How does a pulse oximeter work?

The oximeter measures the amount of oxygen in the blood. This measurement is used to monitor the condition of patients with respiratory disorders or diseases of the respiratory system.

PHYSIOLOGY

The principle used for the operation of the pulse oximeter is based on the capacity of blood to absorb red and infrared light, according to its oxygen saturation. The calculation of the blood oxygen saturation rate noted SpO2 is based on the ratio between CHbO2 and CHb.

The CHbO2 is the blood concentration of oxyhemoglobin and the CHb is the total concentration of hemoglobin in the blood. Hemoglobin is a blood molecule containing iron atoms and has a high affinity for oxygen. Red blood cells are made up of about 33% hemoglobin. This is what gives them the ability to transport oxygen from the lungs to the various tissues of the body.

When hemoglobin captures oxygen in the lungs, it becomes oxyhemoglobin and turns bright red and when this oxygen is released in the tissues, it becomes deoxyhemoglobin. These two types of hemoglobins have different absorption rates for red and infrared light. Oxyhemoglobin absorbs infrared light better and deoxyhemoglobin absorbs red light better.

OPERATION

The principle of absorbance will determine the rate of oxygen saturation of a medium. Indeed, the amount of light absorbed by a medium is proportional to its concentration in a given chemical species, according to the law of Beer-lambert. The sensor which is placed at the end of the finger is equipped with a transmitter and a receiver of light.

The emitter allows the emission of an infrared light and a red light thanks to two Led. The red light has a wavelength of 660 nm, the infrared light has a wavelength of 950 nm. These two lights will cross the skin and will be captured by a receiver, constituted by a photodiode, which will quantify them.

A calculation on the quantity of absorbed light will allow to determine the blood saturation in oxygen. The saturation of the blood (SpO2), is expressed in percentage and will allow to have an estimation of the state of a patient. The normal value is between 90% and 100%. The oximeter will also allow to measure the heart rate, by measuring the variation of the different blood flows in the extremities.

In addition to the sensor that is placed at the extremities, the oximeter consists of a monitor and a cable when it is not compact. The cable is used to connect the sensor to the monitor, which has a computer that will analyze and translate the data received from the sensor. The sensor has the shape of a fingernail or finger clip and is placed at the level of the fingers, ears and wings of the nose for adults or at the level of the toes and foot for pediatric models.