Hazard [Health Devices Nov 1998;27(11):402-4]
ECRI has investigated several incidents of patient exposure to carbon monoxide (CO) during the administration of inhalation anesthetics through semiclosed circle anesthesia systems. In one of the incidents, patient injury resulted. In each case, after ruling out other possible sources of CO, we concluded that dangerous levels of the gas were generated within the anesthesia system under the conditions present during the incidents. These conditions included the presence of excessively dry carbon dioxide (CO2) absorbent in an anesthesia system
being used to deliver halogenated anesthetic agents for the first case of the
Similar incidents have been reported in
the literature, with one common characteristic being the timing of the
exposures. Many incidents have occurred during Monday morning cases, and all
appear to be associated with the first delivery of an anesthetic after a lengthy
period (e.g., overnight, over a weekend) of anesthesia machine
The Dangers of Carbon Monoxide
Carbon monoxide is very toxic, even in
low concentrations. Once in the blood, CO binds tightly with hemoglobin, forming
carboxyhemoglobin and diminishing the ability of hemoglobin to transport and
release oxygen. The level of CO exposure will be a function of both the inhaled
concentration and the exposure duration. The specific effect on the patient will
vary depending on the patient's cardiovascular condition and the level of oxygen
administered before and during administration of the anesthetic.
Circle Anesthesia Systems and Carbon
To understand how CO exposures can occur, readers will need a basic understanding of circle anesthesia systems and the role of CO2 absorbers within these systems.
Inhalation anesthetics are usually administered through semiclosed circle
anesthesia systems, although closed circle systems are sometimes used. In either
type of circle anesthesia system, some portion of the gas exhaled by the patient
is recirculated through the system and back to the patient, thus conserving
medical gases, vaporous anesthetics, and expired water vapor.
To prevent dangerous levels of CO2 from accumulating in the recirculating gas mixture, anesthesia machines that employ circle systems include an integral CO2 absorber to remove the CO2 exhaled by the patient. These absorbers typically consist of two stacked canisters containing granular absorbent materials that chemically neutralize CO2 as the exhaled gas passes through. Commonly used absorbent materials include soda lime (e.g., Sodasorb) and barium hydroxide lime (e.g., Baralyme). When the ability of these materials to neutralize CO2 becomes exhausted, the
absorbent is replaced. For most absorbents, the current basis for determining
when replacement is needed is the change in color of a pH indicator impregnated
in the absorbent material.
Although the exact chemical mechanism by which CO can be generated is not clear, published studies have indicated that a reaction between halogenated anesthetic agents and commonly used CO2 absorbents can produce CO if the CO2 absorbent is excessively dry. Drying out of the absorbent material can occur when 1) an anesthesia machine has been sitting idle, such as over a weekend, and 2) there is a continuous flow of medical gas (which is very dry) through the CO2 absorber. When dry, the absorbent becomes highly reactive in the presence of certain halogenated agents, resulting in the production of CO as the agent flows through the machine's CO2 absorber. Desflurane (Suprane) appears to be
the most reactive of the halogenated anesthetic agents, although other
agents—particularly enflurane and isoflurane—have also been reported
to produce CO. The reaction between the agent and the absorbent material can
continue for many minutes.
Complicating matters is the fact that
identifying patient exposure to CO when it does occur can be difficult because
carboxyhemoglobin levels are not monitored during anesthesia. Monitoring devices
such as pulse oximeters and blood gas analyzers are not intended to detect
carboxyhemoglobin; in fact, pulse oximeters will usually detect
carboxyhemoglobin as oxyhemoglobin. Similarly, medical mass spectrometers are
not configured to detect CO. And while whole blood co-oximeters can distinguish
carboxyhemoglobin from oxyhemoglobin, these devices require a fresh blood sample
and cannot provide real-time monitoring. As a result, CO exposure may go
undiscovered unless patient morbidity leads to a comprehensive clinical and
In the cases investigated by ECRI, anesthetists identified all the incidents of CO exposure indirectly. For example, in the incident that resulted in an injury, the patient's pulse oximetry readings had become erratic, but the heart rate and ECG waveform remained normal. After the same results were obtained using another pulse oximeter (of the same model) and a new probe, blood was drawn for a blood gas analysis, which revealed a high partial pressure of oxygen (PaO2 greater than 600 mm Hg). Suspecting a problem with the
anesthesia machine, the staff switched to a different machine. The blood sample
was then analyzed by co-oximetry, which revealed a carboxyhemoglobin level of
60% to 70% (values that grossly exceed normal levels); thus, the cause of the
patient's condition was determined to be CO exposure.
One further complication is that it can
be difficult to determine when CO exposure is likely to occur because there
appears to be no readily available, convenient, or reliable means of monitoring
moisture within an absorber or of rehydrating absorbent that has dried out.
Thus, to prevent the conditions under which CO can be produced from developing,
users will need to ensure that the absorbent does not dry out. To do this, they
need to ensure that the flow of medical gas is discontinued whenever an
anesthesia machine is not in use on a patient; it is particularly important that
the gas flow be stopped at the end of the workday.
It should be stressed that the reactions that produce CO within an anesthesia system do not occur while the machine is idle; rather, they occur only when agent vapor flows through the absorber. Therefore, flushing the breathing circuit with fresh gas before use (such as during a pre-use check) will not prevent or relieve the problem. It should also be stressed that CO exposures are unlikely to be detected intraoperatively; thus, healthcare facilities need to ensure that the conditions under which CO can be produced during inhalation anesthesia do not occur. Specifically, users must be sure to discontinue the flow of medical gas whenever an anesthesia machine will not be promptly used on another patient. ECRI recommends that the absorbent material in both canisters of an absorber be replaced whenever there is reason
to believe that a machine has been left idle with gas flowing for an
undetermined time. Fresh absorbent materials are sufficiently hydrated and
normally remain hydrated by exhaled water vapor in the circle system, thereby
preventing reaction with halogenated agents.
There is still much to be learned both chemically and clinically about the phenomenon of CO production associated with the interaction of halogenated anesthetic agents and CO2 absorbent materials. ECRI will continue to assess relevant new
findings in the medical literature and to evaluate changes in anesthesia
monitoring and delivery systems. Given the present technology and knowledge of
the problem, all efforts to prevent CO exposure must be directed at detecting
and protecting against unintended medical gas flow when anesthesia systems are
not in use.
- Alert anesthesia and other
appropriate personnel to the problem and to our report.
- Ensure that medical gas is turned
off when an anesthesia machine will not be promptly used for another
procedure. At the end of every day, verify that the gas is off for all
- Before performing a pre-use check for the first case of a day, determine if there is any flow of medical gas. If there is, replace the absorbent material in both absorbent canisters before using the machine. Identify and
address the cause of the gas flow.
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Fang ZX, Eger EI 2nd, Laster MJ, et al. Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane, and sevoflurane by sodalime and baralyme. Anesth Analg 1995 Jun;80(6):1187-93.
Frink EJ, Nogami WM, Morgan SE, et al. High carboxyhemoglobin concentrations occur in swine during desflurane anesthesia in the presence of partially dried carbon dioxide absorbents. Anesthesiology 1997 Aug;87(2):308-16.
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- Anesthesia Unit Absorbers, Carbon
- Anesthesia Unit Carbon Dioxide
- Anesthesia Units
Cause of Device-Related
Device factor: Device
User errors: Incorrect clinical use;
Incorrect control settings
External factor: Medical gas and vacuum
Mechanism of Injury or
Exposure to hazardous gas;