3.1 Oxygen Devices and Cylinder Safety
Key Takeaways
- On the TMC, Troubleshooting and Quality Control of Devices / Infection Control is 20 of 140 scored items (~14%); the full exam is 160 items (140 scored + 20 pretest) in 3 hours, CRT cut score 86 and RRT-eligible cut score 92.
- Low-flow devices deliver variable FiO2 that falls as inspiratory demand rises; fixed-performance devices (air-entrainment/Venturi, blenders, HFNC) hold a set FiO2 regardless of breathing pattern.
- A full E cylinder reads ~2200 psig; duration = (gauge psig - reserve) x cylinder factor / flow, with E = 0.28 L/psig and H/K = 3.14 L/psig.
- When SpO2 falls, verify delivery (source open, tubing patent, reservoir inflated, ports clear) before escalating the ordered FiO2 or device.
Oxygen Devices and the TMC Equipment Domain
Oxygen-delivery items live inside the official Troubleshooting and Quality Control of Devices / Infection Control domain. On the Therapist Multiple-Choice (TMC) examination this domain is 20 of 140 scored items, about 14% of your score. The full exam is 160 items (140 scored + 20 unscored pretest) delivered in 3 hours; a score of 86 earns the Certified Respiratory Therapist (CRT) credential and 92 makes you eligible for the Clinical Simulation Examination (CSE) toward the Registered Respiratory Therapist (RRT). The detailed content outline is effective through December 31, 2026.
The domain rewards practical equipment judgment, not memorized device lists.
Start every oxygen stem by separating patient status from equipment function. If the patient is unstable, support oxygenation now while the source and interface are checked. If the patient is stable, verify the order, gas source, flowmeter, tubing, humidifier (if used), and device assembly before increasing therapy.
Low-Flow vs. Fixed-Performance Logic
Low-flow oxygen systems supply only part of the patient's inspiratory flow, so delivered fraction of inspired oxygen (FiO2) swings with rate, tidal volume, mouth breathing, and inspiratory flow. A rough rule: each 1 L/min by nasal cannula adds ~4% to room air, so 1 L/min ≈ 24%, 2 L/min ≈ 28%, up to 6 L/min ≈ 44%. A simple mask runs 5-10 L/min (35-50%); a partial rebreather 10-15 L/min (60-80%); a true nonrebreather 10-15 L/min (60-80%, sometimes higher with a tight seal and one-way valves).
Fixed-performance (high-flow) systems either entrain a predictable air-to-oxygen ratio or deliver flow exceeding the patient's peak inspiratory demand, so FiO2 stays stable. The air-entrainment (Venturi) mask uses precise jet/port sizing — common settings are 24%, 28%, 31%, 35%, 40%, and 50%, with lower FiO2 entraining MORE room air (e.g., 24% air:O2 ≈ 25:1; 40% ≈ 3:1). High-flow nasal cannula (HFNC) delivers heated, humidified gas up to 60 L/min at a blender-set FiO2.
Device Selection and Equipment Checks
| Device | Flow / FiO2 | Equipment Check | Exam Trap |
|---|---|---|---|
| Nasal cannula | 1-6 L/min, ~24-44% | Prongs in nares, tubing patent, flow present | FiO2 estimate varies with breathing pattern |
| Simple mask | 5-10 L/min, 35-50% | Flow high enough to flush exhaled CO2 | Below 5 L/min causes CO2 rebreathing |
| Partial rebreather | 10-15 L/min, 60-80% | Bag stays >1/3 inflated on inspiration | Confused with nonrebreather (no valves) |
| Nonrebreather | 10-15 L/min, 60-80%+ | Reservoir inflated, one-way valves move | Collapsed bag = troubleshoot flow/valves first |
| Air-entrainment (Venturi) | Set 24-50% | Correct jet/adapter, labeled flow, clear ports | Blocked entrainment ports raise FiO2 |
| HFNC | up to 60 L/min, set FiO2 | Blender, heater, water level, cannula size | Dry gas or loose fit defeats it |
Oxygen Source Troubleshooting
A flowmeter reading does not prove gas reaches the patient. Trace the path from wall outlet or cylinder to the interface — critical after a room change, during transport, or near multiple outlets.
| Finding | Likely Equipment Issue | First Corrective Check |
|---|---|---|
| Bobbin/ball reads zero | Source closed, no wall pressure (should be 50 psig), flowmeter not seated | Open valve or reseat flowmeter in correct outlet |
| Flow shows but no gas at patient | Kink, leak, loose tubing, disconnect | Follow tubing source to interface |
| Nonrebreather bag collapses | Flow too low or valve problem | Raise flow to keep bag inflated; inspect valves |
| Venturi FiO2 inconsistent | Wrong adapter, wrong flow, blocked port | Match the labeled jet/flow; clear air ports |
| Blender alarm | Air or O2 inlet pressure lost | Check both 50-psig sources before using output |
| Portable concentrator alarms | Battery, sieve bed, or cannula flow | Place patient on backup O2, then troubleshoot |
Worked example: a COPD patient on a 28% Venturi suddenly desaturates and you hear extra entrainment noise with bedding over the ports. The fix is to clear the ports and confirm jet/flow — not to crank the flow (which on a Venturi does NOT raise FiO2).
Cylinder Safety and Duration Calculations
Compressed oxygen is stored at high pressure, so handling rules are tested. Keep cylinders secured in an approved holder, away from heat and open flame; never use oil or grease on valves/regulators (oxygen + hydrocarbon = combustion risk); use protective caps in storage; and read the label, not the color, to confirm contents. Open valves slowly, confirm the yoke washer is present, and leak-check with an approved method (a hissing regulator or a falling gauge after setup means correct it before transport).
Know the cylinder factors and a full-tank pressure of about 2200 psig:
| Cylinder | Factor (L/psig) | Full volume (approx) |
|---|---|---|
| D | 0.16 | ~350 L |
| E | 0.28 | ~625-660 L |
| G | 2.41 | ~5300 L |
| H / K | 3.14 | ~6900 L |
Duration formula: usable minutes = (gauge psig - reserve psig) x cylinder factor / flow (L/min). Subtract the safety reserve (many services keep 500 psig) BEFORE multiplying.
Worked example: an E cylinder reads 1600 psig, the service reserve is 500 psig, flow is 4 L/min. Usable = 1600 - 500 = 1100 psig; 1100 x 0.28 / 4 ≈ 77 minutes. If the question gives no reserve, use the full gauge reading.
TMC Decision Pattern
For oxygen-equipment stems, pick the choice that verifies delivery before escalating therapy. A falling SpO2 may be disease progression, but the TMC frequently buries a closed cylinder, an empty tank, a loose flowmeter, a wrong Venturi adapter, or a disconnected tube in the scenario. Confirm the gas is actually reaching the patient first.
A transport patient is receiving 3 L/min by nasal cannula from an E cylinder. The gauge reads 1400 psig, the service keeps a 500 psig reserve, and the E-cylinder factor is 0.28. About how long can the planned oxygen supply be used?
A patient on a nonrebreather mask has a reservoir bag that flattens completely during each inspiration. Which action best fits the equipment problem?
A COPD patient is ordered a precise 28% FiO2 by air-entrainment (Venturi) mask. The therapist hears extra entrainment noise and sees bedding covering part of the air-entrainment ports. What should be done first?