Oxygen therapy is measured using two concepts: flow rate and concentration. Flow rate is quantified in Liters per Minute (L/min), indicating the volume of gas delivered. Concentration is measured by the Fraction of Inspired Oxygen (\(\text{FiO}_2\)), representing the percentage of oxygen in the inhaled air. Ambient air contains \(21\%\) oxygen, which is the baseline \(\text{FiO}_2\). Supplemental oxygen, often delivered via a low-flow device like a nasal cannula, aims to increase this concentration above \(21\%\).
The Baseline and the Estimation Rule
For a nasal cannula, the flow rate is not a direct measurement of \(\text{FiO}_2\) because the patient breathes in room air simultaneously. Clinicians use a standard estimation rule: for every 1 L/min increase in oxygen flow, the \(\text{FiO}_2\) is estimated to increase by approximately \(4\%\) above the \(21\%\) baseline. This clinical approximation is used for quick adjustments in non-intensive care settings.
To determine the estimated \(\text{FiO}_2\) delivered at 2 L/min via nasal cannula, one starts with the \(21\%\) room air baseline and adds the oxygen increment. Based on the standard \(4\%\) estimation rule, the total inspired oxygen concentration at 2 L/min is estimated to be \(28\%\). This calculation allows providers to quickly gauge the potential concentration. The estimation rule shows a linear increase in \(\text{FiO}_2\) up to 6 L/min, the typical maximum flow for a standard nasal cannula.
| Flow Rate (L/min) | Estimated \(\text{FiO}_2\) |
| :—: | :—: |
| 1 | 24% |
| 2 | 28% |
| 3 | 32% |
| 4 | 36% |
| 5 | 40% |
| 6 | 44% |
At 6 L/min, the estimated \(\text{FiO}_2\) reaches about \(44\%\), the highest concentration a nasal cannula typically delivers. This chart represents a prediction, not a precise measurement, as the actual concentration is heavily influenced by the patient’s breathing patterns.
Factors Influencing Delivered Oxygen Concentration
The \(4\%\) estimation rule is challenged by the patient’s breathing mechanics, which dictate how much room air mixes with the supplemental oxygen. A primary factor is the respiratory rate. Rapid breathing results in higher minute ventilation, meaning the patient inhales a greater volume of air each minute.
A faster breathing rate means the patient inhales more room air, diluting the concentration and lowering the actual \(\text{FiO}_2\) below the \(28\%\) estimate for 2 L/min. Conversely, a slower, deeper breathing pattern may result in a slightly higher delivered \(\text{FiO}_2\) because less room air is needed. Shallow breaths (smaller tidal volume) also impact the final concentration.
The peak inspiratory flow rate is another determinant. Since standard low-flow devices deliver a fixed flow of oxygen, if the patient’s inspiratory flow rate is very high, the oxygen flow makes up only a small portion of the total inhaled volume. This causes significant room air to be entrained, leading to a lower overall \(\text{FiO}_2\) than predicted. These physiological variables confirm that the \(4\%\) rule is a convenient guideline rather than a precise measurement.
Comparing Low-Flow Oxygen Delivery Devices
The nasal cannula is a low-flow oxygen delivery system, relying on the patient inhaling ambient air to meet their total inspiratory demand. It delivers an estimated \(\text{FiO}_2\) range of \(24\%\) to \(44\%\) at flow rates up to 6 L/min. This device is preferred for comfort and for patients requiring a low to moderate increase in oxygen concentration.
For patients needing higher concentrations, a simple face mask delivers \(\text{FiO}_2\) in the \(35\%\) to \(55\%\) range (5 to 12 L/min). Flow rates below 5 L/min are avoided with a simple mask to prevent the patient from rebreathing exhaled carbon dioxide. The non-rebreather mask provides the highest concentration among low-flow devices, achieving \(\text{FiO}_2\) levels of \(60\%\) to \(90\%\) (8 to 15 L/min).
The non-rebreather mask uses a reservoir bag and one-way valves to minimize the mixing of exhaled air and room air with the oxygen supply. These low-flow systems differ significantly from high-flow systems, such as Venturi masks. High-flow devices provide a total gas flow that meets or exceeds the patient’s inspiratory demand, offering a more precise and consistent \(\text{FiO}_2\) that can range up to \(100\%\).
Clinical Monitoring and Safety Considerations
Because the \(\text{FiO}_2\) delivered by a nasal cannula is an estimation, clinicians do not rely on the flow rate alone to manage oxygen levels. Instead, the flow rate is adjusted, or “titrated,” based on the patient’s response, which is continuously monitored using a pulse oximeter. This non-invasive device measures the oxygen saturation in the peripheral blood (\(\text{SpO}_2\)).
For most patients, the oxygen flow rate is adjusted to maintain an \(\text{SpO}_2\) target between \(94\%\) and \(98\%\). The \(\text{SpO}_2\) reading is the primary driver for changing the L/min setting. For specific populations, such as those at risk for hypercapnic respiratory failure, a lower \(\text{SpO}_2\) target (\(88\%\) to \(92\%\)) is maintained to prevent the suppression of their respiratory drive.
Avoiding over-oxygenation (hyperoxemia) is important, as excessive oxygen can be harmful. It may lead to oxygen toxicity, which damages lung tissue, or absorption atelectasis, where high concentrations of oxygen cause alveoli to collapse. Therefore, the goal of oxygen therapy is to provide only the minimum flow rate, such as 2 L/min, necessary to achieve the target \(\text{SpO}_2\) and ensure patient safety.