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The Science Behind Pulse Oximeters

The Covid-19 has wreaked havoc worldwide. The total infected cases is 21 million and counting, while the number of deaths currently stand at 774K. Such an unprecedented scenario has seen a rise in the sale of heath-tech products. One such product is the Pulse Oximeter. A device used for measuring blood saturation level and pulse rate, it is an extremely handy device. With such a grim situation looming around, Pulse Oximeter has joined the ranks of devices like thermometers and BP measuring machines, which are found in almost every household. Let us therefore look at what Pulse Oximeters are, and the science behind them.

Below are the list of topics we will cover. Click on any of them to directly jump to the exact section.

 

What is a Pulse Oximeter & how does it work?

A pulse oximeter is a small and lightweight device, used to monitor the level of oxygen in a person’s body (blood). It is a non-invasive tool, which means there is no incision or wound involved. A pulse oximeter attaches painlessly to a person’s fingertip. It does so by sending two wavelengths of infrared light into the capillaries of the finger and Pulse Oximetermeasures how much light is reflected off the gases, thereby displaying the pulse rate and how much oxygen is there in the system. You just have to put your finger (index finger preferred) inside the cavity and power ON the device. The pulse oximeter displays the SpO2 level and the pulse rate, where SpO2 is the ‘Peripheral Oxygen Saturation’ level, and indicates the level of oxygen in a person’s blood. The test done on a modern pulse oximeter usually has an error limit of 2 percent, although there are more accurate devices available as well. It means the actual blood oxygen level may be as much as 2 percent higher or lower than the readings. Nail polish can hamper the readings of a pulse oximeter as well. They usually lead to a lower that normal reading. Therefore, it is always advisable to remove any polish from the nails, and even trim the nails before using the device. We will look at the various reasons that can lead to faulty readings in the upcoming sections. As the pulse oximeter is a non-invasive device, it can be used even without medical supervision. Although one might need to consult a doctor to interpret the results and further diagnosis.

 

Pulse Oximetry – The Science Behind Pulse Oximeters

Pulse Oximetry is a non-invasive easy method of measuring the amount of peripheral arterial oxygen saturation (SpO2)- in patients who are hypoxic or have pulmonary problem (problems related to lungs) that may affect their gas exchange. Oxygen saturation simply means the percentage of total hemoglobin carrying oxygen. The measurement is carried out using a light emitter and a light detector, placed opposite to each other. The patient’s finger, or earlobe is placed between these two.

Pulse Oximetry is based on a variation of the Beer-Lambert Law. It states that the absorption of light of a given wavelength passing through a non-absorbing solvent, which contains an absorbing solute, is proportional to the product of the solute concentration, the light path length, and an extinction coefficient. The Beer-Lambert law can readily be applied to co-oximeters in a laboratory setting because the light path length is known and hemoglobin is in solution. However, it must be modified for pulse-oximetry to overcome the obstacles associated with interference from tissue and pulsatile flow. This modification involves measuring absorbance at two different wavelengths, one to detect oxyhemoglobin (hemoglobin with oxygen) and the other to detect deoxyhemoglobin (hemoglobin without oxygen).

Here, the light absorbing solvent is blood, and the light is actually absorbed by the hemoglobin. It means, higher the hemoglobin concentration, higher will be the absorption of light. This is the crux of Beer’s Law. On the other hand, if the artery is wider, light will have to travel more distance to reach the light detector. The more wider the artery, more will be the absorption (as the light wave will encounter more hemoglobin). This is in essence the Lambert’s Law.

The Pulse Oximeter uses two wavelengths of light- RED (wavelength = 650 nm) and INFRARED (wavelenght = 950 nm). The oxyhemoglobin and deoxyhemoglobin absorb the red and infrared lights in a different ways.

Absorption behavior of Oxyhemoglobin and Deoxyhemoglobin towards Red and Infrared light.
Photo Credit: howequipmentworks.com. Absorption behavior of Oxyhemoglobin and Deoxyhemoglobin towards Red and Infrared light.

From the above absorption curve, we can clearly see that,

  • Deoxyhemoglobin (Deoxy Hb) absorbs more Red light than Infrared light.
  • Oxyhemoglobin (Oxy Hb) absorbs more Infrared light than Red light.

The Pulse Oximeter calculates the blood oxygen saturation level by making a comparison between how much red light is absorbed by blood vs how much infrared light is absorbed by blood. In short, it compares the amount of Oxyhemoglobin present in blood and Deoxyhemoglobin present in blood, which leads to a ratio, and this ratio is our blood saturation level.

The Issue of Presence of Blood Tissues

As discussed in an earlier paragraph, there can be issues with finger pulse oximeter, as besides arterial bllod there are blood tissues as well. These tissues can hamper correct reading. Fortunately, there is a way out. As the pulse oximeter analyzes arterial blood, it can ignore other stuffs. As a matter of fact, arterial blood is pulsating in nature, and therefore leads to change in the level of absorption. The pulse oximeter can detect such a variation, and immediately knows that this is due to arterial blood. Whereas, blood tissues do not pulsate, and leads to constant level of absorption. Pulse Oximeters know that if the absorption level is constant, it has to be ignored. The pulse oximeter mitigates this problem in a simple an effective.

 

What Does The Pulse Oximeter Reading Tell?

The readings of the pulse oximeter tell us the blood saturation level and pulse rate.  Blood saturation level (SpO2 level) can be broadly classified into two categories- normal, and below normal.

Normal
Type of Person Blood Oxygen Level
Healthy 95 to 100 percent
Suffering from COPD/asthma 85 to 90 percent
Below Normal (Hypoxemia)
Blood Oxygen Level Below 92 percent
Can lead to Chest pain; breathlessness; increased heart rate
The pulse rate reading differs as per the person’s age.
Age Group Normal Pulse Rate (in beats per minute, i.e. bpm)
3 to 4 years 80 to 120
5 to 6 years 75 to 115
7 to 9 years 70 to 110
Above 10 years and adults 60 to 100

There is a detailed article written on normal/abnormal blood oxygen by healthline.com. It talks about the diagnosis and prognosis of various ailments related to abnormal blood oxygen level.

Link to the article: https://www.healthline.com/health/normal-blood-oxygen-level

Medical News Today also published an informative article on heart rate a few years ago. It talks about resting heart rate, target heart rates during exercise and how to maintain a normal heart rate.

Check out the article: https://www.medicalnewstoday.com/articles/235710

What Hampers The Correct Reading of a Pulse Oximeter?

As pointed above, the readings of a Pulse Oximeter can get hampered due to various reasons. The error limit usually varies between 2% (above & below) the actual level. Although, there are a few research papers which suggest some obstructions can cause the error to jump beyond 2%. The common reasons for wrongs reading can be:

  • Faulty device
  • Painted Nails
  • Untrimmed Nails
  • Motion Artifacts (it occurs due to voluntary or involuntary patient movement)
  • Low Perfusion (reduced peripheral arterial blood flow)

Anesthesiology, which is the official journal of the American Society of Anesthesiologists, published a paper in 2018, on pulse oximetry and the effectiveness of various oximeters at detecting Hypoxia (reduced level of tissue oxygenation) during obstructions. The study found that all oximeters detected hypoxemia during motion and low-perfusion conditions, but motion impaired performance at all ranges. Lower perfusion degraded performance as well. In simple terms, what it means is – ‘when using a pulse oximeter, stay still’.

Click here if you interested in reading the above mentioned research paper.

In the year 2013, a research paper with the name ‘Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations’ was published in the famous medical journal Respiratory Medicine (previously known as ‘British Journal of Diseases of the Chest’) which listed down the common causes and mechanisms of unreliable SpO2 readings. These common causes are:

1. Causes of intermittent drop-outs or inability to read SpO2
2. Causes of falsely normal or elevated SpO2
3. Causes of falsely low SpO2
  • Venous pulsations
  • Excessive movement
  • Intravenous pigmented dyes
  • Inherited forms of abnormal hemoglobin
  • Fingernail polish
  • Severe anemia (with concomitant hypoxemia)
4. Causes of falsely low or high SpO2
5. Causes of falsely low FO2Hb as measured by a co-oximeter

If you are interested in an in-depth reading on how the above listed causes lead to unreliable readings in a Pulse Oximeter, click here to checkout the research paper.

Conclusion

Pulse oximetry has revolutionized the ability to monitor oxygenation in a continuous, accurate,and non-invasive fashion. It has become so widely prevalent in medical care, that it is often regarded as the fifth vital signal. Its application in the finger pulse oximeter has been quite handy for medical professionals as well as the common-man. The current situation world over, has created a demand of health-tech products, and pulse oximeter is on that list for sure. If you are interested in a Buying Guide for Pulse Oximeter and how Pulse Oximeter has become a necessity during Covid-19,  check out our other blog Buying Guide for Pulse Oximeter – A necessity during Covid-19.

 


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