What is blood oxygen saturation? How to measure blood oxygen saturation?

Blood oxygen saturation is a very important medical concept, and you must have heard of it often, but what does this term mean? Today, let's talk about what blood oxygen saturation is and how to measure it.

Image source: pixabay

01 What is blood oxygen saturation?

First, you need to know how oxygen is transported through the blood. You might think that gases can be dissolved in liquids, but the oxygen dissolved in the blood only accounts for a small part of the blood oxygen content, about 1.5% of the total. So, how is the rest of the oxygen transported in the blood? This requires another way, called chemical binding. There is a component in the blood called hemoglobin. When the blood is exchanging gases in the lungs, the oxygen in the air will combine with hemoglobin to form oxygenated hemoglobin.

To use an analogy, the circulatory system is a railway, and hemoglobin is a train running on the railway. Its function is to transport oxygen to various parts of the body. When it is loaded with goods, it is equivalent to oxygenated hemoglobin. The blood that carries more goods is arterial blood, and because oxygenated hemoglobin is bright red, our arterial blood is particularly bright. The blood that carries less goods is venous blood, and because hemoglobin is purple-blue, venous blood looks darker.

But the question is, how much cargo does arterial blood and venous blood carry? The most widely used, popular, and familiar indicator is blood oxygen saturation. Blood oxygen saturation is the ratio of oxygen content to oxygen capacity. The so-called oxygen capacity refers to the maximum amount of oxygen that hemoglobin can bind to in 100 ml of blood. Using the analogy we just used, it is the maximum capacity of 100 ml of blood to transport cargo. The so-called oxygen content refers to the actual amount of oxygen that hemoglobin can bind to in 100 ml of blood, which is the actual load of cargo transported by train.

So, what value should the blood oxygen saturation be maintained at under normal circumstances? When a healthy person is quiet and inactive, the blood oxygen saturation of arterial blood is higher than 95%, about 98%, and the level of venous blood is about 75%. At this point, you can probably understand that hemoglobin is a train that transports oxygen, but when it arrives at the destination, not all carriages unload their cargo, but only a small part of the carriages unload their cargo.

The point is that the difference between the oxygen saturation of arterial blood and venous blood is only 98% to 75%, which is not that big. In other words, when the train carrying oxygen is unloading, it only unloads more than 20% of the entire truckload of goods. When the oxygen saturation of arterial blood decreases, although it seems that the value has not decreased much, it is already a large part of the more than 20%, so it has a great impact on the human body. Therefore, once it is lower than 95%, we should be alert. If it is lower than 90%, we definitely need to go to the hospital for treatment.

02 How to measure blood oxygen saturation?

There are two commonly used methods. The first is blood gas analysis, which is often used in hospitals. The second method can be used in hospitals and at home, which is to measure fingertip oxygen saturation.

Let's talk about blood gas analysis first. Since it is to measure the oxygen saturation in arterial blood, the most direct method is of course to draw arterial blood for measurement. This is of course very accurate, but it is an invasive test. Whether we usually check blood routine or liver and kidney function, we draw blood from the veins. The blood pressure in the veins is relatively low and the flow rate is relatively slow, so the blood drawing operation is safer. Arterial blood is different. The arterial pressure is relatively high and the flow rate is relatively fast, so the operation is of course more difficult. Therefore, the advantage of blood gas analysis is that direct blood drawing is highly accurate, but it can usually only be performed in hospitals. The risk of doing it ourselves at home is very high.

Fortunately, we have a second method, which is to measure the oxygen saturation of the fingertips. The principle of this technology is that the ability of hemoglobin to absorb light changes under different oxygen content. So scientists use this property to use light-emitting diodes to emit red light with a wavelength of 660 nanometers and infrared light with a wavelength of 940 nanometers, and then measure the absorption of hemoglobin and oxygenated hemoglobin to these two measurement lights, and then calculate the blood oxygen saturation of arterial blood. This method is called dual-light spectroscopy.

After all, this technology uses light to irradiate the body instead of needles, so it is non-invasive. In layman's terms, it is a very delicate device that is put on the finger, and we can see the value of arterial blood oxygen saturation on the screen. Because it is measured on the finger, this value is often called finger pulse oximetry by doctors.

Comparing the principles of blood gas analysis and finger pulse oximetry, it is easy to find that blood gas analysis is more accurate. However, in most cases, the accuracy of finger pulse oximetry is also very high. At the same time, it is simple to operate and non-invasive, which is why it is widely used. So, under what circumstances may the value of finger pulse oximetry be inaccurate?

The first is the situation of severe hypoxia. This is inevitable because when the machine was originally designed, it was impossible to let patients with severe hypoxia serve as volunteers to calibrate the instrument's values. Those who can serve as volunteers are healthy people, so the finger pulse oxygen machine has this inherent deficiency. In a state of severe hypoxia, its value is inaccurate. Of course, it can at least indicate that the patient is in a state of hypoxia in the early stage. If we suddenly experience shortness of breath, chest tightness, difficulty breathing and other discomforts, we can self-test the finger pulse oxygen saturation. Once it is lower than the normal value, we should go to the hospital immediately for further diagnosis and treatment.

The second situation is easier to understand. If the machine is not connected properly and there is poor contact with the finger, or if you apply black, green, or blue nail polish, or if your skin color is too dark, it may also cause inaccurate readings.

It is enough for us to understand the above two situations. As for other possibilities, we should leave it to professionals, that is, doctors to deal with.

The article is produced by Science Popularization China-Starry Sky Project (Creation and Cultivation). Please indicate the source when reprinting.
Author: Sun Yifei Director of the Medical Education History Research Office of Hebei Medical University Reviewer: Chen Gang Associate Chief Physician of Peking Union Medical College Hospital