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Fingertip Pulse Oximeter:- Application, Limitations, Working Mechanism

Fingertip Pulse Oximeter is a light and portable pulse oximeter that is used on the fingers to get the readings. It is a non-invasive method to measure oxygen saturation (SPO2) in the arterial blood, heart rate, and perfusion index (based on the model). It is generally used alongside with oxygen concentrator or oxygen cylinder.

Fingertip Pulse Oximeter Readings

There can be two or more readings, that are displayed on the screen of the fingertip pulse oximeter. They are as follows

1. Oxygen Saturation (SPO2)

SPO2 = (total no. of oxyhemoglobin/total no. of hemoglobin) * 100. In an RBC, if there are 8 hemoglobin molecules bound to the oxygen out of 10 molecules then that means SPO2 is 80%.

SPO2

Status

Remarks

94% to 99% Normal No need to worry

90% to 94%

Might be a normal patient

Consult the doctor

Below 90%

Critical patient

Supplementary oxygen is needed

2. Heart Rate

It denotes the number of beats the heart makes in a minute. Normal heart rate ranges from 60 to 100 beats per minute (BPM).

3. Perfusion Index

Perfusion Index is defined as the ratio of pulsatile blood flow to the non-pulsatile blood flow in our tissue. In other words, a high perfusion index means a good supply of oxygenated blood through the blood vessels and vice versa.

The perfusion index ranging from 0-5 can be considered as low value. Similarly, above 5 might be considered as a good value of perfusion index.

How to use a fingertip pulse oximeter?

  • Insert your finger inside the pulse oximeter and wait for few seconds to get the reading
  • Do not expose oximeter to excessive light
  • Do not move your finger while using
  • Should not use nail police
  • For most of the models of a fingertip pulse oximeter, measurement of SPO2 below 75% will not be accurate.

Which finger is best for the fingertip pulse oximeter?

Few kinds of research have shown that the middle finger of the right hand gives the best accuracy of SPO2 reading. But you can also use any other finger to measure SPO2 as the difference in accuracy is not much significant among the fingers.

Why do you need to buy a fingertip pulse oximeter along with the oxygen concentrator?

When you use an oxygen concentrator the level of SPO2 will generally increase in your blood. But, if your concentrator has stopped giving purified oxygen and instead gives out normal air, the pulse oximeter will help to know if your machine needs any maintenance.

Working Principle of Fingertip Pulse Oximeter

A fingertip pulse oximeter is based on the principle of differential wavelength absorption. To understand this principle we need to, first of all, understand the basic components of a pulse oximeter.

Components

1. Biological Variables

Single red blood cell (RBC) consists of around 270 million hemoglobin in it. Each hemoglobin molecule can bind up to four oxygen molecules in its binding site. The oxyhemoglobin is red in color while deoxyhemoglobin is dark red in color.  Oxyhemoglobin absorbs more infrared light (IR) and allows red light to pass through it and vice versa.

2. Sensors and Photodetector

Two LEDs emit monochromatic red light (660 nm) and IR light (940 nm). These wavelengths may slightly differ from one company to another.

 Working Principle of Fingertip Pulse OximeterIf you see through the pulse oximeter you will notice the blinking of red light while infrared rays (IR) are invisible to our naked eyes. They blink several hundred times in a second but both of them are not ON at the same time. The repetitive LED sequences for two diodes are ON/OFF, OFF/ON, and OFF/OFF. The OFF/OFF stage is for the measurement of ambient light to reduce the error during measurement. These energy beams are detected by photodetectors.

3. Integrator

The integrator is based upon these two principles:

  • Beer-Lambert Law: This principle states that absorbance is directly proportional to the concentration of the substance and the distance the light has to travel to reach the detector

          i.e. Absorbance (A) α Concentration (c) *distance (d)

          or A α cd

  • When light is passed through the sample (in this case nose, fingers, ears, etc.), it will undergo absorption, reflection, or transmission.

This is because a certain amount of light Light Absorption Diagram will be absorbed by parts such as bones, tissues, leukocytes, etc. The nature of light absorption does not change for these factors and is denoted by (DC). On the other hand pulsatile arterial blood flow is denoted by (AC)

  • If deoxyhemoglobin > oxyhemoglobin then absorption of red light is more than that of the IR beam. So, a low amount of red light will be transmitted to the detector.
  • If oxyhemoglobin > deoxyhemoglobin then absorption of IR light is more than that of red light. So, a low amount of IR light will be transmitted to the detector.

Once the values of AC and DC are measured experimentally, then the ratio is calculated among them.

R=  ((Red AC)(IR DC))/((Red DC)(IR AC))

This ratio is analyzed by the processor.

4. Output

Once the ratio is analyzed by the processor, the result will be displayed on the monitor.

Limitations of the Fingertip Pulse Oximeter

  • Does not tell us about the presence of CO2. This might give an error.
  • The motion of the probe causes an alteration in the blood flow which will create noise in the blood.
  • In the case of poor peripheral perfusion, the oximeter may not be able to detect the hemoglobin.
  • It might not be able to differentiate between abnormal hemoglobin such as carboxyhemoglobin and oxyhemoglobin.

 

Once the pulse oximeter gets damaged, do not waste your time trying to repair it.

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