Thursday, September 2, 2021

What is Oximetry ?

Oximetry and Oxygen Saturation


What is Oximetry ?

Currently, the oxygen saturation checking is still mostly done manually. Manual heart rate measurement can be done by counting the pulse on the wrist and then counting it per minute. Measurements in this way also cannot detect heart rate activity and oxygen saturation automatically and cannot be performed on people who live far from the hospital. So to get this information, a system is needed that can intercept information on heart rate and oxygen saturation and can work in remote areas of the body. Therefore, a system was created to carry out monitoring activities that can continuously check oxygen saturation with the Oximetry device.

Oximetry is one of the most important tests (written in the WHO Surgical Safety Checklist) to determine oxygen saturation in the blood. This examination is a universal standard found in the ICU, Emergency Unit, Neonatal, and even in the operating room. Because the Pulse Oximeter is used to continuously detect the lack of oxygen in the body, medical personnel can know when and how much oxygen should be added. If a person is left deprived of oxygen (below the average limit) even if only for three minutes, it can cause brain damage and heart failure. Likewise, this measurement is also useful in making quick medical decisions but for disease diagnosis, it is still assisted by a series of other medical tests.

About Oxygen Saturation

Oxygen is a gas in the air we breathe, it is necessary for human life. Some people with respiratory disorders cannot get enough oxygen naturally. They may need supplemental oxygen, or what is called oxygen therapy. To determine if a person needs to benefit from oxygen therapy, doctors will test the amount of oxygen in their arterial blood. Another way to check is to use a pulse oximeter which indirectly measures oxygen levels, or saturation, without the need for continuous, real-time monitoring of blood samples.

Oxygen saturation is a measurement and calculation of the percentage of oxyhemoglobin (HbO2) in arteries, Oxygen saturation is defined by the ratio of oxyhemoglobin and deoxyhemoglobin. Then it can be shown by the following equation:

Oxygen Saturation

Oxyhemoglobin (HbO2) is hemoglobin that completely binds oxygen. While Deoxyhemoglobin (Hb) is hemoglobin that does not completely bind oxygen. SaO2 is the oxygen saturation of the arteries. While SpO2 is oxygen saturation detected by the pulse oximeter.

The monitoring system is a system that is acquired and displayed on a screen that can be directly accessed by examiners. The purpose of monitoring is to ensure that the right treatment can be given to the patient, and can carry out an examination before a more severe condition occurs. Therefore, monitoring is an activity that provides an early indication of changes in examiner status in assisting for early intervention and makes it possible to record, monitor, and control the results of these interventions. The parameter that is displayed continuously in the manufacture of this tool is the measurement of oxygen saturation in the blood. Oxygen saturation is the percentage of hemoglobin bound to oxygen in the arteries, normal oxygen saturation is between 93 – 100%.

About Oximetry

Oximetry or Pulse Oximetry is a device to measure the value of oxygen levels in the blood or measure the concentration of oxygen in the blood without the need for any tools to enter the body (non-invasive). In the oximetry measurements, the level of oxygen in the blood is usually referred to as SpO2 and the measurement results are displayed in percent value units (%), besides that the pulse oximetry can also display measurements of heart rate (pulse rate), and the measurement is usually displayed in units of beats per minute (beats per minute) BPM.

Oximetry is used to measure the level of oxygen in the blood. This device is very useful and can be used in various situations and conditions. Among others :

  • During or after surgery or while using sedatives (anesthesia)
  • To monitor the performance of drugs during treatment on the lungs
  • To know one's ability when coping with increasing activity level
  • To monitor the use of the ventilator machine
  • To monitor whether a person experiences moments of stopping breathing during sleep (sleep apnea)
  • Pulse Oximetry can also be used to check a person's health for health conditions related to oxygen levels in the blood such as : Heart Attack, Heart Failure, Chronic obstructive pulmonary disease (COPD), Anemia, Lung Cancer, Asthma, and Other.
  • For the current now, condition of the Virus epidemic in the world, this device is very useful for monitoring the condition of oxygen saturation (SPO2) of infected patients.

How Does Pulse Oximetry Work 

This measurement has 2 methods, namely transmittance mode and reflectance mode. If in transmittance mode, the red and infrared LEDs (transmitters) are placed across the fingers as measured by the photodiode sensor (receiver). Unlike the reflectance mode, red and infrared LEDs (transmitters) are installed parallel to the photodiode sensor (receiver). As shown in the following figure:

How Does Pulse Oximetry Work

In Transmittance mode, the light source (LED and Infrared) passes through the arteries on the finger that is placed to measure oxygen saturation, then the light is received by the photodiode sensor, but in this mode, the measurement is limited to fingers, toes, and ears. While in reflectance mode, light sources (LED and Infrared) are placed parallel to pass through the arteries and then reflected and then received by the photodiode sensor. With this mode, measurements can be applied almost anywhere on the skin area. And in principle, blood cells that have a lot of oxygen will absorb more light from infrared than light from a red LED and from the absorption of the light then the rest will be detected by the photodiode sensor, and the results of the light captured by the photodiode sensor will be converted into analog values ​​so that later will be processed as data. Red LED has a wavelength of 660nm and infrared light has a wavelength of 960nm which is the difference in absorption of the two wavelengths of oxyhemoglobin and deoxyhemoglobin.

Pulse Oximetry

In the picture above, blood containing oxygen (oxyhemoglobin) absorbs more infrared light than red light, while blood that does not contain oxygen (deoxyhemoglobin) absorbs more red light than infrared light.

The output of the Photodiode sensor consists of an AC component which is a change in blood volume caused by heart rate and a DC component generally associated with skin tissue absorption. The output is then filtered with a low pass first to remove noise at frequencies above 50 Hz and then filtered again with a high pass filter to remove low frequencies. The heart rate calculation uses the input signal from the high pass filter to find the frequency between peaks to peaks and then multiplied by 60 seconds or 1 minute. Calculation of Root Mean Square or RMS is used to calculate oxygen saturation. The RMS calculation process is to add up the squares of each of the 15 red LED and infrared LED signal data then.


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