saturation (SaO2). Pulse oximetry is a very easy-to-use method and can be applied ubiquitously (everywhere). Initial readings are available after only a few seconds and allow meaningful follow-up of hemodynamics (circulatory function) and pulmonary function.
Indications (areas of application)
In principle, any situation requiring monitoring of oxygen saturation or pulmonary function forms an indication for the use of pulse oximetry. The procedure is most commonly used in perioperative monitoring, whenever narcotics (anesthetic substances) are used, and in emergency medicine. Especially in anesthesia, its use is mandatory in some cases:
- Pulse oximetry screening for newborns – to detect critical congenital heart defects (vitia); optimal time: 24th-48th hour of life [early detection examination: U1].
- Obesity permagna – Extreme obesity defined by a BMI (body mass index) greater than 40.
- Anesthesia in patients with Z. n. pulmonary resection (surgical removal of a lung lobe).
- Analgesia – administration of an analgesic (painkiller) in combination with a sedative (tranquilizer) to perform minor surgery. Unlike anesthesia, the patient breathes independently.
- Wake-up phase (phase following anesthesia).
- Respiratory patients in intensive care.
- Impaired lung function – e.g., in chronic obstructive pulmonary disease (COPD).
- High-frequency ventilation – form of ventilation characterized by a very high frequency (60-600/min).
- Pediatric anesthesia – monitoring of premature infants, newborns or babies.
- Obstructive sleep apnea syndrome (OSAS) – This is characterized by airway narrowing and apneas (cessations of breathing) or hypopneas (periods when the patient does not breathe or breathes too little during sleep) and often snoring (rhonchopathy). The disease leads to daytime sleepiness, microsleep and secondary hypertension (high blood pressure).
- Transport of intensive care patients
- Transport of emergency patients
- Cyanotic Heart Defects – Congenital heart defects that result in severely decreased oxygen saturation and subsequent bluish coloration of the skin (cyanosis).
- Magnetic resonance imaging (MRI).
Contraindications
Pulse oximetry is a non-invasive diagnostic procedure, so there are no contraindications to mention. In some situations, pulse oximetry measurement has limited validity and should be used with caution. These situations are described in more detail in the following text section.
Before the examination
Pulse oximetry examination is a noninvasive diagnostic method that does not require any preparation of the patient. Only the body site for attachment of the device should be checked for minor injuries to avoid pain or irritation.
The procedure
Pulse oximetry is based on photometric measurement of the light absorption of hemoglobin. Deoxygenated (without oxygen molecule) and oxygenated (with oxygen molecule) hemoglobin have different absorption spectra, so their relative concentrations can be calculated using a physical law (Lambert-Beer law). The absorption maximum of deoxyhemoglobin is 660 nm in the red light range, that of oxyhemoglobin is 940 nm in the infrared light range. For the measurement, a light-emitting diode and a photodiode are required, which are arranged opposite each other.The earlobes, the fingertips (most common site of application) or the toes are best suited for implementation. In special cases, the pulse oximeter can also be attached to the nose, tongue, hand and foot. The pulse oximeter is designed in the form of a clamp, so that the device can be clamped to the mentioned parts of the body. The light from the light emitting diode passes through the tissue and a portion of the light is absorbed by the hemoglobin in the passing erythrocytes. The remaining part is registered by the photodiode (transmission principle). To avoid falsification of the measurement by the light absorption of the surrounding tissue, a pulsatile blood flow is necessary. From the so-called systolic peak absorption caused by the arterial blood, the background absorption caused by the surrounding tissue can be subtracted. Oxygen saturation (SpO2) is expressed as a percentage and should be above 95%; a saturation of 98% is normal. The physical measurement principle of pulse oximetry is subject to some limitations. For example, if pulsatile blood flow is absent, the measurement will not work. This is possible in the case of:
- Arrhythmias (cardiac arrhythmias).
- Hypothermia (hypothermia)
- Hypovolemia (decreased blood volume)
- Hypotension (decreased blood pressure)
- Vasoconstriction in the context of shock
The following factors also reduce the usefulness of pulse oximetry:
- Physical exercise
- Nail polish
- Dark skin color
- Dyes – e.g. methylene blue
- Bright ambient lighting
- Anemia (anemia)
- Extracorporeal circulation (replacement of natural circulatory function by a heart–lung machine).
- Fetal hemoglobin (HbF; hemoglobin form of the unborn child).
- Carbon monoxide intoxication (poisoning by CO, which is produced, for example, during combustion processes. Carbon monoxide binds to hemoglobin with an affinity (binding force) many times higher than oxygen, blocking oxygen transport to cells) – Hemoglobin bound with carbon monoxide (COHb), has a similar absorption capacity as oxyhemoglobin, so that falsely high values are measured as oxygen saturation decreases. This situation can lead to a fatal misjudgment of the patient’s oxygenation.
- Methemoglobinemia – Hemoglobin contains divalent iron, if this is oxidized to trivalent, for example by drugs, methemoglobin is formed.
- Edema (water retention) – e.g. venous congestion of the tissue under study.
- Venous pulsations – e.g., in tricuspid regurgitation (leakage with reflux of blood from the heart valve between the right atrium and the right ventricle).
- Magnetic resonance imaging (MRI) – For use during an MRI scan, a nonmagnetic device must be used.
After the examination
Following the examination, usually no special measures on the patient are necessary. Depending on the results of the examination, medicinal or other therapeutic measures may have to be carried out. In case of erroneous measurements, artifacts, or inconclusive results, a repetition of the examination should be considered or possible limitations of the informative value (see above) should be evaluated. After long-term use of the device, check the attachment site for pressure necrosis and change the site if necessary.
Potential complications
Because this is a noninvasive procedure, complications are not usually expected. The device is attached using a clamp, so mechanical irritation and even pressure necrosis (death of tissue due to pressure) may occur. For this reason, the device should be alternately placed in different locations.
