Pulse Oximeter: Principle and Uses
In recent times, when the world is still experiencing the repercussions of the devastating COVID-19 pandemic, ‘pulse oximeter’ is a common term heard in every household. In case of a fever, shortness of breath, or even a simple cold, we tend to reach out for these devices to check blood oxygen levels to gauge the severity of our condition.
By clipping a pulse oximeter onto our finger or any other part of the body that enables the device to read blood flow, we can compute our pulse as well as the oxygen saturation of haemoglobin. During the COVID-19 pandemic, there was an increase in demand for pulse oximeters, along with COVID-19 test kits, as people wished to supervise their own health at the safety of their homes.
The instrument is widely used to monitor the blood oxygen saturation of patients with conditions that affect oxygen levels, such as:
· Cardiac arrest
· Asthma
· Chronic obstructive pulmonary disease (COPD)
· Pneumonia
· Lung cancer
· Anaemia
In hospitals, anaesthesiologists monitor anaesthesia cases with the help of pulse oximeters. In addition to this, patients on opioids require such supervision as these medications can impact respiration negatively. Ambulances, general hospitals, intensive care units and emergency rooms also use this device to keep track of the patient’s pulse and oxygen levels while being transported to the hospital, during surgery, and while the patient is recovering.
While it might seem effortless to use, the pulse oximeter executes a series of internal mechanisms to provide accurate readings. Let’s take a look at the working parts of a pulse oximeter and how they combine to perform the task efficiently.
How does a pulse oximeter work?
The primary principle the oximeter works on is the difference in absorption of light by oxygenated blood and deoxygenated blood. Two light-emitting diodes (LEDs), embedded opposite to a photodiode in the device, emit red light of wavelength 660nm and infrared of wavelength 940nm respectively. While oxygen-rich haemoglobin tends to absorb more infrared, less saturated haemoglobin absorbs a greater amount of red light. This means that oxyhaemoglobin transmits red light and deoxyhaemoglobin is penetrable to infrared light.
The LEDs alternatively turn on and off, with a short period in between when neither are on. This cycle occurs thirty times per second and the light that passes through the finger is detected by the SpO2 sensor.
The output light oscillates in time as each heartbeat increases the amount of arterial blood present. This helps eliminate the effect of skin, bones and other tissues on the transmission of light as the minimum light that has passed through can be deducted from the peak value, ensuring the reading taken is only for the blood passing through the finger at that time.
In order to find the ratio of oxygenated blood to deoxygenated blood, the processor calculates the ratio of red light to infrared light that has been transmitted. Aided by a lookup table derived from the Beer-Lambert Law, which states that the absorbance of light is directly proportional to the concentration of the sample and the path length, the processor converts the ratio into SpO2, which is the oxygen saturation displayed on the screen, along with the heart rate, on the pulse oximeter.
Given below are the formulae for the Absorbance and the calculation of SpO2:
Absorbance = molar absorptivity x path length x sample concentration
SpO2 = [HbO2] / [Hb] + [HbO2]
Where, [HbO2] is the concentration of oxygenated haemoglobin and [Hb] is the concentration of haemoglobin.
References:
https://medicine.uiowa.edu/iowaprotocols/pulse-oximetry-basic-principles-and-interpretation
