Carbon monoxide production from degradation of desflurane, enflurane, isoflurane, halothane, and sevoflurane by soda lime and Baralyme

Investigation and Possibilities of Reuse of Carbon Dioxide Absorbent Used in Anesthesiology

Absorbents used in closed and semi-closed circuit environments play a key role in preventing carbon dioxide poisoning. Here we present an analysis of one of the most common carbon dioxide absorbents—soda lime. In the first step, we analyzed the composition of fresh and used samples. For this purpose, volumetric and photometric analyses were introduced. Thermal properties and decomposition patterns were also studied using thermogravimetric and X-ray powder diffraction (PXRD) analyses. We also investigated the kinetics of carbon dioxide absorption under conditions imitating a closed-circuit environment.

Do the Manual or Computer-Controlled Flowmeters Generate Similar Isoflurane Concentrations in Tafonius?

Introduction: Tafonius is an anesthesia machine with computer-controlled monitor and ventilator. We compared the isoflurane fluctuations in the circuit with manual (MF) or computer-driven (CF) flowmeters, investigated the origin of the differences and assessed whether isoflurane concentration time course followed a one-compartment model.

Material and Methods: A calibrated TEC-3 isoflurane vaporizer was used. Gas composition and flows were measured using a multiparametric monitor and a digital flowmeter. Measurements included: (1) Effects of various F_iO₂ with MF/CF on the isoflurane fraction changes in the breathing system during mechanical ventilation of a lung model; wash-in kinetic was fitted to a compartmental model; (2) Gas outflow at the common gas outlet (CGO) with MF/CF at different F_iO₂; (3) Isoflurane output of the vaporizer at various dial settings with MF/CF set at different flows without and with reduction of the CGO diameter.

Results: (1) The 3% targeted isoflurane concentration was not reached; additional time was required to reach specific concentrations with CF (lowest F_iO₂, longer time). The exponential course fitted a two-compartment model; (2) Set and measured flows were identical with MF. With CF at 0.21 F_iO₂, flow was intermittently 7.6 L min⁻¹ or zero (mean total: 38% of the set flow); with CF at 1.00 F_iO₂, flow was 10.6 L min⁻¹ or zero (mean: 4–5.3 L min⁻¹); with 0.21 < F_iO₂ < 1.00, combined flow was intermittent (maximum output: 15.6 L min⁻¹); (3) With MF, isoflurane output was matching dial setting at 5 L min⁻¹ but was lower at higher flows; with CF generating intermittent flows, isoflurane output was fluctuating. With the 4 mm diameter CGO, isoflurane concentration was close to dial setting with both MF and CF. With a 14 G CGO, isoflurane concentration was lower than dial setting with MF, higher with CF.

Conclusions and Clinical Relevance: Using MF or CF led to different isoflurane fraction time course in Tafonius. Flows were lower than set with CF; the TEC-3 did not compensate for high/intermittent flows and pressures; the CGO diameter influenced isoflurane output.

Important role of calcium chloride in preventing carbon monoxide generation during desflurane degradation with alkali hydroxide-free carbon dioxide absorbents

We investigated whether calcium chloride (CaCl₂), a supplementary additive in carbon dioxide (CO₂) absorbents, could affect carbon monoxide (CO) production caused by desflurane degradation, using a Japanese alkali-free CO₂ absorbent Yabashi Lime^®-f (YL-f), its CaCl₂-free and 1% CaCl₂-added derivatives, and other commercially available alkali-free absorbents with or without CaCl₂. The reaction between 1 L of desflurane gas (3-10%) and 20 g of desiccated specimen was performed in an artificial closed-circuit anesthesia system for 3 min at 20 or 40 °C. The CO concentration was measured using a gas chromatograph equipped with a semiconductor sensor detector. The systems were validated by detecting dose-dependent CO production with an alkali hydroxide-containing CO₂ absorbent, Sodasorb^®. Compared with YL-f, the CaCl₂-free derivative caused the production of significantly more CO, while the 1% CaCl₂-added derivative caused the production of a comparable amount of CO. These phenomena were confirmed using commercially available absorbents AMSORB^® PLUS, an alkali-free absorbent with CaCl₂, and LoFloSorb™, an alkali-free absorbent without CaCl₂. These results suggest that CaCl₂ plays an important role in preventing CO generation caused by desflurane degradation with alkali hydroxide-free CO₂ absorbents like YL-f.

Anesthesia-Related Carbon Monoxide Exposure: Toxicity and Potential Therapy

Exposure to carbon monoxide (CO) during general anesthesia can result from volatile anesthetic degradation by carbon dioxide absorbents and rebreathing of endogenously produced CO. Although adherence to the Anesthesia Patient Safety Foundation guidelines reduces the risk of CO poisoning, patients may still experience subtoxic CO exposure during low-flow anesthesia. The consequences of such exposures are relatively unknown. In contrast to the widely recognized toxicity of high CO concentrations, the biologic activity of low concentration CO has recently been shown to be cytoprotective. As such, low-dose CO is being explored as a novel treatment for a variety of different diseases. Here, we review the concept of anesthesia-related CO exposure, identify the sources of production, detail the mechanisms of overt CO toxicity, highlight the cellular effects of low-dose CO, and discuss the potential therapeutic role for CO as part of routine anesthetic management.

Carbon monoxide and anesthesia-induced neurotoxicity

The majority of commonly used anesthetic agents induce widespread neuronal degeneration in the developing mammalian brain. Downstream, the process appears to involve activation of the oxidative stress-associated mitochondrial apoptosis pathway. Targeting this pathway could result in prevention of anesthetic toxicity in the immature brain. Carbon monoxide (CO) is a gas that exerts biological activity in the developing brain and low dose exposures have the potential to provide neuroprotection. In recent work, low concentration CO exposures limited isoflurane-induced neuronal apoptosis in a dose-dependent manner in newborn mice and modulated oxidative stress within forebrain mitochondria. Because infants and children are routinely exposed to low levels of CO during low-flow general endotracheal anesthesia, such anti-oxidant and pro-survival cellular effects are clinically relevant. Here we provide an overview of anesthesia-related CO exposure, discuss the biological activity of low concentration CO, detail the effects of CO in the brain during development, and provide evidence for CO-mediated inhibition of anesthesia-induced neurotoxicity.

Performance of a new carbon dioxide absorbent, Yabashi lime® as compared to conventional carbon dioxide absorbent during sevoflurane anesthesia in dogs

In the present study, we compare a new carbon dioxide (CO2) absorbent, Yabashi lime(®) with a conventional CO2 absorbent, Sodasorb(®) as a control CO2 absorbent for Compound A (CA) and Carbon monoxide (CO) productions. Four dogs were anesthetized with sevoflurane. Each dog was anesthetized with four preparations, Yabashi lime(®) with high or low-flow rate of oxygen and control CO2 absorbent with high or low-flow rate. CA and CO concentrations in the anesthetic circuit, canister temperature and carbooxyhemoglobin (COHb) concentration in the blood were measured. Yabashi lime(®) did not produce CA. Control CO2 absorbent generated CA, and its concentration was significantly higher in low-flow rate than a high-flow rate. CO was generated only in low-flow rate groups, but there was no significance between Yabashi lime(®) groups and control CO2 absorbent groups. However, the CO concentration in the circuit could not be detected (≤5ppm), and no change was found in COHb level. Canister temperature was significantly higher in low-flow rate groups than high-flow rate groups. Furthermore, in low-flow rate groups, the lower layer of canister temperature in control CO2 absorbent group was significantly higher than Yabashi lime(®) group. CA and CO productions are thought to be related to the composition of CO2 absorbent, flow rate and canister temperature. Though CO concentration is equal, it might be safer to use Yabashi lime(®) with sevoflurane anesthesia in dogs than conventional CO2 absorbent at the point of CA production.

The modern integrated anaesthesia workstation

Over the years, the conventional anaesthesia machine has evolved into an advanced carestation. The new machines use advanced electronics, software and technology to offer extensive capabilities for ventilation, monitoring, inhaled agent delivery, low-flow anaesthesia and closed-loop anaesthesia. They offer integrated monitoring and recording facilities and seamless integration with anaesthesia information systems. It is possible to deliver tidal volumes accurately and eliminate several hazards associated with the low pressure system and oxygen flush. Appropriate use can result in enhanced safety and ergonomy of anaesthetic delivery and monitoring. However, these workstations have brought in a new set of limitations and potential drawbacks. There are differences in technology and operational principles amongst the new workstations. Understand the principles of operation of these workstations and have a thorough knowledge of the operating manual of the individual machines.

Inhaled anesthetics in horses

Inhaled agents represent an important and useful class of drugs for equine anesthesia. This article reviews the ether-type anesthetics in contemporary use, their uptake and elimination, their mechanisms of action, and their desirable and undesirable effects in horses.

The safety and efficacy of minimal-flow desflurane anesthesia during prolonged laparoscopic surgery

BACKGROUND

Minimal-flow anesthesia can meet the demands of a modern society that is more sensitive to environmental protection and economic burdens. This study compared the safety and efficacy of minimal-flow desflurane anesthesia with conventional high-flow desflurane anesthesia for prolonged laparoscopic surgery.

METHODS

Forty-six male patients (ASA physical status II or III) undergoing laparoscopic urologic surgery for more than 6 hours were randomly divided into two groups: the high-flow (HF) group and the minimal-flow (MF) group. The HF group was continuously administered a fresh gas flow of 4 L/min. In the MF group, a fresh gas flow of 4 L/min was administered for the first 20 minutes and was thereafter lowered to 0.5 L/min. Inspiratory and expiratory desflurane concentrations, respiratory variables, and hemodynamic variables were continuously monitored during administration of anesthesia. Measurements of carboxyhemoglobin (COHb) concentration and arterial blood gas analysis were performed every 2 hours during anesthesia. Serum aspartate aminotransferase (AST), alanine aminotransferase (ALT), blood urea nitrogen (BUN) and creatinine were measured on the first and second day after the surgery.

RESULTS

Demographic data and duration of anesthesia were not different between the two groups. Significant differences were not observed between the two groups in terms of hemodynamic variables, respiratory variables, and inspiratory and expiratory desflurane concentrations. Inspiratory O(2) concentration was maintained lower in the MF group than in the HF group (43-53% vs. 53-59%; P < 0.05). Compared with the HF group, COHb concentrations was higher (P < 0.05), but not increased from the baseline value in the MF group. Serum AST, ALT, BUN, and creatinine were not significantly different between the two groups.

CONCLUSIONS

In prolonged laparoscopic surgery, no significant differences were found in safety and efficacy between minimal-flow and high-flow desflurane anesthesia.

Desflurane - revisited

The search for an ideal inhalational general anesthetic agent continues. Desflurane, which was recently introduced in the Indian market, possesses favorable pharmacokinetic and pharmacodynamic properties and is closer to the definition of an ideal agent. It offers the advantage of precise control over depth of anesthesia along with a rapid, predictable, and clear-headed recovery with minimal postoperative sequelae, making it a valuable anesthetic agent for maintenance in adults and pediatric patients in surgeries of all durations. The agent has advantages when used in extremes of age and in the obese. Its use may increase the direct costs of providing anesthetic care. Methods or techniques, such as low-flow anesthesia, to reduce the overall cost and along with minimal environmental implications must be followed.

The mechanisms of carbon monoxide production by inhalational agents

Carbon monoxide can be formed when volatile anaesthetic agents such as desflurane and sevoflurane are used with anaesthetic breathing systems containing carbon dioxide absorbents. This review describes the possible chemical processes involved and summarises the experimental and clinical evidence for the generation of carbon monoxide. We emphasise the different conditions that were used in the experimental work, and explain some of the features of the clinical reports. Finally, we provide guidelines for the prevention and detection of this complication.

Effects of inspiratory oxygen concentration on endtidal carbon monoxide concentration

OBJECTIVE

Carbon monoxide (CO) is eliminated mainly via the lungs so that exhaled carbon monoxide concentration reflects endogenous production. In this context, we studied the effects of inspiratory oxygen concentration and endotracheal intubation on endtidal CO concentrations.

METHODS

In patients undergoing general anaesthesia, endtidal CO concentrations were measured while breathing room air, oxygen as well as after induction of general anaesthesia and endotracheal intubation. To exclude time-dependent effects, patients were assigned to two groups. Patients in group 1 (n = 20) were preoxygenated for 5 minutes, whereas patients in group 2 (n = 20) were preoxygenated for 10 minutes. We also studied the effects of different inspiratory oxygen concentrations in volunteers (n = 20) breathing room air, 50% and 100% oxygen.

RESULTS

Breathing oxygen for 5 minutes increased endtidal carbon monoxide concentrations in all patients (in group 1 from 7.6+/-4.9 to 12.6+/- 5.0 ppm, p < 0.001; in group 2 from 7.1+/-6.1 to 16.4 +/- 8.6 ppm, p < 0.001). No further change of CO concentration was detected after 10 minutes of preoxygenation (16.4 +/- 9.0 vs. 16.4 +/- 8.6 ppm, p > 0.05). Endtidal CO values however significantly increased with induction of anaesthesia and endotracheal intubation (in group 1 to 21.5 +/- 6.3 ppm, p < 0.001, in group 2 to 26.1 +/- 13.1 ppm, p < 0.001). In volunteers, mean endtidal CO values increased from 10.7 +/-5.9 to 14.8+/-7.3 ppm after breathing 50% oxygen for 3 minutes (p < 0.001). Breathing pure oxygen had no additional effect on endtidal CO values (16.0 +/- 6.0 ppm, p > 0.05).

CONCLUSIONS

Endtidal carbon monoxide levels are influenced by inspiratory oxygen concentrations. Induction of anaesthesia and endotracheal intubation further increases endtidal CO concentrations beyond the effects attributable to preoxygenation alone.

Inhalation Anesthesiology and Volatile Liquid Anesthetics: Focus on Isoflurane, Desflurane, and Sevoflurane

Clinical pharmacists rarely are involved in the selection and dosing of anesthetic agents. However, when practicing evidence-based medicine in a cost-conscious health care system, optimizing drug therapy is imperative in all areas. Thus, we provide general information on anesthesiology, including the different types of breathing systems and the components of anesthesia machines. Modern inhalation anesthetics that are predominantly used in clinical practice include one gas--nitrous oxide--and new volatile liquid agents--isoflurane, desflurane, and sevoflurane. Desflurane and sevoflurane are the low-soluble inhalation anesthetics, and they offer some clinical advantages over isoflurane, such as fast induction and faster recovery with long procedures. However, efficient use of isoflurane can match the speed of induction and recovery of the other agents in certain cases. In addition, the patient characteristics, duration and type of procedure, type of breathing system, and efficiency in monitoring must be considered when selecting the most optimal therapy for each patient. Maximizing the clinical advantages of these agents while minimizing the waste of an institution's operating room and pharmacy budget requires an understanding of the characteristics, pharmacokinetics, and pharmacodynamics of these anesthetic agents and the collaborated effort from both the anesthesia and pharmacy departments. An anesthetic agent algorithm is provided as a sample decision-process tree for selecting among isoflurane, desflurane, and sevoflurane.

Effect of breathing low concentrations of volatile anaesthetic agents on incidence of adverse airway events

The effect of breathing 0.1 minimum alveolar concentrations (MAC) of desflurane or isoflurane for three minutes on the incidence of adverse airway events on a subsequent breath of 2 MAC was investigated. Twenty-five volunteers known to develop an adverse airway event to desflurane or isoflurane took part in the study. Each volunteer was exposed to isoflurane and desflurane at least 24 h apart. Volunteers were assessed for adverse airway events while breathing 2 MAC inhalational anaesthetic following breathing 100% O(2) for 3 min. This was repeated with 0.1 MAC inhalational anaesthetic in oxygen instead of 100% O(2). Adverse airway events decreased from 88% to 40% when tests were conducted with desflurane (p = 0.002). With isoflurane, the reduction from 60% to 52% was not statistically significant (p = 0.774). Breathing low concentrations of desflurane decreases the incidence of adverse airway events on subsequent inhalation of higher concentration of desflurane.

Carbon monoxide production from desflurane and six types of carbon dioxide absorbents in a patient model

BACKGROUND

Desflurane is known to produce high concentrations of carbon monoxide (CO) in desiccated sodalime or Baralyme (Allied Healthcare Products, St. Louis, MO). Desiccated absorbents without strong bases like potassium hydroxide or sodium hydroxide are reported to produce less or no CO at all. The purpose of this study is to compare the concentration of CO in an anesthesia circuit for desflurane with six different types of completely desiccated CO(2) absorbents with less strong bases than sodalime.

METHODS

A patient model was simulated using a circle anesthesia system connected to an artificial lung. Completely desiccated CO(2) absorbent (950 g) was used in this system. A low flow anesthesia (500 ml min(-1)) was maintained using desflurane. For immediate quantification of CO production a portable gas chromatograph was used.

RESULTS

Peak concentrations of CO were very high in Medisorb (Datex-Ohmeda, Hoevelaken, The Netherlands) and Spherasorb (Intersurgical, Uden, The Netherlands) (13317 and 9045 p.p.m., respectively). It was lower with Loflosorb (Intersurgical, Uden, The Netherlands) and Superia (Datex-Ohmeda, Hoevelaken, The Netherlands) (524 and 31 p.p.m., respectively). Amsorb (Armstrong, Coleraine, N. Ireland) and lithium hydroxide produced no CO at all.

CONCLUSION

Medisorb and Spherasorb are capable of producing large concentrations of CO when desiccated. Loflosorb and Superia produce far less CO under the same conditions. Amsorb and lithium hydroxide should be considered safe when desiccated.

Carbon monoxide production from five volatile anesthetics in dry sodalime in a patient model: halothane and sevoflurane do produce carbon monoxide; temperature is a poor predictor of carbon monoxide production

Background

Desflurane and enflurane have been reported to produce substantial amounts of carbon monoxide (CO) in desiccated sodalime. Isoflurane is said to produce less CO and sevoflurane and halothane should produce no CO at all.

The purpose of this study is to measure the maximum amounts of CO production for all modern volatile anesthetics, with completely dry sodalime. We also tried to establish a relationship between CO production and temperature increase inside the sodalime.

Methods

A patient model was simulated using a circle anesthesia system connected to an artificial lung. Completely desiccated sodalime (950 grams) was used in this system. A low flow anesthesia (500 ml/min) was maintained using nitrous oxide with desflurane, enflurane, isoflurane, halothane or sevoflurane. For immediate quantification of CO production a portable gas chromatograph was used. Temperature was measured within the sodalime container.

Results

Peak concentrations of CO were very high with desflurane and enflurane (14262 and 10654 ppm respectively). It was lower with isoflurane (2512 ppm). We also measured small concentrations of CO for sevoflurane and halothane. No significant temperature increases were detected with high CO productions.

Conclusion

All modern volatile anesthetics produce CO in desiccated sodalime. Sodalime temperature increase is a poor predictor of CO production.

Formation and toxicity of anesthetic degradation products

Toxic degradation products are formed from a range of old and modern anesthetic agents. The common element in the formation of degradation products is the reaction of the anesthetic agent with the bases in the carbon dioxide absorbents in the anesthesia circuit. This reaction results in the conversion of trichloroethylene to dichloroacetylene, halothane to 2-bromo-2-chloro-1,1-difluoroethylene, sevoflurane to 2-(fluoromethoxy)-1,1,3,3,3-pentafluoro-1-propene (Compound A), and desflurane, isoflurane, and enflurane to carbon monoxide. Dichloroacetylene, 2-bromo-2-chloro-1,1-difluoroethylene, and Compound A form glutathione S-conjugates that undergo hydrolysis to cysteine S-conjugates and bioactivation of the cysteine S-conjugates by renal cysteine conjugate beta-lyase to give nephrotoxic metabolites. The elucidation of the mechanisms of formation and bioactivation of degradation products has allowed for the safe use of anesthetics that may undergo degradation in the anesthesia circuit.

Fires from the Interaction of Anesthetics with Desiccated Absorbent

Rarely, fire and patient injury have resulted from the degradation of sevoflurane by desiccated carbon dioxide absorbent. Desiccated absorbent also can degrade desflurane and isoflurane, and in the present investigation we sought to determine whether a danger of fire also arose with their use in the presence of desiccated absorbent. Baralyme was desiccated by heating and directing a 10 L/min flow of oxygen through the absorbent. Approximately 1200 g of this desiccated absorbent was used to fill a standard absorber placed in a standard anesthetic circuit to which we directed a 6 L/min flow of oxygen containing 1.5 or 3.0 MAC desflurane, isoflurane, or sevoflurane. A 3-L reservoir bag served as a surrogate lung, and we ventilated this lung with a minute ventilation of 10 L/min. With desflurane or isoflurane, at both 1.5 MAC and 3.0 MAC, temperatures increased in 30 to 70 min to a peak of approximately 100 degrees C and then decreased. With 1.5 MAC sevoflurane (3.0 MAC was not studied), temperatures increased to over 200 degrees C, and in 2 of 5 studies, flames appeared in the anesthetic circuit. In a separate study, we found that concurrent delivery of carbon dioxide and desflurane did not increase peak temperatures. We conclude that the interaction of desflurane or isoflurane with desiccated absorbent is not likely to produce the conflagrations possible with sevoflurane.

Inhalational anaesthesia

Sevoflurane and desflurane have important advantages over isoflurane and halothane. Disadvantages, which the clinician should keep in mind, include the degradation of both agents by soda lime under certain circumstances during closed circuit anaesthesia. As a result compound A and carbon monoxide (CO) may be generated in soda lime canisters and may be inhaled by patients. The extent to which this constitutes a significant problem during routine anaesthesia in humans is not clear. Recent developments in absorbent technology have the potential to reduce any hazard to negligible proportions. Other undesirable properties of the newer inhalation agents include agitation with sevoflurane in children and cardiovascular and airway effects with desflurane.

Sevoflurance: approaching the ideal inhalational anesthetic. a pharmacologic, pharmacoeconomic, and clinical review

Sevoflurane is a safe and versatile inhalational anesthetic compared with currently available agents. Sevoflurane is useful in adults and children for both induction and maintenance of anesthesia in inpatient and outpatient surgery. Of all currently used anesthetics, the physical, pharmacodynamic, and pharmacokinetic properties of sevoflurane come closest to that of the ideal anesthetic (200). These characteristics include inherent stability, low flammability, non-pungent odor, lack of irritation to airway passages, low blood:gas solubility allowing rapid induction of and emergence from anesthesia, minimal cardiovascular and respiratory side effects, minimal end-organ effects, minimal effect on cerebral blood flow, low reactivity with other drugs, and a vapor pressure and boiling point that enables delivery using standard vaporization techniques. As a result, sevoflurane has become one of the most widely used agents in its class.

The role of new anesthetic agents

The three anesthetic drugs introduced most recently to the market are sevoflurane, desflurane, and ropivacaine. Sevoflurane and desflurane are both inhalational anesthetic agents and ropivacaine is a local anesthetic agent. Sevoflurane provides a rapid onset and offset of action; it is well tolerated with little airway irritation. It is hemodynamically stable, with low potential for toxicity. Concerns about its interaction with soda lime during low-flow anesthesia with the production of Compound A have not proved to be a clinical problem. While desflurane also provides rapid onset and recovery from anesthesia, it is not as hemodynamically stable as sevoflurane, and also causes airway irritation. Ropivacaine is a unique local anesthetic in that it is supplied as the pure S-enantiomer. It is at least as effective as bupivacaine, with lower toxicity and less motor block for the same degree of sensory block.

Carbon dioxide absorption: toxicity from sevoflurane and desflurane

The degradation of volatile anaesthetics by desiccated carbon dioxide absorbents can result in adverse outcomes. Desiccated carbon dioxide absorbent reacting with desflurane can cause potentially life-threatening intraoperative carbon monoxide exposure; the reaction with sevoflurane can cause the formation of several toxic breakdown products, e.g. compound A. Compound A-related renal toxicity in humans is still a matter of controversy.

Performance of an electrochemical carbon monoxide monitor in the presence of anesthetic gases

OBJECTIVE

The passage of volatile anesthetic agents through accidentally dried CO2 absorbents in anesthesia circuits can result in the chemical breakdown of anesthetics with production of greater than 10000 ppm carbon monoxide (CO). This study was designed to evaluate a portable CO monitor in the presence of volatile anesthetic agents.

METHODS

Two portable CO monitors employing electrochemical sensors were tested to determine the effects of anesthetic agents, gas sample flow rates, and high CO concentrations on their electrochemical sensor. The portable CO monitors were exposed to gas mixtures of 0 to 500 ppm CO in either 70% nitrous oxide, 1 MAC concentrations of contemporary volatile anesthetics, or reacted isoflurane or desflurane (containing CO and CHF3) in oxygen. The CO measurements from the electrochemical sensors were compared to simultaneously obtained samples measured by gas chromatography (GC). Data were analyzed by linear regression.

RESULTS

Overall correlation between the portable CO monitors and the GC resulted in an r2 value >0.98 for all anesthetic agents. Sequestered samples produced an exponential decay of measured CO with time, whereas stable measurements were maintained during continuous flow across the sensor. Increasing flow rates resulted in higher CO readings. Exposing the CO sensor to 3000 and 19000 ppm CO resulted in maximum reported concentrations of approximately 1250 ppm, with a prolonged recovery.

CONCLUSIONS

Decrease in measured concentration of the sequestered samples suggests destruction of the sample by the sensor, whereas a diffusion limitation is suggested by the dependency of measured value upon flow. Any value over 500 ppm must be assumed to represent dangerous concentrations of CO because of the non-linear response of these monitors at very high CO concentrations. These portable electrochemical CO monitors are adequate to measure CO concentrations up to 500 ppm in the presence of typical clinical concentrations of anesthetics.

The response of anesthetic agent monitors to trifluoromethane warns of the presence of carbon monoxide from anesthetic breakdown

OBJECTIVE

Trifluoromethane and CO are produced simultaneously during the breakdown of isoflurane and desflurane by dry CO2 absorbents. Trifluoromethane interferes with anesthetic agent monitoring, and the interference can be used as a marker to indicate anesthetic breakdown with CO production. This study tests representative types of gas monitors to determine their ability to provide a clinically useful warning of CO production in circle breathing systems.

METHODS

Isoflurane and desflurane were reacted with dry Baralyme at 45 degrees C. Standardized samples of breakdown products were created from mixtures of reacted and unreacted gases to simulate the partial degrees of reaction which might result during clinical episodes of anesthetic breakdown using 1% or 2% isoflurane and 6% or 12% desflurane. These mixtures were measured by the monitors tested, and the indication of the wrong agent or a mixture of agents due to the presence of trifluoromethane was recorded and related to the CO concentration in the gas mixtures.

RESULTS

When presented with trifluoromethane from anesthetic breakdown, monochromatic infrared monitors displayed inappropriately large amounts of isoflurane or desflurane. Agent identifying infrared and Raman scattering monitors varied in their sensitivity to trifluoromethane. Mass spectrometers measuring enflurane at mass to charge = 69 were most sensitive to trifluoromethane.

CONCLUSION

Monochromatic infrared monitors were unable to indicate anesthetic breakdown via interference by trifluoromethane, but did indicate falsely elevated anesthetic concentrations. Agent identifying infrared and Raman monitors provided warning of desflurane breakdown via the interference of trifluoromethane by displaying the wrong agent or mixed agents, but may not be sensitive enough to warn of isoflurane breakdown Some mass spectrometers provided the most sensitive warnings to anesthetic breakdown via trifluoromethane, but additional data processing by some patients monitor units reduced their overall effectiveness.

Implications of chemical and physical properties of desflurane for longer surgery

Increased duration of anaesthetic administration has implications for recovery from anaesthesia, has cardiovascular effects, and potential for toxicity through metabolism and breakdown of the anaesthetics. Recovery of function after desflurane or sevoflurane anaesthesia, because of the low blood gas and tissue solubilities of these agents, is more rapid than after halothane, isoflurane or enflurane, with recovery being most rapid after desflurane. Increased duration of anaesthesia amplifies the differences in rate of recovery because of the additional anaesthetic (greater with more soluble agents) dissolved in tissues. Increased duration of anaesthesia lessens the cardiovascular depression associated with most halogenated inhaled anaesthetics including desflurane, but not isoflurane. Increased duration of anaesthesia allows for greater metabolism of anaesthetics and greater exposure to metabolites and potentially toxic breakdown products. Desflurane is the least metabolised of the available anaesthetics and is stable in soda lime and Baralyme. Thus, it has exceedingly low potential for toxicity. Sevoflurane undergoes considerable metabolism, producing free fluoride ion, with plasma concentrations proportional to dose and duration of anaesthesia exceeding 50 microM in approximately 7% of patients. In rats, the effects of a toxic breakdown product of sevoflurane, CF2 = C(CF3)OCH2F (compound A), are also dose- and duration-dependent, with lower concentrations producing toxic effects as duration of exposure increases. The clinical importance of the metabolism and in vitro breakdown of sevoflurane has still to be adequately tested.

Desflurane. A review of its pharmacodynamic and pharmacokinetic properties and its efficacy in general anaesthesia

Desflurane is a halogenated ether inhalation general anaesthetic agent with low solubility in blood and body tissues, and approximately one-fifth the potency of isoflurane. The pharmacodynamic properties of desflurane generally resemble those of isoflurane; thus, it produces dose-dependent depression of the central nervous and cardiorespiratory systems, and tetanic fade at the neuromuscular junction. The alveolar equilibration of desflurane is rapid (90% complete at 30 minutes compared with 73% for isoflurane). Both desflurane and isoflurane are distributed to various tissues to a similar extent. Desflurane is resistant to chemical degradation and undergoes negligible metabolism (approximately equal to 10% of that seen with isoflurane). Desflurane 'wash-out' is approximately equal to 2 to 2.5 times faster than that of isoflurane in the first 2 hours after discontinuation of anaesthesia. The low solubility of desflurane facilitates a rapid induction of anaesthesia and precise control of the depth of anaesthesia (during maintenance). Results from a few clinical studies indicate that emergence from desflurane is significantly earlier (by approximately equal to 2 to 6 minutes) than that from propofol anaesthesia, whereas other studies do not concur. In comparison with isoflurane, emergence from desflurane anaesthesia is significantly earlier (by 5 minutes) after ambulatory and approximately equal to 50% earlier (also significant) after nonambulatory surgical procedures. Limited comparative studies with halothane or sevoflurane also suggest an earlier time of emergence from desflurane anaesthesia. Comparative studies of desflurane and propofol, and other inhalation agents, indicate that the times to toleration of oral fluids, sitting and discharge from recovery room are similar, regardless of the general anaesthetic agent administered. However, some limited data in elderly patients (aged > 65 years) suggest that this patient group spends a significantly shorter time in the postanaesthesia care unit after desflurane than after isoflurane anaesthesia. Differences, if any, in the recovery of cognitive and psychomotor functions after desflurane or propofol anaesthesia remain unclear. However, in comparison with isoflurane anaesthesia, recovery of these functions (up to 45 minutes post-operatively) occurs earlier after desflurane. Significantly fewer patients are subjectively impaired (i.e. drowsy, clumsy, fatigued or confused) upon recovery from desflurane than from isoflurane anaesthesia. Likewise, significantly fewer adult patients are delirious when recovering from desflurane than from isoflurane anaesthesia, though in paediatric patients delirium is more likely when recovering from desflurane than from halothane anaesthesia. Haemodynamic stability during coronary artery surgery is as well maintained with desflurane as with isoflurane, and the drug does not worsen the adverse postoperative outcomes.(ABSTRACT TRUNCATED AT 400 WORDS)