Guide — Oxygen Therapy
Oxygen Delivery Devices
The device you reach for decides not just how much oxygen the patient gets, but how predictable that dose is. This guide sorts every common appliance into three families — low-flow, high-flow, and reservoir — gives the flow and FiO₂ for each, and lays out the rules for matching a device to the patient in front of you.
10 min read · Oxygen Therapy
Written by Apex Respiratory Editorial Team
Educational use only. This material supports respiratory therapy education and exam review. It is not medical advice and is not a substitute for clinical judgment, institutional protocols, or physician orders. Always follow facility policies and current provider orders, and verify calculations independently before clinical use.
Overview
Oxygen devices are grouped by one question: does the device supply the patient’s entire inspiratory demand, or only part of it? That single distinction — not the number on the flowmeter — separates a precise, reproducible FiO₂ from an estimate.
- Low-flowdevices deliver less gas than the patient inspires. Room air is entrained on every breath, so the delivered FiO₂ rises and falls with the patient’s rate and depth. Nasal cannula and simple mask live here.
- High-flow devices supply gas faster than the patient can inhale, so no room air is entrained and the FiO₂ is fixed and reproducible. Venturi masks and HFNC live here.
- Reservoir devices add a bag that stores oxygen during exhalation, letting the patient draw a high-concentration breath without a high source flow. Partial rebreathers and non-rebreathers live here.
Key Concepts — The Device Table
| Device | Flow | Approx. FiO₂ | Notes |
|---|---|---|---|
| Nasal cannulaLow-flow | 1 – 6 L/min | ~24 – 44% | Variable FiO₂ (≈ +4% per L above 21%); drying and uncomfortable above 4 L without humidification. |
| Simple maskLow-flow | 5 – 10 L/min | ~35 – 50% | Minimum 5 L/min to flush exhaled CO₂ from the mask; FiO₂ still varies with breathing pattern. |
| Venturi (air-entrainment) maskHigh-flow | 2 – 15 L/min per adapter | Precise 24 / 28 / 31 / 35 / 40 / 50% | Fixed-performance — delivered FiO₂ stays constant. Best choice for titrated, capped oxygen in COPD. |
| Partial rebreatherReservoir | 10 – 15 L/min | ~60 – 80% | Keep the bag at least one-third inflated throughout the breath; no one-way valve between bag and mask. |
| Non-rebreather (NRB)Reservoir | 10 – 15 L/min | ~60 – 80% (often cited up to ~90% with a good seal) | One-way valves on the bag and exhalation ports. Emergency high-FiO₂ device; bag must stay inflated. |
| High-flow nasal cannula (HFNC)High-flow | Up to 60 L/min, heated & humidified | 21 – 100% | Independently set flow and FiO₂; generates a modest PEEP-like effect, washes out anatomic dead space, well tolerated. |
The single idea that ties the table together: every FiO₂ in the low-flow and reservoir rows is an approximationthat depends on the patient’s inspiratory flow and breathing pattern, while the Venturi and HFNC rows deliver a consistent, setFiO₂. When the number has to be exact — titrating a CO₂ retainer, calculating a P/F ratio — you need a fixed-performance device.
Assessment & Findings
Choosing and confirming a device is a bedside assessment, not a one-time order. Read these together:
- SpO₂ against the target. Most acutely ill adults sit at 92–96%; known CO₂ retainers at 88–92%. The device should hold the target without driving the patient above it.
- Work of breathing and respiratory rate. A fast, deep breather on a low-flow device is entraining a great deal of room air — the real FiO₂ is lower than the chart implies, and a fixed-performance device may be the honest choice.
- Reservoir bag behavior.On a partial or non-rebreather, watch the bag: it should not fully collapse on inspiration. A bag that empties means the flow is too low for the patient’s demand.
- Comfort, secretions, and mucosa. Above 4 L/min a dry nasal cannula cracks the mucosa and is poorly tolerated; heated humidification (or HFNC) keeps secretions mobile and the patient compliant.
RT Priorities & Interventions
- Match the device to the FiO₂ need. Climb the ladder rather than jumping: room air → nasal cannula → simple or Venturi mask → HFNC or non-rebreather. Escalate when the current device can no longer hold the target.
- Humidify above 4 L/min. Add humidification to any nasal cannula running above 4 L/min, and use heated humidification for HFNC by design — dry, high flows are both uncomfortable and damaging to the airway.
- Use fixed-performance for COPD. Reach for a Venturi mask to titrate a CO₂ retainer to the 88–92% window in small, known FiO₂ steps instead of chasing a drifting low-flow number.
- Keep reservoir bags inflated. Set partial and non-rebreathers at 10–15 L/min and verify the bag stays partly inflated through inspiration — that is the whole mechanism for the high FiO₂.
- Reassess after every change. Document the device, flow, FiO₂, and SpO₂ each time, and recheck after the patient has settled — the delivered dose on a low-flow device only makes sense alongside the resulting saturation.
Common Pitfalls
- Running a simple mask below 5 L/min, which lets exhaled CO₂ accumulate in the mask and get rebreathed.
- Leaving a non-rebreather with a collapsed reservoir bag — without an inflated bag the patient draws in room air through the exhalation valves and the high FiO₂ never materializes.
- Treating a stated FiO₂ from a low-flow device as the truth. The “40% on 6 L nasal cannula” on the chart is an average, not the dose your fast-breathing patient is actually receiving.
- Driving a CO₂ retainer above the 88–92% target on a high-flow non-rebreather when a Venturi mask would have capped the dose.
Board Exam Pearls
- The nasal cannula rule of thumb is +4% FiO₂ per liter above room air: 1 L ≈ 24%, 2 L ≈ 28%, up through 6 L ≈ 44%. Above 6 L the relationship breaks down and you switch devices.
- When a stem asks for a precise or fixedFiO₂ — especially in COPD — the answer is the Venturi (air-entrainment) mask, not a reservoir mask.
- Simple mask minimum flow is 5 L/min to flush CO₂; the partial and non-rebreather both run at 10–15 L/min with the bag kept inflated.
- HFNC delivers heated, humidified gas up to 60 L/min with FiO₂ set independently, generating a small PEEP-like effect and washing out dead space — the textbook reasons it outperforms a standard cannula.
FAQ
What is the difference between a low-flow and a high-flow oxygen device?
It is not about the liter flow on the dial — it is about whether the device meets the patient's full inspiratory demand. A low-flow device (nasal cannula, simple mask, reservoir masks) delivers less gas than the patient inspires, so room air is entrained and the delivered FiO₂ varies with the patient's breathing pattern. A high-flow device (Venturi mask, HFNC) delivers gas faster than the patient can inhale, so the FiO₂ is fixed and predictable regardless of how the patient breathes.
Why does the FiO₂ from a nasal cannula change between patients?
A nasal cannula delivers a fixed liter flow into a reservoir (the nasopharynx) that the patient then dilutes with entrained room air on every breath. A patient breathing fast and deep entrains more room air and gets a lower FiO₂; a patient breathing slow and shallow entrains less and gets a higher FiO₂. The +4% per liter rule of thumb is a rough average, not a guarantee — which is exactly why you titrate to SpO₂, not to a number you assume.
Why is a Venturi mask preferred for a COPD patient?
Patients with chronic CO₂ retention need a precise, capped FiO₂ to stay in the 88–92% target without over-oxygenating. The Venturi mask is fixed-performance: a 28% adapter delivers 28% whether the patient is breathing quietly or working hard. That predictability lets you titrate oxygen in small, known steps instead of guessing at a low-flow device whose real FiO₂ drifts with the patient's effort.
What is the minimum flow for a simple mask, and why?
Run a simple mask at a minimum of 5 L/min. Below that, exhaled CO₂ accumulates in the mask body and is rebreathed on the next breath. The minimum flow flushes that dead space. This is the most common simple-mask error and a frequent exam stem.
Put it to work
Once a device and flow are set, know how long the supply lasts. The oxygen tank duration calculator turns cylinder size, pressure, and liter flow into the minutes you actually have.
Open the Oxygen Tank Duration calculator →Related Resources
Sources
- Kacmarek RM, Stoller JK, Heuer AJ. Egan's Fundamentals of Respiratory Care. 12th ed. Elsevier; 2021. Medical gas therapy chapter.
- American Association for Respiratory Care. AARC Clinical Practice Guideline: Oxygen therapy for adults in the acute care facility — 2002 revision & update. Respir Care. 2002;47(6):717-720.
- Nishimura M. High-flow nasal cannula oxygen therapy in adults. Respir Care. 2016;61(4):529-541.