Physical factors affecting the production of carbon monoxide from anesthetic breakdown

Background: Parameters determining carbon monoxide (CO) concentrations prod uced by anesthetic breakdown have not been adequately studied in clinical s ituations. The authors hypothesized that these data will identify modifiabl e risk factors. Methods: Carbon monoxide concentrations were measured when partially desicc ated barium hydroxide lime was reacted with isoflurane (1.5%) and desfluran e (7.5%) in a Draeger Narkomed 2 anesthesia machine with a latex breathing bag substituting for a patient. Additional experiments determined the effec ts of carbon dioxide (0 or 350 ml/min), fresh gas flow rates (1 or 4 1/min) , minute ventilation (6 or 18 1/min), or absorbent quantity (1 or 2 caniste rs). End-tidal anesthetic concentrations were adjusted according to a monoc hromatic infrared monitor. Results: Desflurane produced approximately 20 times more CO than isoflurane when completely dried absorbents were used. Peak CO concentrations approac hed 100,000 ppm with desflurane, Traces of water remaining after a 66-h dry ing time (one weekend) markedly reduced the generation of CO compared with 2 weeks of drying. Reducing the quantity of desiccated absorbent by 50% red uced the total CO production by 40% in the first hour. Increasing the fresh gas flow rate from 1 to 4 1/min increased CO production by 67% in the firs t hour but simultaneously decreased average inspiratory concentrations by 5 3%. 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 exposu res. Minute ventilation and carbon dioxide production by the patient are re latively unimportant.