Oximeter
The oximeter measures how much oxygen the blood is saturated with. This measurement is used to monitor the condition of patients with respiratory disorders.
Oximeters are also used in sports, such as hypoventilation training, and in many recreational activities such as scuba diving and aquarium keeping.
What are normal readings, and when should I worry?
Your pulse oximeter will show you your oxygen level as “SpO2.” Normal oxygen levels are at least 95%. Some patients with chronic lung disease or sleep apnea can have normal levels around 90%.
If your home SpO2 reading is less than 95%, call your health care provider.
For people with COVID-19 or to detect contamination with this virus in addition to a PCR test, the oximeter is also used. A normal value is between 98 and 100%. If the measured value is less than 90%, it means respiratory distress and oxygen supply is necessary.
Physiology
The principle used for the operation of pulse oximeters is based on the blood absorption capacity of red and infrared lights, depending on their oxygen saturation. The calculation of the rate of blood oxygen saturation noted as SpO2 is based on the ratio of CHbO2 to CHb.
SpO2 = CHbO2 / CHb
CHbO2 is the blood concentration of oxyhemoglobin and CHb is the total concentration of hemoglobin in the blood. Hemoglobin is a molecule in the blood that contains iron atoms and has a high affinity with oxygen. About 33% of red blood cells are hemoglobin. This is what gives them this ability to transport oxygen captured in the lungs to the various tissues of the body.
When hemoglobin takes up oxygen in the lungs it becomes oxyhemoglobin and turns a bright red color and when this oxygen is released in the tissues it becomes deoxyhemoglobin. These two types of hemoglobins have different absorption rates from red light and infrared light. Oxyhemoglobin absorbs infrared light better, and deoxyhemoglobin absorbs red light better.
Oximeter operation
The principle of absorbance will make it possible to determine the level 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 Beer-lambert’s law. The sensor which is placed at the end of the finger is equipped with a transmitter and a receiver of light.
The transmitter allows the emission of infrared light and red light thanks to two LEDs. Red light has a wavelength of 660 nm, infrared light has a wavelength of 950 nm. These two lights will pass through the skin and will be picked up by a receiver, made up of a photodiode, which will quantify them.
A calculation of the amount of light absorbed will make it possible to determine the blood oxygen saturation. Blood saturation (SpO2) is expressed as a percentage and will provide an estimate of a patient’s condition. The normal value is between 90% and 100%. The oximeter will also make it possible to measure the heart rate, by measuring the variation in the various blood flows at the extremities.
In addition to the sensor that is placed at the ends, the oximeter is composed 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 which will analyze and translate the data received from the sensor. The sensor is shaped like a finger cot or finger clip and is placed on the fingers, ears and wings of the nose for adults or on the toes and feet for pediatric models.
Sources: Minnesota Department of Health, Johns Hopkins, WHO
Photo source: Wikimedia Commons
