以吸氣時二氧化碳濃度的出現來當成更換二氧化碳吸附劑的指標可以安全地延長其使用時數

The Valid Time of Soda Lime Could Be Safely Prolonged According to the Inspired Pressure of Carbon Dioxide

背景：閉鎖循環全身麻醉中，須依靠二氧化碳吸附劑將患者呼出的二氧化碳加以吸收。一般其更換的原則，是以指示劑顏色改變程度或記錄總使用時數來當依據。我們設計以吸氣時二氧化碳濃度的值，來當成更換的指標。方法：有55位患者納入此試驗，所使用的二氧化碳吸附劑皆記錄更換時所使用的時數；更換條件是須吸氣期二氧化碳濃度達到6mmHg。測得更換前與更換後二十分鐘患者的動脈血中與吐氣末二氧化碳濃度，取得更換前後兩者的差距，之後檢驗兩組在統計上的意義。結果：兩組的差距值在統計上並無差異；但血中二氧化碳濃度與吐氣末二氧化碳濃度在更換前後有統計上的差異。而所使用的時數，比平時的更換原則的使用時數，延長地更久，患者也都安全地甦醒拔管。結論：動脈血中二氧化碳濃度與吐氣末二氧化碳濃度兩者的差距，並不因吸附劑是否耗竭而受影響。在有二氧化碳監測的全身麻醉下，吸附劑的更換可以取吸氣期二氧化碳濃度的值達6 mmHg當指標，而非依賴指示劑顏色的改變。

Background: Carbon dioxide (C02) absorbent, a disposable chemical mixture enclosed in a canister of anesthetic breathing systems functions to remove the carbon dioxide expired from the patients during general anesthesia. The timing of replacing the exhausted C02 absorbent is usually decided by discoloration of the indicator dye or the valid time of use. However, these methods are subjective. We designed a study to validate our idea of replacing the exhausted C02 absorbent according to the inspired pressure of carbon dioxide (PiC02) and comparison of the pressure difference between the arterial carbon dioxide tension (PaC02) and end-tidal CO2 in the exhausted and fresh C02 absorbent circuits. Methods: Fifty-five adult patients undergoing general anesthesia for elective surgeries were enrolled in this study. All the canisters containing fresh soda lime were labeled with the date of filling and valid time of clinical use. The soda lime was replaced only when the PiC02 reached 6 mmHg. Before and 20 minutes after the replacement, PaC02 and end-tidal C02 were obtained and designated as group-Pre and group-Post, respectively. The pressure differences between PaC02 and end-tidal C02 in both groups were checked for statistical analysis. Results: The pressure differences in group-Pre and group-Post were 5.8 ± 3.4 mmHg and 6.1 ± 3.3 mmHg, respectively, both of which were not statistically significant (P = 0.62). The PaC02 in group-Pre and group-Post was 43.7 ± 4.2 mmHg and 40.9 ± 4.6 mmHg respectively. The end-tidal C02 in group-Pre and group-Post was 38 ± 3.5 mmHg and 35 ± 3.6 mmHg, respectively. There were significant differences in PaC02 and end-tidal C02 between the two groups (P < 0.001). By this determination the valid time of soda lime in Omeda, Cato and Kion anesthesia machines was 57.3 ± 7.2, 35.6 ± 6.3 and 21.7 ± 4.2 hours, respectively, all of which were much longer than 8 hours of routine use previously delimited. Conclusions: Even though the PiC02 concentration reached 6 mmHg in the rebreathing circuit with exhausted soda lime, the gradient between the PaC02 and end-tidal C02 was of no statistical difference compared with that of the fresh soda lime circuit. Thus the pressure difference was not affected by the exhausted C02 absorbent in spite of a reach of PiC02 to 6 mmHg. Under C02 monitoring, the valid time of soda lime could be safely prolonged until the PiC02 was elevated to 6 mmHg rather than 8 h strictly pursuant to traditional method of discoloration of indicator dye.