Search

Background and Objective: Numerous in vitro studies have shown that volatile anaesthetics react with desiccated carbon dioxide (CO2) absorbents to produce carbon monoxide (CO). The effects of anaesthetic concentration, fresh gas flow rate, and the hydration of absorbent or the excretion of CO2 by patients on CO production have also been investigated. This work aims to identify the most significant one of these factors on CO concentration in a low-flow anaesthesia system, without control of the hydration of the absorbents., Methods: A simulated clinical circle anaesthetic breathing system was used to study the CO concentration under various conditions. Desflurane was used at three different concentrations. Two CO2 flow rates and three fresh gas flow rates were used. The absorbent temperatures and hydration were measured simultaneously., Results: Desflurane degraded to produce CO in the breathing tube, when the CO2 absorbents were not dried beforehand. In this imitation clinical low-flow setting, fresh gas flow affected the CO production more than the CO2 did (31.7% vs. 9.5%). The actual desflurane partial pressure was not a significant factor. The CO2 flow rate explained 18.2% and 54.0% of the variation of the absorbent hydration changes (%) and temperature, respectively., Conclusions: In clinical practice, the CO2 production varies among patients and is uncontrollable, but markedly affects CO production. The only controllable factor is the fresh gas flow rate if the ultimate goal is to reduce the undesirable exposure of patients to CO from the breathing tube according to this bench model without counting the oxygen consumption.

The major sources for intraoperative carbon monoxide (CO) in the breathing circuit are related to patient's hemoglobin catabolism, smoking and the degradation between absorbent and anesthetics. We performed this study to evaluate their combined effects on CO production during low-flow anesthesia. We used a direct-measurement instrument to measure real-time CO concentrations in the breathing circuit during different anesthetic conditions for patients who received desflurane or isoflurane. By applying multiple linear regression models, we determined the significant factors related to CO concentrations in the circuit. We identified patients' smoking status, preoperative smoking and body weight as well as gas flow rates as important factors for affecting peak and time-weighted CO levels. These four factors predicted approximately 44.1% and 42.7% of peak and mean inspiratory CO concentrations respectively. We found that chronic and preoperative smokers and patients with larger body weights are associated with increased CO concentrations, whereas increase in gas flow rates could decrease CO concentrations. After controlling these four important factors, we found that inspiratory CO concentrations were not significantly associated with the choice of anesthetic and its concentration during low-flow anesthesia.