Oxygen Flow Rate and Fraction of Inspired Oxygen (FiO2)

Oxygen Flow Rate and Fraction of Inspired Oxygen (FiO2)

by | Updated: Mar 15, 2024

The life-sustaining importance of oxygen is an undisputed fact in biology. Oxygen, a vital component of the air we breathe, serves as the primary electron acceptor in cellular respiration, driving the production of energy in our bodies.

Among the key measures used to assess the concentration of oxygen we inhale is the fraction of inspired oxygen (FiO2).

Normally, this fraction is approximately 21% under standard atmospheric conditions. However, in certain health scenarios, our bodies may require more than this standard fraction.

In this article, we aim to delve into the intricacies of oxygen administration, the implications of FiO2, and the relationship between FiO2 and oxygen flow rate.

By understanding these elements, we can significantly improve the application of supplemental oxygen therapy in various medical settings.

What is Oxygen Flow Rate?

The oxygen flow rate is a measure of the volume of oxygen supplied to a patient over a period of time, usually measured in liters per minute (L/min).

It determines the speed at which oxygen is delivered from an oxygen source (like a medical oxygen cylinder or a hospital wall outlet) to the patient.

The oxygen flow rate can be adjusted according to the patient’s specific needs, with higher flow rates generally used for patients who have a higher demand for oxygen or are experiencing difficulty breathing.

Oxygen Flowmeter Vector Illustration

What is Fraction of Inspired Oxygen (FiO2)?

The fraction of inspired oxygen (FiO2) is the concentration or percentage of oxygen in the air that is inhaled by a person. In normal room air, the FiO2 is approximately 21%.

However, when supplemental oxygen is provided, the FiO2 can be increased, potentially up to 100% if pure oxygen is supplied.

This is particularly important in clinical settings where patients may need higher oxygen concentrations than what is available in normal room air due to health conditions affecting their oxygen saturation levels.

The goal of increasing FiO2 is to enhance the amount of oxygen that reaches the patient’s bloodstream through the lungs.

What is the Importance of Oxygen in Human Life?

In our day-to-day lives, we depend on a steady supply of oxygen, accounting for 21% of the air we inhale.

The rest of the air is made up of 78% nitrogen and a 1% mix of trace elements such as argon, carbon dioxide, neon, helium, and methane.

Composition of air illustration

In normal health conditions, this proportion is typically sufficient to sustain the body’s metabolic activities.

Our cells constantly use oxygen to generate energy through a process called cellular respiration. In this process, oxygen reacts with glucose to produce energy, with carbon dioxide and water being the by-products.

The oxygen we breathe is transported to cells, where it is used for this vital process.

However, certain medical conditions can disrupt this equilibrium, leading to scenarios where the standard 21% oxygen may not be enough to maintain adequate oxygen saturation.

For instance, individuals suffering from chronic obstructive pulmonary disease (COPD), asthma, pneumonia, and other respiratory conditions may require additional oxygen.

In these situations, supplemental oxygen can be administered via various delivery devices, ranging from simple nasal prongs to more complex systems such as invasive ventilation.

This approach increases the concentration of oxygen that a patient inhales, potentially raising the FiO2 to 100%, thereby ensuring adequate oxygen supply to the body.

Administration of Oxygen Therapy

Historically, FiO2 has not received substantial attention outside of critical care settings. However, the landscape of medical care is constantly evolving, and with it, our approach to oxygen therapy.

In contemporary standard hospital environments, there has been a marked increase in the use of humidified high-flow oxygen therapy.

This therapeutic technique necessitates a comprehensive understanding of the relationship between oxygen flow rate and FiO2.

When patients receive oxygen therapy, it’s crucial to adjust both the flow rate (the speed at which oxygen is delivered) and the FiO2 (the concentration of oxygen).

Patient on oxygen therapy with mask vector illustration

Oxygen Flow Rate and FiO2 Chart

Traditionally, in clinical areas requiring FiO2 documentation, tables are often readily available to guide medical professionals.

These tables typically provide a correlation between oxygen flow rate and FiO2, but it’s important to remember that these are approximate values.

Oxygen Flow Rate and FiO2 Table Chart

Understanding this relationship becomes increasingly critical as we administer oxygen to patients with varied needs.

Furthermore, being aware of why the FiO2 increases with specific increments of oxygen flow rate enriches our understanding of the therapy’s mechanism, thereby enhancing our ability to optimize patient care.

The Nuances of Oxygen Flow Rate and FiO2

To understand how oxygen flow rate affects FiO2, we must first clarify a common misconception. The FiO2 of the oxygen being delivered through the flow meter does not depend on the flow rate.

This may sound counterintuitive, but allow me to explain.

The flow meter is connected to a source that supplies pure oxygen, which means regardless of the flow rate, the oxygen being delivered is always at 100% FiO2.

Hence, whether you set the flow rate at 1 L/min or 15 L/min, you’re delivering pure oxygen — it’s always 100% FiO2.

The variation in FiO2 depicted in the aforementioned chart does not reflect changes in the purity of the oxygen being delivered. Instead, it relates to how the patient’s breath mixes pure oxygen with the surrounding air.

This brings us to the concept of “peak inspiratory flow” (the maximum speed of inhalation) and how it interacts with FiO2.

Our lungs, under normal conditions, have a peak inspiratory flow rate ranging between 20–30 L/min. However, during physical exertion or respiratory distress, the demand for air increases, causing us to inhale more vigorously.

This means that our peak inspiratory flow rate can significantly increase during these times.

The change in FiO2 that we observe in clinical settings occurs because patients inhale a mixture of pure oxygen and room air.

As we increase the oxygen flow rate, patients inhale a larger proportion of pure oxygen relative to room air. If the oxygen flow rate matches or exceeds the patient’s peak inspiratory flow rate, the patient inhales virtually no room air, and the FiO2 approaches 100%.

However, if the oxygen flow rate is less than the patient’s peak inspiratory flow rate, the patient will also inhale room air, and the FiO2 will be less than 100%.

Oxygen Flow Rate and Peak Inspiratory Flow

Let’s delve further into the concept of peak inspiratory flow and how it interacts with FiO2 using a simple mathematical illustration.

Assume you are breathing normally at a peak inspiratory flow rate of 30 L/min, with room air at an FiO2 of 21%.

Under these circumstances, the average FiO2 you are breathing in can be calculated as:

30 L/min * 21% (FiO2) = 630%

630% / 30 L/min = 21% (FiO2)

Now, suppose you are receiving supplemental oxygen at a flow rate of 10 L/min at an FiO2 of 100%, while your peak inspiratory flow rate remains at 30 L/min.

The remaining 20 L/min of air needed to meet your peak inspiratory flow rate will be drawn from the surrounding room air, which has an FiO2 of 21%.

Using the same formula:

(10 L/min * 100%) + (20 L/min * 21%) = 1,420%

1,420% / 30 L/min = 47% (FiO2)

Therefore, despite receiving pure oxygen at a flow rate of 10 L/min, the effective FiO2 is diluted to 47% by the room air drawn in to meet the peak inspiratory flow rate.

Conversely, if your peak inspiratory flow rate decreases to 20 L/min while still receiving supplemental oxygen at a flow rate of 10 L/min, the FiO2 increases:

(10 L/min * 100%) + (10 L/min * 21%) = 1,210%

1,210% / 20 L/min = 60% (FiO2)

Here, even though the supplemental oxygen flow rate has not changed, the decrease in peak inspiratory flow rate results in an increase in effective FiO2.

The “dilution” of pure oxygen by room air decreases, resulting in an increased FiO2.

This example illustrates that FiO2 cannot be determined by the oxygen flow rate alone but also depends on the patient’s peak inspiratory flow rate.

The tables that correlate oxygen flow rate and FiO2 are estimations based on average peak inspiratory flow rates (20–30 L/min).

In practice, each patient’s respiratory condition and needs should be carefully evaluated to determine the most suitable oxygen therapy parameters.

Should You Increase the Patient’s Flow Rate or FiO2?

Understanding the distinction between oxygen flow rate and FiO2 can be crucial in the practical management of patients.

In broad terms, a patient’s needs can be divided into two categories:

  1. Oxygenation: If a patient has an issue with oxygenation, they require a higher FiO2. This need is typically addressed by increasing the oxygen flow rate, which consequently increases the FiO2.
  2. Ventilation: If a patient has a problem with ventilation, they require a higher flow rate to help facilitate better breathing.

For example, an asthmatic patient struggling to breathe. Providing additional flow can alleviate their breathing effort, making it easier for them to draw in air.

In contrast, for a patient with suboptimal oxygen saturations, the standard response is to increase the oxygen flow rate to augment the FiO2 and, thus, the oxygen saturation.

Some patients, however, may experience problems with both oxygenation and ventilation. They would require an adjustment of both FiO2 and flow rate.

Summary: It’s crucial to assess the patient’s specific needs—whether it’s an issue with oxygenation, ventilation, or both—to determine the most effective approach for managing their respiratory condition.

FiO2 and Oxygen Concentrators

An oxygen concentrator is a medical device that helps deliver supplemental oxygen to individuals who require more oxygen than is available in ambient air.

These devices function by taking in air from the environment, filtering it to remove nitrogen, and then delivering concentrated oxygen to the user. This oxygen is typically delivered to the patient through a nasal cannula or a mask.

Oxygen concentrator vector illustration

The fraction of inspired oxygen (FiO2) delivered by an oxygen concentrator can vary depending on the settings and the specific device used.

Generally, oxygen concentrators can deliver FiO2 levels ranging from about 21% (equivalent to room air) up to 95% at different flow rates.

However, the exact FiO2 a patient receives also depends on other factors like the patient’s breathing rate, the fit of the mask or nasal cannula, and the patient’s specific respiratory demand.

That’s why, in practice, although oxygen concentrators can produce up to 95% concentrated oxygen, the actual FiO2 received by the patient is usually lower.

It’s also worth noting that while oxygen concentrators can provide high concentrations of oxygen, they are not suitable for all patients requiring supplemental oxygen.

Some patients with severe respiratory conditions may require other forms of oxygen delivery, such as from high-flow systems or pressurized cylinders that can deliver higher flow rates or more precise oxygen concentrations.

Remember: Oxygen therapy should be administered under the guidance of a healthcare provider, who can best assess the patient’s condition and oxygen requirements.

FAQs About Oxygen Flow Rate and FiO2

What is Oxygen Percentage?

Oxygen percentage, often referred to as the concentration of oxygen, denotes the volume of oxygen present per unit volume of air. It is typically expressed as a percentage.

For instance, if the oxygen percentage in a particular air mixture is 21%, this means that out of 100 molecules of air, 21 molecules are oxygen.

What is the Room Air Oxygen Percentage?

The oxygen percentage in room air, or ambient air, is approximately 21%. This means that out of all the gases present in the air we breathe in a typical indoor or outdoor environment (not considering variations due to altitude, pollution, or other factors), about 21% of it is oxygen.

The majority of the rest is nitrogen (approximately 78%), with the remaining 1% composed of trace gases like argon, carbon dioxide, neon, helium, and methane.

What is the Normal FiO2 Range?

FiO2 stands for Fraction of Inspired Oxygen. This is the concentration of oxygen in the air we inhale. In normal, healthy conditions, the FiO2 of room air (the air most people breathe under normal circumstances) is about 21%.

However, when supplemental oxygen is used, for instance, in medical treatment for people with respiratory conditions, the FiO2 can range from 21% up to 100%.

The exact FiO2 prescribed will depend on the individual’s health status and specific needs as assessed by a healthcare provider.

How to Calculate FiO2?

FiO2 refers to the concentration of oxygen that a person is inhaling. Under normal conditions, the FiO2 of room air is approximately 21%.

However, when supplemental oxygen is being administered, FiO2 can be adjusted according to the oxygen flow rate.

For high-flow oxygen delivery systems, the FiO2 is often directly controllable. In low-flow systems, it’s more complex because the patient breathes a mix of room air and the supplied oxygen.

The exact FiO2 in low-flow systems can be difficult to calculate accurately because it depends on several factors, including the patient’s breathing pattern, the fit of the delivery device, and the specific flow rate.

What is SpO2%?

SpO2% stands for peripheral capillary oxygen saturation. It’s a measure of the amount of oxygen-carrying hemoglobin in the blood relative to the amount of hemoglobin not carrying oxygen.

This value is typically assessed using a device called a pulse oximeter, which can be placed on a person’s finger, toe, or earlobe.

Normal SpO2 values typically range from 94 to 100 percent. Values under 90% are generally considered low and may indicate a need for supplemental oxygen.

How Does SpO2% Help in Determining Oxygen Flow Rate?

SpO2% is a critical measurement in determining whether a person requires supplemental oxygen. If a person’s SpO2% is below the normal range, they may be experiencing hypoxia, a condition where the body or a region of the body is deprived of adequate oxygen supply.

Healthcare providers use SpO2% values to determine the initial oxygen flow rate when starting oxygen therapy.

Subsequent SpO2% readings then guide adjustments to the flow rate. The goal of oxygen therapy is to achieve an SpO2% within a target range specified by a healthcare provider.

This target range can vary depending on the individual’s health status and specific medical condition.

Monitoring SpO2% allows healthcare providers to adjust the oxygen flow rate to ensure that the patient is receiving an adequate amount of supplemental oxygen.

Final Thoughts

Oxygen therapy is not just about providing oxygen to the patients. It’s about understanding the nuances of oxygen flow rate and FiO2 and how they can be tweaked to address patients’ specific needs.

Remember: It’s not only the oxygen content that matters. The rate at which the oxygen is delivered can significantly influence the patient’s respiratory comfort and efficiency.

As healthcare professionals, it’s our responsibility to understand these parameters deeply and apply them effectively in practice.

From asthmatic patients struggling to draw in air, to those dealing with suboptimal oxygen saturations, our ability to manipulate both flow and FiO2 can have a profound impact on patient outcomes.

Despite the complexities, the goal is straightforward: ensure that patients receive the optimal oxygen concentration and flow rate for their unique respiratory needs.

By doing so, we can make breathing easier for them, improve their oxygen saturation levels, and ultimately enhance their overall quality of life.

Key Points:

  • The fraction of inspired oxygen (FiO2) in normal room air is 21%.
  • Oxygen flow rate and FiO2 are closely related, but they cater to different patient needs.
  • Higher FiO2 is required when patients have a problem with oxygenation, while an increased flow rate is needed for those with ventilation issues.
  • Both FiO2 and flow rate need to be increased for patients having trouble with both oxygenation and ventilation.
  • Ensuring appropriate oxygen therapy requires a solid understanding of FiO2 and flow rate, as well as the patient’s unique respiratory needs.

So, when you’re faced with a patient struggling to breathe or a patient with less-than-optimal oxygen saturations, remember: the key may be not just the oxygen itself but also the way in which it is delivered.

John Landry, BS, RRT

Written by:

John Landry, BS, RRT

John Landry is a registered respiratory therapist from Memphis, TN, and has a bachelor's degree in kinesiology. He enjoys using evidence-based research to help others breathe easier and live a healthier life.


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