Cause of Device-Related Incident
Device factors; External factors; Support system failures; User errors

Clinical Specialty or Hospital Department
Clinical/Biomedical Engineering; CSR / Materials Management; Obstetrics and Gynecology; OR / Surgery

Device Factors
Design / labeling error; Failure of accessory; Manufacturing error

Document Type
Hazard Reports

External Factors
*Not stated; Medical gas and vacuum supplies

Mechanism of Injury or Death
Burn (electrical, thermal, chemical); Failure to deliver therapy

Support System Failures
*Not stated; Poor prepurchase evaluation

Tampering and/or Sabotage
*Not stated

User Errors
*Not stated; Failure to perform pre-use inspection

Cryosurgical Units [18-051]; Cryosurgical Units, General Purpose [11-067]; Cryosurgical Units, Ophthalmic [11-068]; Medical Gas Cylinders [16-501]

Siphon Gas Cylinders and Cryosurgical Units

Hazard [Health Devices May 1980;9(7):187-8]


A member hospital reported that nitrous oxide (N2O) gas leaked from the trigger valve of a cryosurgical probe and caused frostbite injuries on the operator's hands. The leak was probably caused by the fact that liquid nitrous oxide had been delivered into the pneumatic circuits of the cryoprobe from a mislabeled N2O siphon tank.

A siphon tank contains an internal tube that extends from the tank's valve to its bottom and is designed to discharge its contents as a liquid. Such a tank should have its neck painted silver. The remainder of the tank is painted the standard color for the gas it contains. Unfortunately, on occasion, a siphon is not properly labeled and painted and is put into service as a tank designed to supply its contents as gas.

We are also aware of a case in which a medical gas supplier was unsure as to whether the requested N2O for a cryosurgical unit should be supplied in a regular or siphon tank. The supplier was preparing to ship the latter when he decided to call the manufacturer of the cryosurgical unit and learned that it uses gaseous N2O.


A siphon tank may be mistakenly installed on a cryosurgical unit if it is not properly labeled or if medical personnel do not know that siphon tanks should not be used with their units. In either case, liquid cryogen in the circuits of a gaseous cryoprobe poses an operating hazard. This problem is not specific to any single brand of cryosurgical unit.

Most cryosurgical probes use N2O or carbon dioxide (CO2) as the cryogen and require that it be supplied in the gaseous form, typically from standard E cylinders of medical grade gas, which must be used in the upright position. (A few units are designed specifically for use with liquid N2O supplied from siphon tanks.) The fittings and seal of pneumatic circuits of the cryosurgical units that operate on gaseous cryogen may not be capable of containing the cryogen in its liquid form. If a leak should develop, liquid cryogen may possibly cause frostbite injury to the operator or patient. The risk of frostbite is not as great with a gaseous cryogen leak because the gas has a lesser cooling effect than the liquid. Leaks are also less likely to occur when the form of cryogen used is appropriate for the unit.

Since it is possible that a siphon tank may be mislabeled and supplied for use on a cryosurgical unit, inspect the tank prior to use. Procedures in the Compressed Gas Association (CGA) Pamphlet P-2, Characteristics and Safe Handling of Medical Gases, may be useful for determining whether N2O or CO2 cylinders contain a siphon. In paragraph 4.4.4 of that document, CGA recommends that a nonflammable gas cylinder be vented prior to use: "After removing (the) valve protection cap, slightly open (the) valve an instant to clear (the) opening of possible dust and dirt." The outlet should be located under the outlet because a small amount of liquid cryogen may spill out of a siphon tank.

Following this procedure with a siphon tank will usually yield a mist of gas vapor and possibly a small amount of liquid from the outlet. A nonsiphon tank will exhaust gas only. We suggest, however, that the gas cylinder valve be opened one-half to one full turn and vented 2-4 seconds. This should ensure that, if liquid cryogen is delivered through a siphon to the valve, the mist is not vaporized by warmth in the valve.


  1. Determine whether your cryosurgical unit is designed for gaseous or liquid N2O or CO2 (no units currently on the market use liquid CO2), and specify your needs on orders to your medical gas supplier. Upon receipt, examine each cylinder for a siphon tank label and/or a silver-colored neck. Use only the correct type of cylinder and cryogen form (i.e., gaseous or liquid) specified by the manufacturer.
  2. Before installing the gas cylinder, open the valve one-half to one full turn for 2-4 seconds. Do this with the cylinder standing upright, with the outlet directed away from personnel, and with no part of the body under the outlet. If a mist is observed, the tank contains a siphon and should be used only in those units specified for use with the liquid cryogen.
  3. Always use gas cylinders in the upright position to prevent delivery of the wrong form of cryogen.
  4. Address any questions about the gas requirements for your cryosurgical units to the manufacturer.


  • Cryosurgical Units [18-051]
  • Cryosurgical Units, General Purpose [11-067]
  • Cryosurgical Units, Ophthalmic [11-068]
  • Medical Gas Cylinders [16-501]

Cause of Device-Related Incident

Device factors: Design/labeling error; Failure of accessory; Manufacturing error

User error: Failure to perform pre-use inspection

External factors: Medical gas and vacuum supplies

Support system failure: Poor prepurchase evaluation

Mechanism of Injury or Death

Burn (chemical); Failure to deliver therapy

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