Are SpO2 and FiO2 the Same?

You may have heard of oxygen saturation (SpO2) and fraction of inspired oxygen (FiO2), but what exactly are they? Are they the same? No. SpO2 and FiO2 are not the same, but they are related. SpO2 determines the amount of oxygen in your blood, while FiO2 is the percentage of oxygen in the air you breathe.

While most people may know about SpO2, FiO2 may be a relatively new term to many. This article looks closely at SpO2 and FiO2 and how they are related.

What Is FiO2?

FiO2 stands for Fraction of Inspired Oxygen. It is the fraction or concentration of oxygen in the gas mixture that a person inhales. Typically, natural room air has a FiO2 of 21 percent, regardless of the person’s altitude.

The air we breathe is composed of three components:

• 21% oxygen
• 78% nitrogen
• 1% other gasses such as argon, carbon dioxide, hydrogen, and neon

The 21% FiO2 is enough to keep your cells healthy when you're in good health. However, you may need more than 21% (0.21) of FiO2 to maintain enough oxygen saturation when sick. In such cases, supplemental oxygen or oxygen therapy is given.  

You may require varying amounts of FiO2 supplementation depending on how sick you are. Generally, FiO2 is kept below 0.5 to avoid oxygen toxicity, which the high partial pressure of inspired oxygen can cause. FiO2 can be increased to 100% (1.00).

PaO2 refers to the partial pressure of oxygen in your blood, indicating how well oxygen is delivered to your cells. A normal PaO2 ranges between 75 and 100 mmHg or 10.5 and 13.5 kilopascals (kPa). The lower the partial pressure, the less oxygen is delivered to cells that need it, leading to hypoxia.

The partial pressure of oxygen (PaO2) changes with the barometric pressure, which varies with altitude. However, FiO2 remains at 21% regardless of altitude.

Why Is the Fraction of Inspired Oxygen Important?

FiO2 ensures your body is getting the oxygen it needs. It is significant because it helps doctors and nurses determine how well patients are oxygenating and if they need more oxygen. Monitoring FiO2 is crucial in preventing low and high oxygen levels, which can lead to adverse effects. Properly managing FiO2 is essential for adequately treating conditions such as acute hypoxemic respiratory failure and severe hypoxemia, where there is a deficiency in the amount of oxygen in the blood.

Measuring FiO2

We have to be careful when measuring FiO2 by looking at the concentration of oxygen in the gas mixture. The standard amount of FiO2 in a gas mixture at room temperature should be at an average of 21%. However, this differs depending on gas exchange levels and several other factors. These factors include altitude, humidity, ventilator settings, and patient-specific factors such as lung function.

Oxygen Flow Rate and the FiO2

When delivering supplemental oxygen, two crucial elements to consider are oxygen flow rate and FiO2. The oxygen flow rate measures how much oxygen flows to your body at a given time. It is typically measured in liters per minute (L/min).

Oxygen delivery devices predict the oxygen flow rate and the FiO2 depending on their algorithm. Typically, the FiO2 increases by 4% for every liter of oxygen delivered. For instance, a nasal cannula with a 1 L/min flow rate provides 28% FiO2.

Generally, FiO2 is never precise, but some guidelines can help achieve an accurate estimation. The actual O2 concentration will depend on breathing rate and tidal volume. However, you can use this simple formula to convert the oxygen flow rate in liters per minute to FiO2.

FiO2 = 20% + (4 x oxygen liter flow)

When you use this formula, you will end up with values similar to the ones in the table below:

There are many devices that deliver oxygen, with each device delivering varying FiO2. Below is a table that can guide you in understanding the amount of oxygen a device can deliver in L/min and the oxygen concentration it can provide in percentage.

What Happens if FiO2 Is High?

If FiO2 remains high for too long, typically because of ventilation with high oxygen concentration levels, it can cause oxygen toxicity that can damage your lungs. When FiO2 is too high, you may experience coughing, difficulty breathing, dizziness, headaches, and nausea. In some critical cases, it can lead to death.

What Happens if FiO2 Is Low?

If your FiO2 is low, the oxygen level in your blood is lower than it should be. Low FiO2 can lead to several health problems, including difficulty breathing, dizziness, fatigue, and even heart problems such as arrhythmias. Low FiO2 can also lead to hypoxia. Proper management and monitoring of oxygen levels are essential to prevent these effects.

What Causes Decreased FiO2?

Several factors, especially environmental ones, lead to low levels of FiO2. At higher altitudes, the partial pressure of oxygen decreases, leading to lower FiO2. In a poorly ventilated or confined space, the room air can be filled with other gases, lowering the relative oxygen concentration.

There are also physiological factors that affect your body’s ability to utilize oxygen efficiently. These factors include airway resistance, hypoventilation, impaired alveolar diffusion, lung compliance, pulmonary shunting, respiration rate, and respiratory drive.

What Is the P/F Ratio?

The P/F ratio, or the PaO2/FiO2 ratio, is calculated by dividing the arterial partial pressure of oxygen (PaO2) by the inspired oxygen concentration (FiO2). The PaO2/FiO2 ratio helps determine the severity of any issue related to how the lungs transfer oxygen to the blood. These issues include acute respiratory distress syndrome (ARDS) and acute lung injury.

The intensive care unit (ICU) frequently employs the PaO2/FiO2 ratio to gather information on the oxygenation status of critically ill patients. Patients in the intensive care unit have a higher risk of mortality and more extended hospital admissions when their PaO2/FiO2 values are low. Pathological conditions, such as acute respiratory distress syndrome (ARDS), pneumonia, and acute pulmonary edema, can lead to a low P/F ratio.

What is SpO2?

SpO2, also known as oxygen saturation, is a measurement of oxygen in the blood at any given time. Measuring SpO2 through pulse oximetry has become a standard procedure in hospitals, as the levels can tell your doctor a lot about your health. Low SpO2 can signify a serious health condition, including heart and lung diseases. 

Some people may confuse SpO2 with SaO2. The main difference is that SpO2 measures hemoglobin in peripheral blood saturated with oxygen. On the other hand, SaO2 is the percentage of hemoglobin in your arterial blood that is saturated with oxygen. It is measured via arterial blood gas (ABG) analysis.

Measuring SpO2

Measuring SpO2 is often done through pulse oximetry, using a small device you clip to your finger. The pulse oximeter shines a light into your finger and measures how much light is absorbed, which corresponds to the amount of oxygen in your blood. 

The regular SpO2 reading through pulse oximetry is 95 to 100%. This means 95 to 100% of your blood is saturated with oxygen. Anything less than 95% in the pulse oximetry device is considered low and may indicate hypoxia. However, clinical relevance may vary, and further testing may be needed to confirm hypoxia.

Pulse oximetry utilization skyrocketed during the COVID-19 pandemic to help monitor oxygen levels, especially for those with respiratory symptoms.

What Is the SpO2/FiO2 Ratio?

The oxygen saturation to fraction of inspired oxygen (SpO2/FiO2), or S/F ratio, is a non-invasive measure that can detect acute respiratory distress syndrome (ARDS) in high-risk patients. The SpO2/FiO2 ratio can be used as a substitute for invasive PF during oxygen supplementation. Studies show a high correlation between the SpO2/FiO2 and PaO2/FiO2 ratios.

Conclusion

Oxygen saturation (SpO2) and fraction of inspired oxygen (FiO2) are not the same, but they are closely related. Despite not being the same, they are both crucial as they can help estimate the degree of hypoxemia and other acute respiratory problems.