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.

Monitoring of isoflurane and desflurane breakdown: interfering gases and infrared detection.

OBJECTIVE: The reaction of isoflurane, enflurane or desflurane with dried CO2 absorbents produces carbon monixide (CO), a highly toxic gas which cannot be detected by gas monitors typically available in the operating room. Trifluoromethane (CHF3) is produced along with CO when this reaction occurs with isoflurane and desflurane, and can be detected by gas monitors. This study will determine the ability of a modified SAM module (Smart Anesthesia Multigas Module, GE/Marquette Medical Systems, Milwaukee, WI) to identify the presence of CHF3, and provide a clinically useful indirect warning of CO production. METHODS: Isoflurane (1.5%) and desflurane (7.5%) were reacted under clinical conditions with desiccated absorbents resulting in CO production. CO and CHF3 concentrations were measured using gas chromatography. The CHF3 concentrations measured by a modified SAM monitor were compared with the measurements obtained by gas chromatography. Alarm limits set on the SAM monitor were used to warn of the presence of CHF3. RESULTS: A concentration of 0.25% CHF3, as measured by the SAM monitor, corresponds to an average CO concentration of 780 ppm for isoflurane and 1700 ppm for desflurane. Lowering the threshold to 0.05% CHF3 would result in an average CO concentration of 155 ppm CO for isoflurane and 345 ppm CO for desflurane. CONCLUSIONS: We have shown that the SAM module is capable of measuring CHF3 due to anesthetic breakdown. With appropriate changes in the display programming and reference cell spectra the monitor would be able to provide an early warning of CO exposure, although the amount of CO would not be reported.

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.

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.

Effect of asthma and ventilatory loading on arterial PCO2 of humans during submaximal exercise.

In humans, attenuating carotid chemoreceptor activity by hyperoxia does not alter arterial PCO2 (PaCO2) during submaximal exercise, yet a transient hypercapnia occurs in carotid chemoreceptor-resected (CBR) asthmatic subjects during submaximal exercise. We hypothesized that this difference was due to asthma and not CBR causing the abnormal response. Accordingly, we determined the temporal pattern of PaCO2 during mild and moderate exercise in chemoreceptor-intact asthmatic (n = 10) and nonasthmatic subjects (n = 10). We also hypothesized that hyperoxia alters PaCO2 during exercise if exercise already has disrupted PaCO2 homeostasis. Accordingly, we studied, during exercise, asthmatic subjects while hyperoxic; nonasthmatic subjects during loaded breathing of room air, which increased PaCO2; and nonasthmatic subjects during loaded breathing while hyperoxic. While breathing room air, neither asthmatic nor nonasthmatic subjects maintained arterial isocapnia during exercise. An increase in PaCO2 between rest and exercise and between mild exercise and 1st min of moderate exercise was greater in asthmatic than in nonasthmatic subjects (P < 0.05). In six asthmatic subjects that were hypercapnic breathing room air during exercise, hypercapnia was accentuated by hyperoxia. The ventilatory load in nonasthmatic subjects resulted in a work load-dependent hypercapnia (P < 0.01) accentuated (P < 0.01) by hyperoxia. We conclude that normally in humans the carotid chemoreceptors contribute minimally to the hyperpnea of submaximal exercise. However, when PaCO2 is increased from resting values during exercise, then the chemoreceptors serve to augment ventilation and thereby minimize the hypercapnia.