Hypoxic gas mixture and oxygen supply failure
Stem
A 46-year-old man, 82 kg, is the first case in a newly renovated operating room for a laparoscopic inguinal hernia repair. After preoxygenation, induction with propofol and rocuronium and an easy intubation, the SpO2 falls from 100% to 84% over two minutes. The inspired oxygen reads 22% and the gas analyzer shows 75% nitrous oxide, although the nitrous flowmeter is off. Breath sounds are equal.
Question
What is happening, and what do you do?
Ideal answer
HeadlineA hypoxic gas through the oxygen supply, most likely a pipeline crossover after the renovation: an emergency. I say it aloud and call for help.
- Off the circuit, self-inflating bag on room air first, safer than anything the machine delivers, or from a separate green oxygen cylinder.1
- Open the backup cylinder; physically disconnect the pipeline hose, because the machine keeps drawing from a connected, pressurized pipeline; opening the cylinder alone does not fix it.
- Reconnect only once the analyzer shows inspired oxygen rising, since the gas, not the pressure, was wrong.2
- Propofol infusion, paralysis kept, surgeon pauses, because he has no volatile on board.
- Warn engineering and the other rooms on the supply, because they breathe the same gas; the outlet stays out of use until tested with an analyzer.1
Watch for: Hypoxic injury afterward; I document and report it.
Breaking down the answer
- The analyzer made the diagnosis — the standard requires an oxygen analyzer with a low-concentration alarm during every general anesthetic on a machine.2 In the PSNet case, the machine had no analyzer and a child died.1
- Room air beats a hypoxic mixture — it is a known gas immediately, and the bag removes every machine variable while I think.1
- Test the pipeline before reuse — OpenAnesthesia also says disconnect and test it;3 cylinder on without pipeline off is the step candidates forget.
- Safety systems are fallible — pin index, diameter index, linked flowmeters and ratio controllers all fail when the wrong gas is in the oxygen pipe.1 The oxygen-pressure alarms cannot detect a crossover, because pressure stays normal.
- Think about the building — other rooms share the pipe, so communication is part of treatment; this is the systems thinking examiners grade.
- Prevention is the checkout — pipeline pressure at least 50 psig, a full backup cylinder, a calibrated oxygen monitor and a working bag every day.4 New or repaired outlets are tested with an analyzer.1
Follow-up question
Ninety minutes later, in the same case on the backup cylinder, the hospital announces a total oxygen supply failure. The cylinder gauge reads 1000 psi and you are running 2 L/min of oxygen. How long do you have and what do you do?
Follow-up answer
HeadlineThe cylinder is my only oxygen now, so I calculate time and tell the surgeon and charge nurse.
I ask for full cylinders. An E cylinder holds about 660 L at 2000 psi, so at 1000 psi about 330 L, which at 2 L/min lasts about two and a half hours: hours ≈ psi ÷ (200 times L/min) = 1000 ÷ 400.3 I lower fresh gas flow to the minimum that keeps the circuit full, use the lowest FiO2 that keeps his saturation safe, and hand-ventilate if the ventilator uses oxygen as its drive gas. Propofol keeps him asleep; the team finishes fast. I watch the gauge every few minutes, the self-inflating bag and room air are the fallback.
Pitfalls that cost points
- Trusting the flowmeter or pipeline pressure gauge over the oxygen analyzer.
- Troubleshooting the machine while he is still breathing the hypoxic gas.
Rapid-fire
Q: What pressure does the oxygen pipeline run at? A: About 50–55 psi.3
Q: When do the oxygen supply failure alarms fire? A: An alarm at 30 psi of oxygen pressure, and other gas flows shut off below 20 psi.3
Q: Why do those alarms miss a crossover? A: The pipeline pressure is normal; only the gas is wrong.
Q: What should be in every operating room regardless of the machine? A: A working self-inflating bag and an auxiliary oxygen cylinder, checked daily.4
Q: What does the checkout say about the oxygen monitor? A: Calibrate it, or verify calibration, and check the low-oxygen alarm.4
Numbers to know
| Item | Value |
|---|---|
| Pipeline pressure | 50–55 psi3 |
| Checkout minimum | Pipeline ≥50 psig4 |
| Full E cylinder (oxygen) | About 660 L at 2000 psi3 |
| Time remaining | psi ÷ (200 × flow in L/min) = hours3 |
| Supply alarm / shutoff | 30 psi alarm; other gases off below 20 psi3 |
Links
- Anesthesia machine checkout and intraoperative failures, Pulse oximetry: principles and limitations, Capnography: waveforms and sudden loss of ETCO2, Intraoperative awareness and depth monitoring
- ASA Standards for Basic Anesthetic Monitoring (2025); 2008 ASA Recommendations for Pre-Anesthesia Checkout Procedures
Sources
- 1.Hypoxic Gas Supply from Cross-Connected Pipelines · AHRQ PSNet, 2024 · psnet.ahrq.govBohringer, Guemidjian, Utter; WebM&M case of an oxygen outlet connected to nitrous oxide.
- 2.Standards for Basic Anesthetic Monitoring · ASA, 2025 · asahq.orgLast amended October 15, 2025.
- 3.Wall Oxygen Failure · OpenAnesthesia · openanesthesia.org
- 4.New Guidelines Available for Pre-Anesthesia Checkout · APSF, 2008 · apsf.orgReproduces the 2008 ASA Recommendations for Pre-Anesthesia Checkout Procedures.
Facts on this page were checked against these sources on Oct 15, 2026. Guidelines change; confirm doses against current institutional protocols.