title: The valid time of soda lime could be safely prolonged according to the inspired pressure of carbon dioxide.
abstract: BACKGROUND
  Carbon dioxide (CO2) 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 CO2 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 CO2 absorbent according to the inspired pressure of carbon dioxide (PiCO2) and comparison of the pressure difference between the arterial carbon dioxide tension (PaCO2) and end-tidal CO2 in the exhausted and fresh CO2 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 PiCO2 reached 6 mmHg. Before and 20 minutes after the replacement, PaCO2 and end-tidal CO2 were obtained and designated as group-Pre and group-Post, respectively. The pressure differences between PaCO2 and end-tidal CO2 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 PaCO2 in group-Pre and group-Post was 43.7 +/- 4.2 mmHg and 40.9 +/- 4.6 mmHg respectively. The end-tidal CO2 in group-Pre and group-Post was 38 +/- 3.5 mmHg and 35 +/- 3.6 mmHg, respectively. There were significant differences in PaCO2 and end-tidal CO2 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 PiCO2 concentration reached 6 mmHg in the rebreathing circuit with exhausted soda lime, the gradient between the PaCO2 and end-tidal CO2 was of no statistical difference compared with that of the fresh soda lime circuit. Thus the pressure difference was not affected by the exhausted CO2 absorbent in spite of a reach of PiCO2 to 6 mmHg. Under CO2 monitoring, the valid time of soda lime could be safely prolonged until the PiCO2 was elevated to 6 mmHg rather than 8 h strictly pursuant to traditional method of discoloration of indicator dye.
authors.affiliation: Department of Anesthesiology, Changhua Christian Hospital, Changhua, Taiwan, ROC.
authors.forename: Yi-Hong
authors.initials: YH
authors.lastname: Chen
authors.forename: Chao-Liang
authors.initials: CL
authors.lastname: Chen
authors.forename: Yung-Tai
authors.initials: YT
authors.lastname: Chung
authors.forename: Liang-Tsai
authors.initials: LT
authors.lastname: Yeh
authors.forename: Kuan-Ta
authors.initials: KT
authors.lastname: Lu
authors.forename: Chung-Yuan
authors.initials: CY
authors.lastname: Lin
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000768:Anesthesia, General / Q000379:methods*; D017610:Calcium Compounds / Q000494:pharmacology*; D019296:Capnography; D002245:Carbon Dioxide / Q000378:metabolism*; D006801:Humans; D008875:Middle Aged; D010087:Oxides / Q000494:pharmacology*; D011312:Pressure; D012972:Sodium Hydroxide / Q000494:pharmacology*; D013997:Time Factors
chemical_list: D017610:Calcium Compounds; D010087:Oxides; D002245:Carbon Dioxide; C004569:soda lime; D012972:Sodium Hydroxide