Performance of Litholyme™ compared with Sodasorb™ carbon dioxide absorbents in a standard clinical setting C. J. Fowler * , M. A. Burbridge, R. A. Jaffe and J. G. Brock-Utne Palo Alto, CA, USA *Corresponding author. E-mail: cedar@stanford.edu EditordIn an era of efficiency and cost considerations within the operating theatre, the search for products with improved performance profiles is critical. The effort even extends to the disposable parts of the anaesthetic machine such as the car- bon dioxide (CO 2 ) absorber. There are currently multiple sup- pliers of CO 2 absorbents. CO 2 absorbent evolution is a study of compromise as manufacturers have attempted to balance the ability to absorb CO 2 with undesired effects such as degrada- tion of volatile anaesthetics, excessive heat generation, and compound A production from sevoflurane. 1 The final decision regarding which CO 2 absorbent an institution uses is multi-factorial and includes tradition, product reliability, ease of use, and cost. Our high-volume academic institution traditionally has used Sodasorb™ (W.R. Grace, Lexington, MA, USA) as a CO 2 absorbent. In the interest of efficiency and cost considerations, our institution decided to compare Sodasorb™ with a newer-generation CO 2 absorber, Litholyme™ (Allied Healthcare Products Inc., St. Louis, MO, USA). An often-noted advantage of Litholyme™ is that it can be disposed of with normal waste instead of medical waste as is required of Sodasorb™, thereby reducing cost significantly. Published data are particularly promising with respect to the CO 2 absorbent capabilities of Litholyme™. 2,3 Data from in vitro experiments using a model anaesthetic set-up suggest that Litholyme™ is capable of absorbing 1.5 times the CO 2 of its nearest competitor. The improved CO 2 absorbent capabil- ities, and an in vivo report, 4 suggest that Litholyme™ will provide lengthened time until canister depletion and outper- form Sodasorb™. The absence of in vivo performance studies of CO 2 absorbents is in part attributable to in vitro data frequently utilising absorbent weight for calculation of litres of CO 2 absorbed. 2,5 This measurement cannot be duplicated in vivo owing to variances in humidity within the canister. Therefore, for our study we used litres of gas processed and CO 2 absorption to compare CO 2 absorbents. We directly compared Litholyme™ and Sodasorb™ in a clinical setting by measuring CO 2 absorbent depletion times and amount of CO 2 absorbed. The institutional review board (IRB)-approved study was conducted at Stanford University Hospital, Stanford, CA, USA, and prospectively compared Litholyme™ with Sodasorb™. Twenty identical CO 2 canisters were randomly filled with Sodasorb™ (n¼10) or Litholyme™ (n¼10) per standard operating theatre policies regarding refilling of the reusable canisters. Canisters were used in Drager Apollo® (Drager Inc., Telford, PA, USA) anaesthetic machines. There was surgery case restriction for enrolment. General tracheal anaesthesia was induced via administration of 10 L min À1 oxygen followed by intravenous induction. After induction, fresh gas flow (FGF) rates were reduced to 2 L min À1 . Flow rates were adjusted to maintain oxygen concentration at approximately 50%. Anaesthesia was maintained with volatile anaesthetic or intravenous anaesthetic as clinically appro- priate. Ventilation was controlled with tidal volumes of 4e6 mL kg À1 , and ventilator rate was adjusted to maintain end- tidal CO 2 of 4.6e5.3 kPA (35e40 mm Hg). After skin closure, the anaesthetic was discontinued and FGF was increased to 10 L min À1 of oxygen until the patient met extubation criteria. The CO 2 absorber was counted as expired when there was evidence of exhaustion as determined by inspired CO 2 fraction (FiCO 2 ) at 1.0% after disconnection and agitation, as per insti- tution policy. Litres of gas processed as a measure of CO 2 absorbent depletion times achieved by the same volume of absorbent and an estimate of CO 2 absorption were the primary end points. CO 2 absorption was estimated from end-tidal CO 2 , FGF, and minute ventilation. Data are expressed as mean (standard deviation, SD), and statistical testing was done using two-tailed Student’s t-test with Welch’s correction. There was no significant difference in patient characteris- tics between groups. The mean age of patients was 55 (39e71) yr, and the mean body mass index was 29 (22e36) kg m À2 .A significant difference existed between CO 2 absorbents with respect to total CO 2 absorbed (p<0.01, Fig. 1a). Sodasorb™ canisters absorbed an average of 353 (321e385) L of CO 2 compared with Litholyme™ canisters, which absorbed an average of 238 (222e254) L. Significant differences were noted in total litres of gas processed (p<0.01, Fig. 1b). Sodasorb™ canisters were able to process 1.4 times as much CO 2 as Lith- olyme™ canisters. Published literature and company publications suggest that Litholyme™ may offer advantages with respect to cost and efficiency. 4 However, the purchase price of Litholyme™ is approximately double the cost of Sodasorb™ for our institu- tion. Using the two CO 2 absorbers (Litholyme™ and Soda- sorb™) in the operating theatres at our institution allowed us to test the performance of the CO 2 absorbents under typical operating theatre conditions. The data suggest that Soda- sorb™ canisters require less frequent changes than Lith- olyme™ canisters of identical size (Fig. 1). Sodasorb™ processed approximately 1.4 times the volume of exhaled gas compared with Litholyme™. Others have found that Litholyme™ canisters require fewer changes and therefore lead to improved efficiency and cost savings. 4 There is no published comparison between Lith- olyme™ and Sodasorb™ in an actual clinical setting. There- fore, we are unable to directly compare our data with those reported in previously published studies. Our data do not account for all factors. We used Drager Apollo® anaesthetic machines and the reusable canisters provided as part of the machine. Litholyme™ is frequently packaged in disposable containers, which may provide addi- tional benefits. Our institutional policy is to change CO 2 Correspondence - e11