Hazard Update [Health Devices Dec 1987;16(12):407-9]
Member hospitals frequently inquire about the need to scavenge
cryosurgical units (CSUs) that use nitrous oxide (N2O) as the cryogen (i.e.,
refrigerant gas). Confusion continues to exist about the rationale for scavenging this
equipment, appropriate scavenging methods, and available options that eliminate the need
ECRI discovered the hazard presented by high CSU N2O
concentrations in 1979 while developing a draft standard under contract to FDA that
specifically required scavenging attachments on CSUs that use N2O.(1) In
"Nitrous Oxide Exhausted from Cryosurgical Units," (Health Devices 8:293-294, October 1979) we fully discussed the issues
and made specific recommendations for addressing problems. We also commented in
article about the need to scavenge
CSUs.(2) Our recommendations are repeated and broadened in this update report. In short, exhausted
N2O must be scavenged.
The primary reasons that CSU manufacturers use N2O as a cryogen
are its properties as an effective freezing agent and its availability in the hospital.
Its analgesic and pharmacologic effects play no role in the operation or performance of
the CSU. Using N2O as a cryogen is acceptable only if the potential
adverse effects to personnel from the drug are understood and measures are taken to
minimize exposure. Repeated exposure to the extremely high concentrations of N2O
that result from CSU use can pose significant risks to clinical and clerical personnel in
the vicinity of the device. Some physicians who use N2O CSUs appear not to
fully appreciate or understand the acute side effects and long-term carcinogenic and
teratogenic effects of chronic exposure.
Exposure to unscavenged CSUs has led to at least two lawsuits against a
CSU manufacturer. In those cases, a physician and his clerical assistant claimed injury
from daily exposure to the gas in the doctor's office. (One of the cases settled out of
court for a substantial sum; the other was pending when this report was originally
published.) Hospitals could also be named as defendants. For example, employees injured
while working could be entitled to Workers' Compensation benefits. It is also conceivable
that a tort action could be brought against a hospital that fails to minimize the risks
associated with N2O CSU use on the basis that the hospital knew or should have
known of the hazards or intentionally failed to notify personnel of the risks.
Although the rate of N2O exhaust from CSUs and the resulting
ambient gas concentrations are not widely appreciated, they are easy to understand. The
current NIOSH-recommended exposure limit for N2O is 25 parts per million (ppm)
averaged over the period of administration.(3) In the OR, anesthesia machines typically
exhaust only 2-4 L/min of N2O during a surgical procedure. Witcher et al.
studied concentrations of N2O exhausted from unscavenged anesthesia machines in
operating and delivery rooms having relatively high room air exchange rates (10-21
changes/hr).(4) At an exhaust rate of 3 L/min, average room air concentrations of N2O
ranged from 71 to 130 ppm. Short-term peak concentrations lasting several seconds ranged
from approximately 500 to 1,200 ppm, depending on the height from the floor and the
distance to the anesthesia machine. At the end of anesthesia administration, the
concentrations usually drop to below 25 ppm in less than 20 min.
Originally, concerns over exhausted N2O were raised based on
the chronic exposure of OR personnel to these relatively low levels of the gas. As a
result, scavenging of anesthesia machines is now the common and accepted practice. The
presence of a scavenger on anesthesia equipment is one of the key requirements of the
Joint Commission on Accreditation of Healthcare Organizations (JCAHO).(5)
If an anesthesia machine exhausts 2 L/min, 120 L of N2O would
be exhausted during a typical 1 hr procedure. In contrast, N2O CSUs typically
exhaust 20-90 L/min during the freeze cycle; some models also exhaust the gas during the
thaw cycle. Therefore, a typical 3 min freeze application at 40 L/min will exhaust the
same amount of N2O as a 1 hr anesthesia procedure.
Cryosurgical equipment is frequently used in treatment rooms in a clinic
or doctor's office where airflow exchange rates are very low (2-4 changes/hr, far lower
than in a typical OR). As such, N2O concentration in that room and in
adjacent rooms and nearby office areas can be excessive. In the treatment
room itself, levels will typically range from 4,000 to 7,000 ppm and may take up to 4 hr to wash
out to the 25 ppm level. Levels in nearby clerical areas can also be high for long
periods. In the lawsuits cited above, it was calculated that the treatment room
concentrations ranged from 8,000 to 12,000 ppm.
Scavenging N2O from CSUs is simple on most units purchased over
the last several years. Most have a scavenging outlet to which the user attaches a long
plastic hose (typically ½-inch diameter). In most ORs, scavenged gas can be easily and
safely vented into a nonrecirculating return air vent. (A "return air
vent" is an air-conditioning vent through which air leaves a room.) However, safely
venting scavenged gas from treatment rooms can be difficult. These rooms have recirculated
ventilation, and therefore the scavenged gas cannot be vented into the return air system.
Recently constructed ORs may also have recirculated ventilation, but they usually also
have a dedicated system for safely venting scavenged anesthetic gases. Depending on the
flow of such a dedicated system, it may also be suitable for scavenged N2O from
a CSU. Options for venting N2O from rooms with recirculated ventilation include
placing the scavenging hose out a window or through a small hole to the outside drilled in
the wall of the room where the CSU is used.
Some hospitals report that CSU manufacturers recommend that the vent hose
be placed down the trap in any convenient sink. Never do this. Such a practice
probably violates most local plumbing codes. In addition, you cannot know where the gas
will eventually exhaust. N2O is heavier than air and may vent from a dry trap
in a sink or drain within the building on a floor below. Because of the high gas flows,
such venting may temporarily pressurize the plumbing system and allow gas to vent through
wet traps as well.
In addition, some CSU manufacturers also recommend that the units be
vented into the hospital's centrally piped medical/surgical suction system. Never do
this. The high CSU gas flows can temporarily compromise an entire suction system if
vented into it. A CSU so vented in one OR could adversely affect the piped suction being
used in a neighboring room and place a surgical patient at risk.
Some older N2O CSUs can be factory-modified or retrofitted with
field kits to permit gas scavenging. However, some older units cannot be easily scavenged,
and replacing them with newer models that can be scavenged or switching to CO2
as the cryogen are the only acceptable options. Physicians experienced with N2O
CSUs may question whether CO2 is cold enough to adequately freeze tissues
because its minimum temperature is -79° C, compared with -89° C for N2O. In fact, CO2 can provide the same
freezing depth for the majority of clinical applications as N2O, although the
freezing time required is somewhat longer. We have found that once physicians understand
this, they are less reluctant to switch to CO2.
Chronic exposure to CO2 exhausted from CSUs does not present a
hazard. Concerns about its use relate to its asphyxial properties during acute,
high-concentration exposures. No carcinogenic or teratogenic risks are associated with its
use. The NIOSH limit for exposure to CO2 is 5,000 ppm averaged over an 8 hr
period.(6) It is extremely unlikely that this limit could be exceeded by a CSU, even under
the most demanding clinical circumstances. While room air concentrations of CO2
may exceed 5,000 ppm during a cryosurgical procedure, based on our studies with N2O,
the concentration in even a poorly ventilated treatment room would drop to below 5,000 ppm
within 15 min or less. If averaged over an 8 hr day, the concentration from one procedure
would be about 150 ppm. A clinician would have to perform in excess of 30 procedures in 8
hr (an unlikely occurrence) in a poorly ventilated room to exceed the limit for average CO2
- Determine whether you own an N2O CSU (some CSUs can be
operated with either N2O or CO2).
- Always scavenge N2O CSUs and vent them
to the outside, away from any air-intake ducts.
Do not vent them into a sink, drain trap, or
the piped medical/surgical suction system.
Contact the manufacturer of your unit to request information on
scavenging the N2O exhaust. If your equipment presently has an N2O scavenging
port, you will probably be able to order the proper size and type of exhaust
hose from the manufacturer. Or, you can order hose from a local supplier. An
exhaust hose is no more inconvenient or obtrusive than an electrical power
cord. If the CSU is used in an OR with nonrecirculated ventilation, place
one end of the exhaust hose 1-2 ft into the return air vent. If vents are inconveniently
located (e.g., near the ceiling), consider permanently installing a short length of
exhaust hose through the vent grill. Equip the exposed hose end with a connector
appropriate for attachment to the CSU exhaust hose. If the OR has a dedicated system for
venting scavenged anesthetic gases, determine the flow capacity for that system. If the
capacity is over 100 L/min (3.5 ft3/mm), the N2O
from the CSU may be vented through that dedicated system if this is more
convenient than through the return air system. For treatment rooms in clinics and physicians' offices (and for ORs where
the N2O cannot be vented as discussed above), the N2O exhaust hose can be vented to the outside through a window
or a small hole drilled in the windowsill or the wall of the room where the
equipment is used. Such holes typically have a pipe of larger diameter than
the vent hose and are capped on the inside when not in use.
- Use CO2 for CSUs if 1) scavenged N2O cannot be
safely or conveniently vented; or 2) the N2O cannot be scavenged because of the
design of the CSU. If you currently have CSUs that can be operated with either N2O
or CO2, strongly consider using CO2,
even if scavenging is possible, because it is intrinsically safer.After switching to CO2, the clinician using the equipment will
have to become accustomed to the changes required in technique (primarily increasing the
freezing time) when using this cryogen. If your unit requires a different gas cylinder
yoke for CO2 tanks, purchase the correct one rather than removing the safety
indexing pins on the existing N2O
yoke. Removing the pins will defeat a standard safety system and create yet
- Remove from service those N2O units that cannot be scavenged
or converted to use with CO2. For future acquisitions of cryosurgical
equipment, purchase units that are operated on CO2. This
eliminates the need for scavenging.
- If you must use an unscavenged N2O CSU while awaiting proper
scavenging modifications or before switching to a CO2 CSU, use it in an
extremely well-ventilated area, such as an OR. Personnel should leave the room as soon as
possible after the procedure is completed and should be exposed to such procedures as
little as possible. In no case should pregnant staff be present during use of an
unscavenged N2O CSU. Pregnant patients are not at risk because they are
not subject to chronic exposure.
- Disseminate this information to physicians on your staff who may use
CSUs in their private practices, particularly gynecologists, dermatologists,
otolaryngologists, and general surgeons.
- Bruley ME. A study of safety and performance
requirements of cryosurgical devices. FDA/BMDDP: Springfield, VA, National
Technical Information Service; 163 pages, 1980 Sep. (PB 81 124943).
- Gonzalez ER. N2O exhaust from cryosurgical units may affect physician
- NIOSH criteria for a recommended standard
occupational exposure to waste anesthetic gases and vapors. Washington,
D.C.:Dept. of Health, Education, and Welfare (HSS), Publication DHEW
(NIOSH) 77-140, 1977.
- Witcher C, et al. Development and evaluation of
methods for the elimination of waste anesthetic gases and vapors in
hospitals. Washington, D.C., Dept. of Health, Education, and Welfare
(HSS), Publication DHEW (NIOSH) 75-137, 1975.
- Accreditation Manual for Hospitals. JCAHO: Chicago, IL, 1986: p. 6.
- NIOSH criteria for a recommended standard occupational exposure to
carbon dioxide. Washington, D.C., Dept. of Health, Education, and Welfare
(HSS), Publication DHEW (NIOSH) 76-194, 1976.
ANSI. Anesthesia Gas Pollution Control, American National Standards
Institute, New York. ANSI Z79.11; 1982. See also, ASA. Waste anesthetic gases in
operating room air: A suggested program to reduce personnel exposure. American Society
of Anesthesiologists Ad Hoc Committee.
Bruce DL, Bach MJ. Effects of trace concentrations of anesthetic gases
on behavioral performance of operating room personnel. Washington, D.C.: Dept. of
Health, Education, and Welfare (HSS), Publication HEW (NIOSH) 76-169.
Cohen EN, et al. Anesthetic health hazards in the dental operatory
(Abstract). Abstracts of Scientific Papers, 1979 Annual Meeting, American Society of
Anesthesiology. Anesthesiology 1979;51(Sep):S-254 (Supplement).
OSHA enforces nitrous oxide recommendation that has not been brought out
as a standard. Hospital Week 1979;15(Aug 24):2.
Wray RP. Cryoprobe leakage of nitrous oxide into operating room air. Anesthesiology
- Cryosurgical Units [18-051]
- Cryosurgical Units, General Purpose [11-067]
- Cryosurgical Units, Ophthalmic [11-068]
Cause of Device-Related Incident
Device factor: Design/labeling error
External factor: Medical gas and vacuum supplies
Support system failure: Poor prepurchase evaluation
Mechanism of Injury or Death
Exposure to hazardous gas