Cause of Device-Related Incident
Device factors; External factors; User errors

Clinical Specialty or Hospital Department
Anesthesia; Clinical/Biomedical Engineering; Facilities Engineering; Pulmonary / Respiratory Therapy

Device Factors
Manufacturing error

Document Type
Hazard Reports

External Factors
*Not stated

Mechanism of Injury or Death

Support System Failures
*Not stated

Tampering and/or Sabotage
*Not stated

User Errors
Failure to perform pre-use inspection

Medical Gas Cylinders [16-501]

Cylinders with Unmixed Helium/Oxygen

Hazard [Health Devices Apr 1990;19(4):146]


A member hospital reported that a medical gas cylinder labeled "certified mixture of 30% oxygen and 70% helium," intended for patient breathing, was connected into a breathing system. When the gas was delivered to a patient, the oxygen monitor in the system indicated that the mixture consisted of less than 21% oxygen. Further investigation showed that the gases had not been mixed before delivery to the hospital, even though the mixture had been labeled as certified. Following this discovery, the remaining cylinders in the lot were tested, and some of them contained unmixed gases.


For accurate control of the components in a mixture, the supplier adds pure gases to a cylinder one at a time. A gravimetric method (weighing the cylinder and contents during filling) provides the most accurate gas mixture, although the gases are stratified and do not quickly mix. Highly compressed gases behave almost like liquids and mix slowly if only diffusion is allowed to occur with no additional method of mixing. Therefore, to accelerate mixing, the cylinders are placed horizontally and rotated on rollers until the components are thoroughly mixed. Once the mixture is homogenized, it will not stratify again at room temperature and can be delivered to users.


  1. Always verify the contents of a cylinder of mixed gases when it is received and before connecting it into a system and administering the gases to a patient. Then do one of the following:
    • If the mixture is not correct and you have the correct equipment (e.g., a drum roller), rotate the cylinder for at least 45 min and then test it again for the actual contents. Test all other cylinders of the same mixture, and notify the supplier of your findings.
    • Alternatively, return all cylinders that show incorrect test results for the intended mixture to the supplier.
  2. Always use an oxygen monitor in a breathing system.


Medical Gas Cylinders [16-501]

Cause of Device-Related Incident

Device factor: Manufacturing error

User error: Failure to perform pre-use inspection

External factor: Medical gas and vacuum supplies

Mechanism of Injury or Death


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