How does a pulse oximeter measure hemoglobin saturation with oxygen

Oxygen saturation is defined as the ratio of oxy-hemoglobin to the total concentration of hemoglobin present in the blood (ie Oxy-hemoglobin + reduced hemoglobin).

Hemoglobin is an iron-containing protein bound to red blood cells and makes up nearly all the oxygen presence (there is a minute amount dissolved in the plasma). Hemoglobin is responsible for transporting oxygen from lungs to other parts of the body, where the oxygen can be used by other cells. Oxy-hemoglobin (HbO2) is the bright red hemoglobin that is a combination of hemoglobin and oxygen from the lungs.

A hemoglobin molecule can carry a maximum of four oxygen molecules. 1000 hemoglobin molecules can carry a maximum of 4000 oxygen molecules; if they together were carrying 3600 oxygen molecules, then the oxygen saturation level would be (3600/4000)*100 or 90%.

Here is an informational video by the New England Journal of Medicine that explains in detail what is oxygen saturation levels or SpO2 and the technology behind pulse oximetry.

When arterial oxy-hemoglobin saturation is measured by an arterial blood gas it is called SaO2. When arterial oxy-hemoglobin saturation is measured non-invasively by a finger pulse oximeter or handheld pulse oximeter, it is called SpO2.

Methods for measuring oxygen saturation level

Pulse Oximeter: A pulse oximeter is a device intended for the non-invasive measurement of arterial blood oxygen saturation and pulse rate. Typically it uses two LEDs (light-emitting diodes) generating red and infrared lights through a translucent part of the body. Bone, tissue, pigmentation, and venous vessels normally absorb a constant amount of light over time. Oxy-hemoglobin and its deoxygenated form have significantly different absorption pattern. The arteriolar bed normally pulsates and absorbs variable amounts of light during systole and diastole, as blood volume increases and decreases. The ratio of light absorbed at systole and diastole is translated into an oxygen saturation measurement.

CO-oximeter: A CO-oximeter is a device for detecting hypoxia and works similar to a pulse oximeter. CO-oximeter measures absorption at several wavelengths to distinguish oxy-hemoglobin from carboxyhemoglobin and determine the oxy-hemoglobin saturation even when the patient has carbon monoxide poisoning.

Capnometer:A capnometer is an instrument for monitoring breathing rate and adequacy of ventilation. It attaches to the endotracheal tube and measure the carbon dioxide content in the inspired and expired air. It uses an infrared light to measure the amount of light absorbed by carbon dioxide molecules during breathing. It detects changes in carbon dioxide concentrations in patients who are hemodynamically stable, but not critically ill.

Arterial blood gas (ABG) analysis: This is a blood test using samples extracted from an artery. The test determines the pH of the blood, the partial pressure of carbon dioxide and oxygen, and the bicarbonate level. Many blood gas analyzers will also report concentrations of lactate, hemoglobin, several electrolytes, oxy-hemoglobin, carboxyhemoglobin and methemoglobin. The arterial blood gas analysis determines gas exchange levels in the blood related to lung function.

Reference

1. Oxygen saturation
2. Definition and notation of hemoglobin oxygen saturation

Oxygen saturation is a measure of how much oxygen is in your blood. Your organs and tissues need oxygen to work. Oxygen can "hitch a ride" in your red blood cells and travel through the bloodstream to get where it needs to go in your body.

For most healthy adults, a normal oxygen saturation level is between 95% and 100%. An "O2 sat" level below this range requires medical attention because it means your body isn't getting enough oxygen.

This article covers conditions that affect the amount of oxygen in your blood and the complications of having low oxygen saturation. You will also learn how to measure your blood oxygen levels and when to seek treatment.

How Blood Gets Oxygenated

To understand how blood gets saturated with oxygen, you have to know about the air sacs in the lungs called alveoli. There are millions of these microscopic air sacs in the lungs. They exchange oxygen and carbon dioxide molecules to-and-from the bloodstream.

As oxygen molecules pass through the alveoli, they bind to a substance in the blood called hemoglobin.

Oxygen "hitches a ride" on the hemoglobin as it circulates until it gets dropped off to the body's tissues. Then, hemoglobin picks up carbon dioxide from the tissues and transports it back to the alveoli. Once there, the cycle can begin all over again.

The level of oxygen in your blood depends on several factors:

• How much oxygen you breathe in
• How well the alveoli swap carbon dioxide for oxygen
• How much hemoglobin is concentrated in red blood cells
• How well hemoglobin attracts oxygen

Most of the time, hemoglobin contains enough oxygen to meet the body's needs. However, some diseases reduce hemoglobin's ability to bind to oxygen.

Each one of your blood cells contains around 270 million molecules of hemoglobin. Any health condition that limits your body's ability to produce red blood cells can result in low hemoglobin levels, which in turn limits the amount of oxygen that can saturate your blood.

Conditions That Affect Oxygen Saturation

Blood disorders, problems with circulation, and lung issues can prevent your body from absorbing or transporting enough oxygen. When this happens, it can lower your blood's oxygen saturation level.

Examples of conditions that can affect your oxygen saturation include:

• Respiratory infections (e.g., a cold, the flu, COVID-19): Any condition that affects your ability to breathe will affect your oxygen intake
• Chronic obstructive pulmonary disease (COPD): A group of chronic lung diseases that make it difficult to breathe
• Asthma: A chronic lung disease that causes airways to narrow
• Pneumothorax: A partial or total collapse of the lung
• Anemia: Not having enough healthy red blood cells
• Heart disease: A group of conditions that affect the heart's function
• Pulmonary embolism: When a blood clot causes blockage in an artery of the lung
• Congenital heart defects: A structural heart condition that is present at birth

How to Measure Your Blood Oxygen Levels

Oxygen saturation is measured in one of two ways: an arterial blood gas test (ABG or Sa02) or pulse oximetry (Sp02).

ABG is usually only done in a hospital, while pulse oximetry can be done in other healthcare settings (like a provider's office) and even at home.

ABG

An ABG value refers to the levels of oxygen and carbon dioxide in blood running through your veins.

During an ABG, a nurse or lab technician draws blood from an artery, such as the radial artery in the wrist or the femoral artery in the groin. The sample is immediately analyzed by a machine or in a lab.

Your ABG value can give your healthcare provider a sense of how efficiently the hemoglobin in your blood exchanges oxygen and carbon dioxide.

Pulse Oximetry

A pulse oximetry reading reflects the percentage of oxygen found in arterial blood.

Unlike the ABG test, pulse oximetry does not involve a needle (non-invasive). Instead, the test uses a sensor to read wavelengths reflected from the blood. The probe is attached to your finger, earlobe, or another place on the body. A pulse oximeter can give results on a screen in just a few seconds.

People can monitor their oxygen saturation levels using wearable pulse oximetry devices—some smartwatches even have this feature. You can also buy a pulse oximetry device at your local pharmacy or online.

What Causes Decreased Oxygen Saturation?

A drop in oxygen saturation in the blood is called hypoxemia. It can be caused by:

• Less oxygen in the air (for example, when you are flying in an airplane)
• Conditions that affect breathing (such as asthma and COPD)
• Conditions that affect oxygen absorption (such as pneumonia)
• Having too few red blood cells or hemoglobin (anemia)
• Breathing in another substance (such as carbon monoxide or cyanide) that binds more strongly to hemoglobin than oxygen does

How to Raise Blood Oxygen Fast

If your O2 saturation level is low, you need to call your provider. If it's very low, you might need to seek emergency medical care.

You can also take a few steps right away to try to increase your blood oxygen levels:

• Sit up straight rather than laying flat
• Get some fresh air or go inside if you are outdoors and it is very hot/very cold
• Cough to loosen up any mucus (like from allergies or a cold/the flu)
• Check that you are not taking "shallow" breaths; you need to take deep, full breaths to get oxygen into your blood

Complications of Low Oxygen Saturation

Low oxygen saturation in the blood can mean there's less oxygen in the body's tissues, including the organs and muscles. When this happens, it's called hypoxia.

Your cells can adapt to a lack of oxygen when the deficiency is small. However, with larger deficiencies, cell damage and cell death can happen.

Hypoxia usually happens because there is not enough oxygen in the blood (hypoxemia). However, it can also happen when:

• There are not enough red blood cells to carry oxygen to the tissues (e.g., from severe bleeding after a trauma or conditions like sickle cell anemia).
• There is inadequate blood flow (e.g., a stroke occurs when there is low blood flow to a region of the brain; a heart attack occurs when there is low blood flow to the heart muscles).
• The tissues require even more oxygenated blood than can be delivered (e.g., severe infections that cause sepsis may result in hypoxemia and eventually organ failure)

Treating Low Oxygen Saturation

Generally speaking, an oxygen saturation level below 95% is considered abnormal. An O2 sat below 90% is an emergency.

If someone's oxygen saturation is dangerously low, they will need oxygen therapy—sometimes urgently.

The brain is the most susceptible organ to hypoxia. Brain cells can begin to die within five minutes of oxygen deprivation. If hypoxia lasts longer, it can lead to coma, seizures, and brain death.

It is very important to find out the cause of low oxygen saturation so the problem can be fixed.

For example, with chronic conditions such as COPD and asthma, the cause of hypoxia is usually low air exchange in the lungs and alveoli. In addition to oxygen therapy, steroids or rescue inhalers (bronchodilators) might be needed to open the airways.

In circulatory conditions like heart disease, inadequate blood flow reduces oxygen delivery. In this case, medications that improve heart function, such as beta-blockers for heart failure or prescription medications to treat heart arrhythmias, can help improve oxygenation.

With anemia, the blood supply to the tissues is reduced because there are not enough healthy red blood cells with hemoglobin to carry oxygen. Sometimes, a red blood cell transfusion is necessary to increase a person's level of healthy red blood cells.

Summary

Oxygen saturation is the measure of how much oxygen is traveling through your body in your red blood cells. Normal oxygen saturation for healthy adults is usually between 95% and 100%.

If you have a chronic health condition that affects your lungs, blood, or circulation, regularly tracking your oxygen saturation is important. An O2 sat level below 95% is not normal. A level under 90% requires emergency care.

Frequently Asked Questions

• What is a dangerously low oxygen saturation level?

  A saturation level of 90% is usually considered the point where your oxygen saturation is dangerously low and you should get immediate medical health. However, if you have pneumonia or another active infection or medical issue, your doctor may direct you to go to the emergency room if it is 92% or lower.

• Which finger is best for an oximeter?

  Most oximeters work best when used on the middle finger. Some studies have shown that the right middle finger works best. In some people, the right thumb may also work well.

• How long does it take for oxygen levels to return to normal after COVID?

  COVID-19 can damage your lungs and affect your breathing. A lot of people who have COVID experience low 02 sats—sometimes dangerously low.

  We are still learning about how COVID affects the lungs. It can take months for the lungs to heal after COVID. Some people develop lasting breathing problems after they have the infection.

• What is a normal O2 sat for a child?

  Normal oxygen saturation levels are the same for kids as they are in adults; between 95% and 100%.

• What oxygen level is normal while sleeping?

  Oxygen levels above 90% are normal when you're asleep. If your O2 sats are lower than that while you're sleeping, it could be a sign that you have a disorder like sleep apnea.

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