What is the pulse oximeter and how does it work?

We require enough oxygen in our blood to remain healthy. The arterial blood is examined to determine the quantity of oxygen in the blood. The quantity of oxygen in the blood may be precisely determined using an arterial blood gas test, often known as an ABG test. However, such technique is not simple. Blood from an artery must be analyzed in a lab. A pulse oximeter, a tiny device that measures the quantity of oxygen in the blood, is a much easier technique. In a couple of seconds, this device can monitor the quantity of oxygen in the blood as well as the heartbeat. There is no need to draw blood from the body for this, nor is a laboratory required. The general characteristics of electromagnetic waves are used to assess oxygen saturation in the blood or blood oxygen saturation. Takuo Aoyagi, a Japanese engineer, developed the current pulse oximeter in 1974.

Hemoglobin in the blood transports oxygen. There is no assurance, however, that all hemoglobin molecules will contain oxygen. When hemoglobin includes oxygen, it is referred to as oxygenated hemoglobin. De-oxygenated or oxygen-free hemoglobin is hemoglobin that does not contain oxygen. Oxygen saturation, often known as saturation, refers to how much of the total hemoglobin in the blood transports oxygen.

The quantity of oxygen saturation is 100 percent when all of the hemoglobin in a given amount of blood contains oxygen. The oxygen saturation will be 70%, if there is only 70% oxygenated hemoglobin and 30% deoxygenated hemoglobin.

The level of saturation may be calculated quickly using a pulse oximeter. How? The pulse oximeter is tiny enough to be clipped to the finger. On one side, there are two light emitting diodes (LEDs). They produce a red light with a wavelength of about 660 nanometers. With our naked eyes, we can see this light. Other LEDs have a wavelength of around 940 nanometers and emit infrared or infrared light. We can’t perceive infrared light with our bare eyes. The electromagnetic wave includes this light. A photosensitive detector is located at the opposite end of the clip, and it produces an electronic signal at a rate proportionate to the electromagnetic wave that interacts with it.

When electromagnetic waves are applied, some of it is absorbed in the body, and some of it penetrates the body and comes out from the other side. How much will be absorbed, how much will be emitted depends on the frequency and energy of the electromagnetic wave. The power of X-rays is greater than the power of visible light. So most of the X-rays come out through the body.

Oxygenated hemoglobin contains more substances than oxygenated hemoglobin. So oxygenated hemoglobin absorbs much more light than oxygen-free hemoglobin. Oxygenated hemoglobin absorbs much more red light than infrared light. Absorption of oxygen-free hemoglobin is relatively low in both cases.

The clip of the oximeter is pressed by the finger. The fingers have both veins and arteries. Arteries contain oxygen. But the carbon-di-oxide in the blood of the veins also contains oxygen. Then that oxygen is supposed to come out as well. So how does an oximeter understand venous blood and arterial blood? We only want to measure the oxygen in the arterial blood.

The role here is the contraction and dilation of our heart or systole and diastole. The amount of arterial blood increases during the contraction of the heart, and the amount of arterial blood decreases during dilation. As a result, the amount of blood in the arteries fluctuates. The frequency of this wave is equal to the pulse of the artery. The number of vibrations per minute from arterial blood waves can also be calculated.

There is no difference in the amount of blood flowing through the veins. As a result no waves are created. The waveform that occurs between arterial and venous blood flow is called a plathismographic trace. This plate has to be very good for accurate diagnosis of pulse oximeter results.

Oxygen saturation in arterial blood is measured by calculating the ratio of oxygenated and oxygen-free hemoglobin. The higher the amount of oxygen in the blood, the higher the absorption of red light, the lower the emission. As a result the detector will detect less light. The absorption of infrared light will occur even more. The detector will detect infrared light even less. Then the ratio of red light to infrared light will be quite low.

Similarly, if the amount of oxygen in the blood is less, the absorption will be less and the excretion will be more. As a result, the ratio of red light and infrared light detected in the detector will be quite large. Thus the ratio of red and infrared light detected in the detector is inversely proportional to the oxygen saturation.

Red and infrared light LEDs are used in the clip of the oximeter. But the other lights in the room during use also enters the finger. Now does that light cause problems in the reading of the oximeter? Not actually. How? After placing the finger on the clip of the oximeter, the red light comes on in the first step. Then the detector detects the light and red light of the room. Then in the second step the infrared light shines. The detector then detects infrared light and room light. In the third stage, both red and infrared lights are turned off. Then just detect the light in the room. The electronic circuit of the oximeter excludes the light signal of this room from the proportional calculation. So the light in the room does not cause any problem in the reading of the oximeter.

However, if the fingernails have dark colored nail polish, the reading of the oximeter may vary slightly. During this time, many people are monitoring the amount of oxygen saturation in the body with the help of pulse oximeter. In a normal healthy body, the oxygen content of the blood is more than 95%.

Source: “The Daily Prothom Alo” Click Here

October 2021



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