We thank Ueyama et al. for their interest and comments regarding Amsorb (Armstrong Medical Ltd., Coleraine, Northern Ireland) and our recent report. 1Although the absolute carbon dioxide (CO2) absorptive capacity of Amsorb is less than that of conventional absorbents, we must emphasize that the true performance of any CO2absorbent is its ability to facilitate low-flow anesthesia safely. 2Retaining a strong base, as within conventional soda lime, carries the risk of carbon monoxide poisoning and the formation of compound A, 3,4both of which substances, unlike CO2, are not detectable in clinical practice.
Major differences exist between our study and the study reported by Ueyama et al. They used significantly larger canisters and a different study method than those described in our paper. 1Beyond this initial work, we have used a model similar to that of to Ueyama et al. and have shown that changes in canister size and design significantly alter the CO2absorptive capacity of both soda lime and Amsorb by almost sixfold. 5Comparing Ueyama et al. ’s data for the two reported canister designs (Dräger, Lübeck, Germany, and Datex Ohmeda, Bromma, Sweden), it is clear that there are marked intracanister differences in CO2absorption capacity for Amsorb and Medisorb both, highlighting potential inefficient use of all absorbents caused by shortcomings in canister design. The convenience of a smaller-sized canister is always a trade-off against efficient use of a CO2absorbent. We argue that a CO2absorption capacity of 900 min (15 h) for 2.4 l Amsorb at a flow rate of 500 ml/min is more than adequate for 1–2 days of anesthesia.
Ueyama et al. state that their model of determining CO2absorptive capacity is based on clinical anesthetic practice. However, in routine practice, the life of a conventional CO2absorbent is limited by safety concerns and the United States Food and Drug Administration Center for Disease Control recommendation regarding this subject is that “All soda lime that has been dormant in the anesthesia machine for more than 24 hours should be changed, and dated.”6Such a restriction does not apply to Amsorb. The authors also state that color change is not a good indicator of exhaustion of soda lime, and, for conventional limes, this is correct because the strong alkali allows regeneration of pH changes within the indicator after the calcium hydroxides’ capacity for CO2absorption has been exceeded. Amsorb does not contain strong alkali, so color change is not reversible and does indicate exhaustion. With concurrent use of capnography, unexpected rebreathing does not occur.
We wish to draw the authors’ attention to a cost analysis of the use of Amsorb in clinical low-flow anesthesia that has shown that the life of the Amsorb (ignoring Anesthesia Patient Safety Foundation recommendations for the changing of soda lime) is about two thirds that of conventional limes. 7
We thank the authors for their interest in our new absorbent, but we stress that to measure this product against soda lime purely on absorptive capacity ignores safety issues and is a retrograde step for low-flow anesthesia.