The Type of Carbon Dioxide Absorbent has no Relation to the Concentration of Carbon Monoxide in the Breathing Circuit during Low-flow Isoflurane Anaesthesia in Smoking and Non-smoking Subjects

The present study was designed to investigate the concentrations of carbon monoxide (CO) in the anaesthetic circuit and of arterial carboxyhaemoglobin (COHb) during low-flow isoflurane anaesthesia in smoking and non-smoking subjects using three kinds of cardon dioxide (CO₂) absorbent. Thirty smoking and 30 non-smoking subjects were selected for this study, and these two groups were each divided into three groups according to the type of CO₂ absorbent used (Wakolime^™ A, Drägersorb^™ Free, and Amsorb^™). Anaesthesia was maintained with 1.0% isoflurane and nitrous oxide (1.0l min^–1)/oxygen (1.0l min^–1). Concentrations of CO in the inspired breathing circuit and concentrations of arterial COHb were measured at 0, 1, 2, 3, and 4 hours after exposure to isoflurane. In the smoking groups there were no significant differences in CO concentrations in the circuit between the groups and the CO concentrations did not change significantly during the study period. There were also no significant differences in the arterial COHb values between the groups and the COHb concentrations remained constant. There was a significant linear correlation between the concentrations of CO and COHb (r=0.86, n=30, P<0.001). In the nonsmoking groups all of the parameters remained constant at low levels that were independent of the type of CO₂ absorbents tested. The major source for increased intraoperative CO exposure is related to the patient's smoking status, and the type of CO₂ absorbent used has no relation to an increase in CO concentration in the breathing circuit.