Marshall B. Dunning

Acute smoking increases ST depression in humans during general anesthesia.

UNLABELLED We tested the hypothesis that acute smoking is associated with ST segment depression during general anesthesia in patients without ischemic heart disease. The carbon monoxide (CO) concentration in expired gas and hemodynamic data was measured during general anesthesia for noncardiac or nonperipheral vascular surgery in patients without symptoms or evidence of ischemic heart disease. Increased expired CO concentrations are indicators of recent smoking. Logistic regression analysis identified significant predictors of ST segment depression > or = 1 mm. Both rate pressure product (odds ratio 1.20 for each increase of 1000, 95% confidence interval = 1.04-1.41, P = 0.007) and expired CO concentration (odds ratio 1.05 for each part per million increase, 95% confidence interval = 1.03-1.08, P = 0.001) were significant predictors of ST segment depression when considered simultaneously. Males demonstrated a lower probability of having an episode of ST depression (odds ratio = 0.16, P = 0.01), but this did not change the relationship between rate pressure product and CO as predictors of ST depression. Approximately 25% of chronically smoking patients smoked on the morning of surgery despite instructions not to smoke. IMPLICATIONS Patients under age 65 without symptoms of ischemic heart disease who smoked shortly before surgery had more episodes of rate pressure product-related ST segment depression than nonsmokers, prior smokers, or chronic smokers who did not smoke before surgery. Females were at greater risk of ST depression than males.

Physical Factors Affecting the Production of Carbon Monoxide from Anesthetic Breakdown

Background Parameters determining carbon monoxide (CO) concentrations produced by anesthetic breakdown have not been adequately studied in clinical situations. The authors hypothesized that these data will identify modifiable risk factors. Methods Carbon monoxide concentrations were measured when partially desiccated barium hydroxide lime was reacted with isoflurane (1.5%) and desflurane (7.5%) in a Draeger Narkomed 2 anesthesia machine with a latex breathing bag substituting for a patient. Additional experiments determined the effects of carbon dioxide (0 or 350 ml/min), fresh gas flow rates (1 or 4 l/min), minute ventilation (6 or 18 l/min), or absorbent quantity (1 or 2 canisters). End-tidal anesthetic concentrations were adjusted according to a monochromatic infrared monitor. Results Desflurane produced approximately 20 times more CO than isoflurane when completely dried absorbents were used. Peak CO concentrations approached 100,000 ppm with desflurane. Traces of water remaining after a 66-h drying time (one weekend) markedly reduced the generation of CO compared with 2 weeks of drying. Reducing the quantity of desiccated absorbent by 50% reduced the total CO production by 40% in the first hour. Increasing the fresh gas flow rate from 1 to 4 l/min increased CO production by 67% in the first hour but simultaneously decreased average inspiratory concentrations by 53%. Carbon dioxide decreased CO production by 12% in completely desiccated absorbents. Conclusion Anesthetic identity, fresh gas flow rates, absorbent quantity, and water content are the most important factors determining patient exposures. Minute ventilation and carbon dioxide production by the patient are relatively unimportant.

Mathematical modeling of Carbon monoxide exposures from anesthetic breakdown : Effect of subject size, hematocrit, fraction of inspired oxygen, and quantity of Carbon monoxide

Background Carbon monoxide (CO) is produced by reaction of isoflurane, enflurane, and desflurane in desiccated carbon dioxide absorbents. The inspiratory CO concentration depends on the dryness and identity of the absorbent and anesthetic. The adaptation of existing mathematical models to a rebreathing circuit allows identification of patient factors that predispose to more severe exposures, as identified by carboxyhemoglobin concentration. Methods From our companion study, the authors used quantitative in vitro CO production data for 60 min at 7.5% desflurane or 1.5% isoflurane at 1 l/min fresh gas flow. The carboxyhemoglobin concentration was calculated by iteratively solving the Coburn Forster Kane equation modified for a rebreathing system that incorporates the removal of CO by patient absorption. Demonstrating good fit of predicted carboxyhemoglobin concentrations to published data from animal and human exposures validated the model. Carboxyhemoglobin concentrations were predicted for exposures of various severity, patients of different sizes, hematocrit, and fraction of inspired oxygen. Results The calculated carboxyhemoglobin concentrations closely predicted the experimental results of other investigators, thereby validating the model. These equations indicate the severity of CO poisoning is inversely related to the hemoglobin quantity of a subject. Fraction of inspired oxygen had the greatest effect in patients of small size with low hematocrit values, where equilibrium and not the rate of uptake determined carboxyhemoglobin concentrations. Conclusion This model predicts that patients with low hemoglobin quantities will have more severe CO exposures based on the attainment of a higher carboxyhemoglobin concentration. This includes patients of small size (pediatric population) and patients with anemia.

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

Objective. The passage of volatile anesthetic agents through accidentallydried CO2 absorbents in anesthesia circuits can result in thechemical breakdown of anesthetics with production of greater than 10 000 ppmcarbon monoxide (CO). This study was designed to evaluate a portable COmonitor in the presence of volatile anesthetic agents. Methods. Two portableCO monitors employing electrochemical sensors were tested to determine theeffects of anesthetic agents, gas sample flow rates, and high COconcentrations on their electrochemical sensor. The portable CO monitorswere exposed to gas mixtures of 0 to 500 ppm CO in either 70% nitrousoxide, 1 MAC concentrations of contemporary volatile anesthetics, or reactedisoflurane or desflurane (containing CO and CHF3) in oxygen.The CO measurements from the electrochemical sensors were compared tosimultaneously obtained samples measured by gas chromatography (GC). Datawere analyzed by linear regression. Results. Overall correlation between theportable CO monitors and the GC resulted in an r2 value>0.98 for all anesthetic agents. Sequestered samples produced anexponential decay of measured CO with time, whereas stable measurements weremaintained during continuous flow across the sensor. Increasing flow ratesresulted in higher CO readings. Exposing the CO sensor to 3000 and 19 000ppm CO resulted in maximum reported concentrations of approximately 1250ppm, with a prolonged recovery. Conclusions. Decrease in measuredconcentration of the sequestered samples suggests destruction of the sampleby the sensor, whereas a diffusion limitation is suggested by the dependencyof measured value upon flow. Any value over 500 ppm must be assumed torepresent dangerous concentrations of CO because of the non-linear responseof these monitors at very high CO concentrations. These portableelectrochemical CO monitors are adequate to measure CO concentrations up to500 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 duringthe breakdown of isoflurane and desflurane by dry CO2absorbents. Trifluoromethane interferes with anesthetic agent monitoring, andthe interference can be used as a marker to indicate anesthetic breakdown withCO production. This study tests representative types of gas monitors todetermine their ability to provide a clinically useful warning of COproduction in circle breathing systems. Methods. Isoflurane anddesflurane were reacted with dry Baralyme® at 45 °C. Standardizedsamples of breakdown products were created from mixtures of reacted andunreacted gases to simulate the partial degrees of reaction which might resultduring clinical episodes of anesthetic breakdown using 1% or 2% isoflurane and 6% or 12% desflurane. These mixtures were measured by the monitors tested, andthe indication of the wrong agent or a mixture of agents due to the presenceof trifluoromethane was recorded and related to the CO concentration in thegas mixtures. Results. When presented with trifluoromethane fromanesthetic breakdown, monochromatic infrared monitors displayedinappropriately large amounts of isoflurane or desflurane. Agent identifyinginfrared and Raman scattering monitors varied in their sensitivity totrifluoromethane. Mass spectrometers measuring enflurane at mass to charge= 69 were most sensitive to trifluoromethane. Conclusions. Monochromaticinfrared monitors were unable to indicate anesthetic breakdown viainterference by trifluoromethane, but did indicate falsely elevated anestheticconcentrations. Agent identifying infrared and Raman monitors provided warningof desflurane breakdown via the interference of trifluoromethane by displayingthe wrong agent or mixed agents, but may not be sensitive enough to warn ofisoflurane breakdown. Some mass spectrometers provided the most sensitivewarnings to anesthetic breakdown via trifluoromethane, but additional dataprocessing by some patient monitor units reduced their overall effectiveness.

Sevoflurane Breakdown Produces Flammable Concentrations of Hydrogen

Background:Fires, explosions, and extreme heat production may occur when sevoflurane reacts with desiccated barium hydroxide lime. The identity of the flammable gas has not previously been published, although carbon monoxide, methanol, formaldehyde, and methyl formate have been identified in low quantities. Methods:The authors reacted sevoflurane with excess desiccated barium hydroxide lime or soda lime at 55°, 100°, 200°, 300°, and 400°C. Formaldehyde, methanol, sodium formate, and hexafluoroisopropanol were reacted with barium hydroxide lime at 300° or 400°C. The authors measured hydrogen production by gas chromatography with a thermal conductivity detector and calculated the molar yield of hydrogen produced. Results:Up to 3 moles of hydrogen were produced per mole of sevoflurane degraded. Each mole of formaldehyde produced up to 2 moles of hydrogen at 400°C. Formate and hexafluoroisopropanol produced up to 1 mole of hydrogen each at 400°C. More than 2 moles of hydrogen were produced by methanol at 400°C. Soda lime and barium hydroxide lime produced similar amounts of hydrogen from sevoflurane above 200°C, but barium hydroxide lime produced more than soda lime at lower temperatures. The temperature above which large amounts of hydrogen were produced seemed to be 300°C. Conclusions:Up to 3 moles of hydrogen are produced by the chemical reaction of sevoflurane with heated, desiccated absorbent. The high yield, ease of ignition, and low tissue solubility of hydrogen make it the most likely fuel in anesthesia machine fires due to the reaction of sevoflurane with desiccated absorbent.

Barium hydroxide lime turns yellow after desiccation.

Ethyl violet is added to carbon dioxide absorbents and normally serves as an indicator of absorbent exhaustion. During the course of several prior studies of anesthetic breakdown, we noted (but did not publish) that barium hydroxide lime (BL), but not soda lime, turns yellow upon desiccation. We hypothesize that ethyl violet undergoes chemical reaction to produce a yellow colorant in desiccated BL. We qualitatively studied the time course of yellow color development during desiccation of these absorbents with dry oxygen. The yellow colorant was extracted from desiccated absorbent with diethyl ether, separated with chromatography, and analyzed with proton nuclear magnetic resonance and combined gas chromatography and mass spectrometry. The yellow color develops after BL has reached nearly complete desiccation. We successfully identified that ethyl violet decomposes into the yellow colorant 4,4′-bis(diethylamino)benzophenone upon desiccation of BL. The color is not intense, is not useful for identifying low levels of absorbent desiccation, and may be difficult to see through tinted canisters. It may be possible for BL to be sufficiently desiccated to allow chemical breakdown of anesthetics, but not yet show yellow coloration. However, if yellow coloration exists, one should assume that it has become desiccated.

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

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.