How Do Pulse Oximeters Work? A Complete Guide

How Do Pulse Oximeters Work? A Complete Guide

Portable pulse oximeter on finger

Think about the last time that you went to the doctor’s office. You likely remember a nurse or a medical assistant gathering information about your health. One of the most vital kinds of information is your vital signs.

Among your vital signs are your temperature, blood pressure, pulse, respiratory rate, and oxygen saturation. The one vital sign that the medical assistant can’t take by themselves is oxygen saturation.

For this reading, they have to depend on pulse oximeters.

Do you remember that little metal object that they place over your finger? That’s it; that’s the pulse oximeter.

To learn more about that little device and how it works, keep reading.

What Are Pulse Oximeters?

Pulse oximeters are small medical devices that have the ability to measure oxygen saturation. These devices do this by detecting how much of the hemoglobin in your blood is carrying oxygen.

All kinds of medical professionals use pulse oximeters to gather information about their patients. You’ll find a finger pulse oximeter in operations rooms, recovery suites, clinics, wards, ambulances, and more.

The Benefits of Using a Pulse Oximeter

Compared to other methods of detecting oxygen saturation, pulse oximeters are the most popular.

First, pulse oximeters are non-invasive. Unlike methods that involve a blood draw, pulse oximeters only have to go on your finger. There’s no pricking involved.

Pulse oximeters are also easy to use. Those without medical experience can still use the devices to take their oxygen saturation at home. All you have to do is place the device over your finger and press the power button.

Because they’re easy to use and commonly found, pulse oximeters are cheap in price. Most people can afford a pulse oximeter, and you can find them at most drug stores.

On top of these, finger pulse oximeters are extremely compact. They can fit in your pocket.

Lastly, pulse oximetry allows you to detect hypoxemia (low oxygen levels) before you develop side effects. One of the most common symptoms of low oxygen perfusion is cyanosis, which occurs when the skin turns blue.

If you have any conditions that affect your oxygen saturation, you should have a pulse oximeter at the ready whenever you feel unwell.

The Importance of Oxygen Saturation

Pulse oximeters measure oxygen saturation. In order to understand how a pulse oximeter works, you should understand what oxygen saturation is.

We all know how important oxygen is to our livelihood, but did you know that we need an oxygen saturation above 95% to have adequate oxygen perfusion? For individuals with chronic conditions, that number may be lower than 95% on a regular basis.

In order to get a higher oxygen saturation in your blood, your lungs need to be able to transfer more oxygen to the hemoglobin in your blood. This process could be interrupted by multiple factors.

First, oxygen enters the lungs as we breathe in. It travels down to the alveoli where gas exchange takes place. This is where the oxygen goes into the hemoglobin in the blood that travels through the vessels around the alveoli.

From there, the blood travels all over our bodies to deliver oxygen to every tissue.

Many people think about this process as a highway. You can think of the bloodstream as the road and the hemoglobin as cars or other vehicles driving on it. The oxygen rides on these vehicles until they reach their destination. 

Hemoglobin with oxygen on it is oxygenated hemoglobin, while hemoglobin without oxygen on it is deoxygenated hemoglobin. Oxygen saturation is the percentage of available hemoglobin in the blood that is carrying oxygen.

How a Pulse Oximeter Works

Pulse oximetry uses light to measure oxygen saturation. Light beams go through your finger into a light detector on the other side of the pulse oximeter. Some of the light beams will make it through the finger, while others will absorb into the finger.

The amount of light that the finger absorbs depends on several factors:

  • The concentration of the substance that’s in between the light beams and the light detector
  • The length of the light path in that substance
  • The amount of oxygenated hemoglobin
  • The amount of deoxygenation hemoglobin

The hemoglobin in our blood absorbs light. Therefore, the amount of light that the finger absorbs is proportional to the concentration of hemoglobin in the blood.

Given that there is an artery and a vein in our finger, one has a high concentration of hemoglobin while the other has a low concentration of hemoglobin, respectively.

The length of the light path depends on the thickness of the artery in the finger. People with thicker arteries are going to absorb more light from the pulse oximeter.

Lastly, we need to cover the difference between the absorption of light in oxygenated hemoglobin and deoxygenated hemoglobin. Oxyhemoglobin absorbs more infrared light than red light, while deoxyhemoglobin absorbs more red light than infrared light.

Since red light and infrared light have different wavelengths, the finger pulse oximeter can detect how much of each wavelength the finger absorbed. Using the ratio between the absorption of red light and the absorption of infrared light, the pulse oximeter can calculate your oxygen saturation.

How to Read a Pulse Oximeter

Pulse oximeter readings are very easy to understand. Every pulse oximeter displays your oxygen saturation in large numbers, so you can read the number without any problems.

Some pulse oximeters also display other vitals like pulse rate. This tells you how fast your heart is beating.

Ordinarily, your pulse rate should be between 60 and 100 beats per minute.

Potential Problems With Pulse Oximeter Readings

Finger pulse oximeters aren’t perfect. Since they are such compact, simple machines, there may be a few problems when you’re trying to use a pulse oximeter. Here are the top ten issues that may happen while measuring your oxygen saturation with a finger pulse oximeter:

  1. Placement
  2. Movement 
  3. Optical shunting
  4. Room light
  5. Electromagnetic interference
  6. Poor peripheral perfusion
  7. Hyperoxia
  8. Calibration
  9. Colored dyes
  10. Abnormal hemoglobin

Each one of these problems is important to consider, especially if you’re working in healthcare.

1. Placement

Placing the pulse oximeter isn’t difficult, but many healthcare providers place it too quickly. The provider may place the meter crooked, making it impossible for the device to read the oxygen saturation correctly.

When you’re placing the device, you should make sure that it is straight on the finger. You should also ensure that the oximeter is placed completely over the end of the finger. There shouldn’t be any space left inside of the oximeter.

2. Movement

If the patient is moving during oxygen measurement, it can cause an interruption in oxygen saturation readings. The person wearing the oximeter should remain still.

Otherwise, the reading won’t be reliable.

3. Optical Shunting

If the probe is the wrong size for the person wearing it, it’s not going to read the oxygen saturation correctly.

Often, this problem occurs when healthcare providers try to use adult meters on pediatric patients.

4. Room Light

Too much light in the room can alter the meter. Make sure that the oximeter remains closed around the patient’s finger so that the room light cannot get into the oximeter.

5. Electromagnetic Interference

If the pulse oximeter is around harmful electric waves, it could alter the meter’s measuring capabilities. Make sure that you aren’t measuring oxygen saturation around these kinds of electromagnetic waves.

6. Poor Peripheral Perfusion

If the patient has poor peripheral perfusion (as seen in hypotension), the pulse oximeter is going to have trouble reading the oxyhemoglobin levels.

Hypotension causes arteries to be less pulsatile. Therefore, the light rays can’t pick up the amount of oxyhemoglobin in the bloodstream.

7. Hyperoxia

Hyperoxia is the opposite of hypoxia. Hyperoxia is the condition of having too much oxygen traveling in the bloodstream.

Unfortunately, pulse oximeters can’t measure an oxygen saturation greater than 100%. Therefore, they aren’t able to detect hyperoxia.

8. Calibration

Pulse oximeters are calibrated by using humans. Therefore, the calibrations can be a little off.

You should make sure that your pulse oximeter is calibrated accurately before using it on patients. If you’re noticing that readings are off for multiple patients, you should consider recalibrating the device if possible.

9. Colored Dyes

If the patient is wearing fingernail polish, this can affect how light travels. Thus, it will affect the ability of the pulse oximeter to read oxygen saturation accurately.

If your patient is wearing fingernail polish, you should remove the polish before placing the pulse oximeter on their finger.

10. Abnormal Hemoglobin

Pulse oximeters cannot detect other forms of hemoglobin, such as carboxyhemoglobin. So, it could give an inaccurate reading for patients that have these altered forms of hemoglobin.

Invest in Pulse Oximetry

Pulse oximeters are an important addition to any household. Even if you don’t have a pulmonary condition, these devices are great for health monitoring when you or a family member is sick.

If you’re looking for an advanced pulse oximetry system, you can count on Infinium’s vital signs monitors. These are great for the office or advanced monitoring at home.

Buy your supplies today so that you can have them the next time a loved one is ill.

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