We have developed a new carbon dioxide absorbent for use during low flow or circle system anesthesia. The material does not contain the strong base sodium and/or potassium hydroxide. This paper discusses the efficiency and chemical inertness of the new material.

Amsorb-a new carbon dioxide absorbent for use in anesthetic breathing systems

BACKGROUND Desflurane, enflurane and isoflurane can be degraded to carbon monoxide (CO) by carbon dioxide absorbents, whereas sevoflurane and halothane form negligible amounts of CO Carbon monoxide formation is greater with drier absorbent, and with barium hydroxide, than with soda lime The mechanism, role of absorbent composition and water, and anesthetic structures determining CO formation are unknown This investigation examined sequential steps in anesthetic degradation to CO METHODS Carbon monoxide formation from anesthetics and desiccated barium hydroxide lime or soda lime was determined at equimole and equiMAC concentrations Carbon monoxide formation from deuterium-substituted anesthetics was also quantified Proton abstraction from anesthetics by strong base was determined by deuterium isotope exchange A reactive chemical intermediate was trapped and identified by gas chromatography-mass spectrometry The source of the oxygen in CO was identified by 18O incorporation RESULTS Desflurane,enflurane,andisoflurane(difluoromethylethyl ethers), but not sevoflurane (monofluoromethyl ether), methoxyflurane (methy-ethyl ether), or halothane (alkane) were degraded to CO The amount of CO formed was desflurane > or = enflurane > isoflurane at equiMAC and enflurane > desflurane > isoflurane at equimole concentrations Proton abstraction from the difluoromethoxy carbon was greater with potassium than with sodium hydroxide, but unmeasurable with barium hydroxide Carbon monoxide formation was correlated (r = 095-100) with difluoromethoxy (enflurane > desflurane > isoflurane > or = methoxyflurane = sevoflurane = 0) but not ethyl carbon proton abstraction Deuterium substitution on enflurane and desflurane diminished CO formation Chemical trapping showed formation of a difluorocarbene intermediate from enflurane and desflurane Incorporation of H2(18)O in barium hydroxide lime resulted in C18O formation from unlabeled enflurane and desflurane CONCLUSIONS A difluoromethoxy group is a structural requirement for haloether degradation to CO Results are consistent with initial base-catalyzed difluoromethoxy proton abstraction (potassium > sodium hydroxide, thus greater CO formation with barium hydroxide lime vs soda lime) forming a carbanion (reprotonated by water to regenerate the anesthetic, hence requirements for relatively dry absorbent), carbanion decomposition to a difluorocarbene, and subsequent difluorocarbene reaction to form CO

Mechanistic aspects of Carbon monoxide formation from volatile anesthetics

Background: Consequences of volatile anesthetic degradation by carbon dioxide absorbents that contain strong base include formation of compound A from sevoflurane, formation of carbon monoxide (CO) and CO toxicity from desflurane, enflurane and isoflurane, delayed inhalation induction, and increased anesthetic costs. Amsorb® (Armstrong Ltd., Coleraine, Northern Ireland) is a new absorbent that does not contain strong base and does not form CO or compound A in vitro. This investigation compared Amsorb®, Baralyme® (Chemetron Medical Division, Allied Healthcare Products, St. Louis, MO), and sodalime effects on CO (from desflurane and isoflurane) and compound A formation, carboxyhemoglobin (COHb) concentrations, and anesthetic degradation in a clinically relevant porcine in vivo model. Methods: Pigs were anesthetized with desflurane, isoflurane, or sevoflurane, using fresh or partially dehydrated Amsorb, Baralyme(, and new and old formulations of sodalime. Anesthetic concentrations in the fresh (preabsorber), inspired (post-absorber), and end-tidal gas were measured, as were inspired CO and compound A concentrations and blood oxyhemoglobin and COHb concentrations. Results: For desflurane and isoflurane, the order of inspired CO and COHb formation was dehydrated Baralyme > > sodalime > Amsorb®. For desflurane and Baralyme, peak CO was 9,700 ± 5,100 parts per million (ppm), and the increase in COHb was 37 ± 14%. CO and COHb increases were undetectable with Amsorb®. Oxyhemoglobin desaturation occurred with desflurane and Baralyme® but not Amsorb or sodalime. The gap between inspired and end-tidal desflurane and isoflurane did not differ between the various dehydrated absorbents. Neither fresh nor dehydrated Amsorb® caused compound A formation from sevoflurane. In contrast, Baralyme and sodalime caused 20-40 ppm compound A. The gap between inspired and end-tidal sevoflurane did not differ between fresh absorbents, but was Amsorb® < sodalime < Baralyme® with dehybrated absorbents. Conslusion: Amsorb caused minimal if any CO formation, minimal compound A formation regardless of absorbent hydration, and the least amount of sevoflurane degradation. An absorbent like Amsorb®, which does not contain strong base or cause anesthetic degradation and formation of toxic products, may have benefit with respect to patient safety, inhalation induction, and anesthetic consumption (cost).

Comparison of Amsorb, sodalime, and Baralyme degradation of volatile anesthetics and formation of carbon monoxide and compound a in swine in vivo.

UNLABELLED Isoflurane, enflurane, sevoflurane, and especially desflurane produce carbon monoxide (CO) during reaction with desiccated absorbents. Of these, sevoflurane is the least studied. We investigated the dependence of CO production from sevoflurane on absorbent temperature, minute ventilation (VE), and fresh gas flow rates. We measured absorbent temperature and in vitro CO concentrations when desiccated Baralyme reacted with 1 minimum alveolar anesthetic concentration of (2.1%) sevoflurane at 2.3-, 5.0-, and 10.0-L VE. Mathematical modeling of carboxyhemoglobin concentrations was performed using an existing iterative method. Rapid breakdown of sevoflurane prevented the attainment of 1 minimum alveolar anesthetic concentration with low fresh gas flow rates. CO concentrations increased with VE and with absorbent temperatures exceeding 80 degrees C, but concentrations decreased with higher fresh gas flow rates. Average CO concentrations were 150 and 600 ppm at 2.3- and 5.0-L VE; however, at 10 L, over 11,000 ppm of CO were produced followed by an explosion and fire. Methanol and formaldehyde were present and may have contributed to the flammable mixture but were not quantitated. Mathematical modeling of exposures indicates that in average cases, only patients 10% during the first 60 min of anesthesia. IMPLICATIONS Sevoflurane breakdown in desiccated absorbents is expected to result in only mild carbon monoxide (CO) exposure. Completely dry absorbent and high minute ventilation rates may degrade sevoflurane to extremely large CO concentrations. Serious CO poisoning or spontaneous ignition of flammable gases within the breathing circuit are possible in extreme circumstances.

Carbon monoxide production from sevoflurane breakdown: modeling of exposures under clinical conditions.

Filter technology Current airway dust filter technology emanates from the USA mining industry during the 1930s. It was further developed and improved by the USA military during World War II to prevent workers from inhaling very fine radioactive particles in the nuclear industry. These later filters we now recognize as high efficiency particulate air filters (HEPA). The efficiency of filters varies according to the number and size of particles they capture. Filters may be classified commonly as 95, 99.95, and 99.97% efficient (95, 99 and 100, respectively). The higher the efficiency the higher the classification number used. The efficiency of filters may also be affected by volatile chemicals which may be inhaled with the particles. Industrial filters which are categorized by the above system may also have a prefix of N, R, or P; N for Not resistant to oil, R for Resistant to oil, and P for oil Proof; thus, a filter would be classified as for example a P100. Requirements for military and environmental biohazard filters are based on these classifications and surprisingly may only achieve 95% efficiency. A generally perceived increased threat from biological hazards that may be inhaled, including bioterrorism threats, especially in hospital environments, has increased interest in respiratory filters. 94 Protecting personnel from inhaled biological hazards requires a filter efficiency of several orders of magnitude greater than the industrial dust filters used in respirators alluded to above. As few as 10 inhaled smallpox viruses may be sufficient to infect patients with the disease. 94 This has resulted in the sub-classification of

Hidden hazards and dangers associated with the use of HME/filters in breathing circuits. Their effect on toxic metabolite production, pulse oximetry and airway resistance

Background: Carbon monoxide forms via reaction of isoflurane, enflurane, and desflurane with dried CO2 absorbents. The authors hypothesize that interventions by nonphysician support personnel to decrease absorbent drying will decrease the exposure rate of patients to carbon monoxide from anesthetic breakdown. Methods: In the control group, all anesthetizing personnel were made aware of the factors enabling CO generation from anesthetic breakdown, and prevention techniques were left to the anesthetizing personnel. After data collection was complete, the following interventions were initiated to reduce absorbent drying: Anesthesia technicians and housekeeping personnel were instructed to turn off all anesthesia machines after the last case of the day in each room, and the CO2 absorbent was changed each morning if fresh gas was found flowing. Baralyme[registered sign] was used in all phases of this study. Results: Five cases of intraoperative carbon monoxide exposure occurred among 1,085 (0.46%) first cases in the control group. Postintervention, patient carbon monoxide exposures decreased (P <0.05), with one exposure among 1,961 (0.051%) first cases in the main operating room. Two exposures among 68 (2.9%) first cases occurred in remote locations (P < 0.001) versus main operating room. Predisposing factors for absorbent drying include the prolonged use of anesthesia machines for monitored anesthesia care, inappropriate drying techniques for expiratory flowmeters, understaffing of support personnel, and anesthesia in remote locations. Conclusions: These interventions reduced patient exposure to carbon monoxide. Monitoring for carbon monoxide exposures during general anesthesia may be necessary to recognize and end patient exposures that occur despite preventative measures.

Harvey J. Woehick

Papers

Reduction in the Incidence of Carbon Monoxide Exposures in Humans Undergoing General Anesthesia