Pulse oximeter is one of the most commonly employed monitoring device in the critical care setup. It is important device for respiratory monitoring.
Over the last few years, numerous studies have focused on the technical aspects of pulse oximeters and found that these instruments have a reasonable degree of accuracy.
The human eye is poor at recognizing hypoxemia. Even under ideal conditions, skilled observers cannot consistently detect hypoxemia until the oxygen (O2) saturation is below 80%.
In a lung disease, blood oxygen level may be lower than normal. This is important to know because
when oxygen level is low, the cells in the body can have problem working normally.
Oxygen is the food burning gas that makes body run and if there is no oxygen or lack of oxygen then body does not run smoothly.
Having a very low blood oxygen level also can put a strain on heart and brain. Most people need an oxygen saturation level of at least 89% to keep their cells healthy.
Having an oxygen level lower than this for a short time is not believed to cause damage. However, body cells can be strained or damaged if low oxygen levels happen many times.
Need of Pulse Oximeter
Most people do not need normally, though during the COVID-19 pandemic, many people are using them to check their oxygen levels.
Some people are prescribed a pulse oximeter if they have or could have periods of low oxygen; for example, when you are exercising or if you travel to high altitude.
Having a pulse oximeter in these cases allow to monitor blood oxygen level and know when need to increase supplemental oxygen flow rate to maintain.
If oxygen level is low on room air, may need to use supplemental oxygen source. The oximeter can be
used to help see how much oxygen you need and when need it.
For example, some people need more oxygen when asleep than when awake. Some need more oxygen with activity than when at rest.
Principles of pulse oximeter
It provide a spectrophotometric assessment of functional arterial hemoglobin oxygenation (SpO2).
It is based on the following two principles:
1) hemoglobin (Hb) and oxygenated hemoglobin (HbO2) differ in their absorption of red and infrared light.
2) The volume of arterial blood in tissue and therefore light absorption by the hemoglobin changes during the pulse.
Device passes red and infrared light into an arteriolar bed, measures changes in light absorption, and determines SpO2.
How Pulse Oximeter Works
Device have red and infrared low voltage light emitting diodes (LED) witch serve as light sources. The
emitted light is transmitted through the tissue.
Later collected by the photodetector, converted into digital signals and sent to the microprocessor of the
All constituents of the human body, venous and arterial blood, and tissue absorb light. The pulsating of arterial blood results in changes in the absorption to to added hemoglobin (Hb) and oxygenated hemoglobin (HbO2) in the path of the light.
Since HbO2 and Hb absorb light to varying degrees, this varying absorption is translated into plethysmographic waveforms at both red and infrared wavelengths.
The relationship of red and infrared plethysmographic signal amplitude can be directly related to arterial oxygen saturation.
For example, when the plethysmographic amplitude ad 660nm and 910nm are equal and the ratio
R/IR=1, the SpO2 is approximately 85%.
Calibration and Accuracy
The light absorption by hemoglobin is wavelength dependent. Therefore red and infrared wavelengths are tightly controlled for each individual sensor.
The LED intensity is auto-magically adjusted for amplitude. This allows device sensors to be use interchangeably without calibration at the time of use.
The measured oxygen level is reasonably accurate. Most devices give a reading 2% over or 2%
under what saturation would be if obtained by an arterial blood gas.
For example, if oxygen saturation reads 92%, it may be actually anywhere from 90 to 94%. To get a good reading, need to allow some time (a few seconds) for stable reading to capture pulsations adequately.
There are many factors that can reduce the accuracy of reading, including:
1 Cold hands
2 Not holding still
3 Some foreign objects like nail polish (especially black, blue or green) present, or wearing artificial nails.
4 Oxygen saturation is below 80%.
5 Skin is thicker than average.
6 Skin pigment can also impact the accuracy. Recent studies show that in those with darker skin pigments, may miss below normal oxygen saturations.
7 For smokers, the reading on device may be higher than the actual oxygen saturation. This is because smoking increases carbon monoxide levels in your blood.
The oximeter cannot tell the difference between the gases carbon monoxide and oxygen.
Truly, pulse oximetry is the fifth vital sign, essential to complete patient monitoring. It may be used in a variety of situations that call for monitoring oxygen level and pulse rates.
This increase patient safety by alerting the hospital staff to the onset of hypoxia during or following surgery.
Also confirm adequate oxygenation during mechanical ventilation. Physician and dental clinics utilize for instant checking of respiratory status, as well as for monitoring during procedures that call for sedation.
Oximeter versus an arterial blood gas (ABG)
An oximeter indirectly measures the amount of oxygen that is carried by your blood. It does not
measure the carbon dioxide level in the blood flow.
An arterial blood gas (ABG) directly measures both the amount of oxygen carried by blood and the actual amount of gases (oxygen and carbon dioxide) that are in blood flow.
To get an ABG, blood is taken directly out of your artery (usually from the wrist) and can be painful. Oximetry is painless but is not as accurate as an ABG.
Information Resources: American Thoracic Society
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