Supplemental oxygen therapy is a common medical treatment prescribed when a patient cannot take in enough oxygen from the air. This therapy delivers oxygen at a specific rate, measured in Liters Per Minute (LPM), to raise the concentration of gas reaching the lungs. A prescription of “5 liters of oxygen” refers directly to this flow rate, which is considered a moderate level of supplemental oxygen delivery. Understanding 5 LPM requires knowing how the flow rate differs from the actual percentage of oxygen a person breathes and how different delivery devices affect the final result.
Defining Liters Per Minute (LPM)
LPM measures the speed at which oxygen flows from the source—such as a tank or concentrator—to the patient. It is a direct measure of volume over time. When a flow meter is set to 5 LPM, five liters of pure oxygen are released into the delivery device every sixty seconds.
The flow rate is not the same as the concentration of oxygen a patient ultimately inhales, which is known as the Fraction of Inspired Oxygen (FiO2). Ambient air normally contains an FiO2 of approximately 21%. Supplemental oxygen increases this total concentration.
A low-flow system, such as a nasal cannula, typically uses flow rates between 1 and 6 LPM, placing 5 LPM near the higher end of its range. This continuous flow is distinct from pulse-dose systems, which deliver oxygen in short bursts based on the patient’s breathing pattern. Continuous flow at 5 LPM provides a steady stream of gas to the patient’s airway.
How 5 LPM Translates to Oxygen Concentration
The resulting FiO2, or the actual percentage of oxygen breathed, is not a fixed number for a 5 LPM prescription. This inspired concentration depends on the type of delivery device used and the patient’s breathing pattern. For instance, faster or deeper breathing dilutes the supplemental oxygen with room air, lowering the effective FiO2.
When 5 LPM is delivered through a standard nasal cannula, the oxygen mixes freely with room air before reaching the lungs. The concentration of inspired oxygen is estimated to be around 40%. This calculation is based on the rule that each additional liter of flow increases the FiO2 by approximately 4% above the 21% room air concentration.
Using a simple face mask, 5 LPM results in a higher concentration because the mask acts as a small reservoir, containing more pure oxygen near the patient’s nose and mouth. A simple mask at 5 LPM can provide an estimated FiO2 in the range of 35% to 55%. A flow rate of at least 5 LPM is required for simple masks to flush out exhaled carbon dioxide, preventing rebreathing.
For devices like a non-rebreather mask, the translation is more dramatic due to a large reservoir bag and one-way valves. These features restrict the patient from inhaling room air and prevent exhaled breath from mixing with the pure oxygen supply. Consequently, a non-rebreather mask can deliver a much higher FiO2, potentially reaching 60% to 80% or more. The choice of device is a medical decision that determines the final oxygen percentage breathed, even when the flow rate is set to 5 LPM.
Medical Goals and Monitoring Oxygen Therapy
The prescription of 5 LPM is a directed attempt to achieve an acceptable blood oxygen saturation level (SpO2). SpO2 is the percentage of hemoglobin carrying oxygen in the blood, measured non-invasively using a pulse oximeter. For most acutely ill patients, the clinical target is typically an SpO2 between 92% and 96%.
For patients with certain chronic respiratory conditions, such as severe Chronic Obstructive Pulmonary Disease (COPD), the target saturation range is often lower, typically between 88% and 92%. This lower goal prevents hypercapnea, where excessive oxygen can lead to carbon dioxide retention, potentially worsening the patient’s condition. The 5 LPM flow rate is often a starting point used to titrate the dose until the patient’s saturation falls within the prescribed range.
Because oxygen is considered a medical treatment, the flow rate should not be adjusted by the patient without a healthcare provider’s direction. Too little oxygen fails to treat low blood oxygen, while too much can be harmful. Delivering oxygen at 5 LPM, especially through a nasal cannula, can lead to dryness and irritation of the nasal passages. Humidification is often considered for flows exceeding 4 or 5 LPM to add moisture and improve patient comfort.