Pulse oximetry and importance of oxygen

  • Category
    Science & Technology
  • Published
    17th May, 2021

Context

In this second wave of COVID-19 pandemic pulse oximeters have become a daily household item and regrettably hypoxia has become the leading cause of death.

Background

  • The Karnataka’s Covid Technical Advisory Committee has recommended that government provide medical kits for all patients in home isolation which must include pulse oximeters along with other drugs
  • This device measures SpO2 of blood working on the principle of pulse oximetry and falling oxygen saturation level is a signal of hypoxia.

Analysis

What is pulse oximetry?

Pulse oximetry is a non-invasive method for monitoring a person's oxygen saturation in blood. There are two types of pulse oximetry- transmissive and reflectance.

What is transmissive pulse oximetry?

  • Asensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or an infant's foot from which EM radiation can pass easily.
  • Fingertips and earlobes have higher blood flow rates than other tissues, which facilitates heat transfer, thus giving accurate reading.
  • The device will pass two different wavelengths of light through the body part to the photodetector.
  • It measures the change in absorbance at each of the wavelengths, allowing it to determine the absorbances due to the “pulsing arterial blood” alone, excluding venous blood, skin, bone, muscle and fat and hence calculating the SpO2

What is reflectance pulse oximetry?

  • It is a less popular alternative to transmissive pulse oximetry.
  • This method does not require a thin section of the body and therefore it is well suited to a universal application such as the feet, forehead, and chest, but it also has some limitations.

Effects of Non-functional Haemoglobin on Oxygen Saturation Measurements

  • If the patient has a large amount of non-functional haemoglobin, the reading is not accurate.
  • Non-functional haemoglobin is defined as haemoglobin which is incapable of carrying oxygen, but does include carboxyhaemoglobin (HbCO) and methaemoglobin (METHb).
  • Functional haemoglobin is defined as haemoglobin capable of carrying oxygen.
  • It includes oxygenated haemoglobin (HbO2) and deoxygenated haemoglobin (Hb).

Common issues with pulse oximetry

  • Light Interference
    • Sometimes external light sources may cause inaccurate readings when they interfere with the light (radiation) with different wavelengths.
  • Movement Artifacts
    • Movement artifacts, such as shivering, have been overcome with the latest SpO2 algorithms. However, unusually rapid and vigorous movement may cause movement artifact in the pulse.
  • Sensor application
    • The sensor should fit the application site snugly. If the sensor is too tight, it might cause venous pulsation. If the sensor is too loose, the light emitting from the emitters may not pass completely through the site and may cause absurd readings.If adhesive sensors are not of the right size, the emitter and detector may not line up correctly.
  • Inadequate Blood Flow
    • Blood pressure cuffs, tight clothing or restraints may interfere with blood flow. Use another application site or loosen clothing
  • Nail Polish
    • Some nail polish and false fingernails may cause false readings. If possible, switch to an unpolished nail, or consider another application site.

Why is oxygen important for body?

  • Fuel for cells: Oxygen is fuel for our cells and helps provide the basic building constituents that our bodies need to survive. The cells in our body combine oxygen with hydrogen and nitrogen to produce various proteins that build new cells/tissues. When oxygen is combined with hydrogen and carbon, we get carbohydrates that provide energy to our bodies that is necessary for our survival.
  • Constructing replacement cells: Oxygen is also a vital element for constructing replacement cells for our bodies. Each day, about 700 billion cells in our bodies wear out and must be replaced, without oxygen our bodies cannot build these new cells.
  • Immune system: Oxygen is also an important part of our immune system. It is used to help kill bacteria, and it fuels the cells that make up our body's defences against viruses and other invaders. Air that has passed through (UV air sanitizers) is particularly good for our body's immune system, as it has been cleansed of bacteria and other agents before it enters our respiratory systems.
  • Finally, it is important to note that the human eye is in particular need of oxygen to function well. However, the eye receives oxygen in a manner that is unique from the rest of the body. Few blood vessels travel to the eye, so our eyes absorb much of the oxygen they need directly through the cornea. The cornea is built in such a way to diffuse oxygen directly into the body from the air.

Conclusion

Pulse oximetry results may not be as accurate for people with darker skin. Their oxygen levels are sometimes reported as higher than they really are. This possibility should be considered when interpreting pulse oximetry results. A person’s oxygen levels may be low if they feel short of breath, are breathing faster than usual, or feel too sick to do their usual daily activities, even if a pulse oximeter says their oxygen levels are normal.Supplementaloxygen or other treatments may be needed if they have shortness of breath or their SpO2 number is less than 95%.

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