System leaks and the loss of sevoflurane

We thank Tokumine et al. for the valuable insights on the potential contribution of system leaks on the loss of sevoflurane [1]. If, as their findings convincingly suggest, the system leaks should be considered negligible, the question remains as to how the missing sevoflurane escaped the system [2]. Given that substantially higher systemic absorption is unlikely, incomplete capture by CONTRAfluran™ (Baxter International, Deerfield, IL, USA) appears to be one of the few plausible explanations.

Since the interaction between activated carbon and sevoflurane is driven by non-covalent forces (primarily Van der Waals interactions which are reversible) it is plausible that, particularly under high fresh gas flow and low partial pressure of sevoflurane, there is incomplete capture or even dissociation and re-release of sevoflurane. It is noteworthy that, to enable the recycling of the adsorbed volatile anaesthetics, the substrate of CONTRAfluran must be selected such that the molecular binding is not excessively strong, allowing desorption at a sufficiently low temperature to avoid degradation of the sevoflurane molecules [3].

In our study protocol, we aimed to achieve maximum elimination of sevoflurane with the patients' trachea intubated. Immediately after stopping sevoflurane administration, we maintained a fresh gas flow of 9 l.min^-1 for a considerable duration until the patient was responsive. A certain amount of desorption and re-release may have occurred during this phase. A possible recommendation could be to use a lower fresh gas flow during the elimination phase of volatile anaesthesia to minimise desorption. This reflects that there is still considerable room for further optimisation of the technology.