Oxygen Delivery Systems and Supplemental Oxygen Therapy

Why Supplemental Oxygen Matters

Oxygen is one of the most frequently used therapeutic agents in emergency medicine. When a patient's tissues do not receive enough oxygen—a condition called hypoxia—cells begin to die within minutes. Your job as an EMT or paramedic is to recognize when supplemental oxygen is indicated, select the right delivery device, set an appropriate flow rate, and apply the device correctly. Using too little oxygen can harm a hypoxic patient; using high-flow oxygen unnecessarily on a patient who does not need it carries its own risks. Match the device to the patient.

Fraction of Inspired Oxygen (FiO2)

Room air contains approximately 21% oxygen. The fraction of inspired oxygen, or FiO2, describes how much of the gas a patient breathes is oxygen. Different delivery devices deliver different FiO2 values depending on flow rate, mask fit, and patient breathing pattern. These values are approximate because actual delivered FiO2 varies with respiratory rate and tidal volume.

Common Oxygen Delivery Devices

Nasal Cannula (NC): A nasal cannula consists of two small prongs that rest inside the nostrils connected to oxygen tubing. It delivers a low-to-moderate FiO2—roughly 24–44% at flow rates of 1–6 liters per minute (L/min). Use it for patients who need mild supplemental oxygen, are breathing adequately, and cannot tolerate a mask (such as patients who feel claustrophobic or are eating). Flow rates above 6 L/min do not meaningfully increase FiO2 and may dry and irritate nasal mucosa.

Simple Face Mask (SFM): The simple face mask covers the nose and mouth and has open exhalation ports on the sides. It delivers approximately 35–60% FiO2 at 6–10 L/min. A minimum flow of 6 L/min is required to flush exhaled carbon dioxide (CO2) from the mask. Never run a simple face mask below 6 L/min.

Partial Rebreather Mask (PRM): A partial rebreather mask adds a small reservoir bag to the simple face mask. The patient rebreathes the first portion of exhaled gas (rich in oxygen) from the reservoir before that gas mixes with room air through the side ports. It delivers approximately 35–60% FiO2 at 6–10 L/min. Keep the reservoir bag at least one-third to one-half inflated at all times.

Non-Rebreather Mask (NRM): The non-rebreather mask is the highest-FiO2 device available without specialized equipment. It adds one-way flutter valves over the exhalation ports and between the mask and reservoir bag. These valves prevent exhaled gas from entering the reservoir. When properly fitted and run at 10–15 L/min, it can deliver FiO2 values approaching 90% or higher in ideal conditions. Keep the reservoir bag fully inflated—never let it fully collapse on inhalation. Use it for any seriously ill or injured patient who may be hypoxic: major trauma, suspected carbon monoxide poisoning, respiratory distress, altered mental status with unknown cause.

Venturi Mask: The Venturi mask uses a color-coded adaptor that entrains a precise, fixed ratio of room air. This produces a consistent, predictable FiO2 regardless of the patient's breathing pattern. Common delivered concentrations range from approximately 24–50%, depending on the adaptor used. The Venturi mask is the preferred device when precise oxygen titration is required, such as in patients with chronic obstructive pulmonary disease (COPD) who may be sensitive to high oxygen levels. Always match the adaptor color to the prescribed FiO2 and set the flow rate printed on that adaptor.

Humidified High-Flow Systems: High-flow nasal cannula and aerosol humidification systems deliver heated, humidified oxygen at higher flow rates and can achieve higher FiO2 values. In the prehospital setting these systems are less common but are encountered in interfacility transport and some advanced EMS systems. Humidification reduces mucosal drying during prolonged oxygen therapy.

Oxygen Cylinders: Safety and Duration

Oxygen is stored as a compressed gas. Cylinders are color-coded green in the United States and must be handled with care—a damaged cylinder can become a projectile. Always secure cylinders upright or in an approved carrier. Keep cylinders away from heat and flames. Never use oxygen near an open flame or in the presence of petroleum-based lubricants.

To open a cylinder, crack the valve briefly away from yourself and others to clear dust, then attach the regulator and open fully. To close, turn the valve clockwise until seated, then bleed remaining pressure from the regulator.

The two cylinder sizes you will use most often in the field are the D cylinder (approximately 350 liters capacity when full) and the E cylinder (approximately 625 liters capacity when full). Full pressure is approximately 2,000 psi for both sizes, though always read the actual gauge.

Estimate remaining cylinder time using this formula:

Duration (minutes) = (Gauge pressure in psi × Cylinder constant) ÷ Flow rate (L/min)

Cylinder constants: D = 0.16, E = 0.28.

Applying Devices Safely

Before applying any device: confirm the cylinder is open, confirm flow before placing the device on the patient, and check that tubing is connected and not kinked. After application, reassess the patient's respiratory rate, effort, and SpO2 (if pulse oximetry is available). A mask that is too loose delivers far less FiO2 than expected. Facial hair, trauma, and anatomy all affect mask seal.

Always have a backup device ready. If a non-rebreather mask fails or is not tolerated, a bag-valve mask with reservoir is the next step for a critically hypoxic patient.