Accuracy of calculated volatile agent consumption from fresh gas content

Case-by-case assessment of volatile anesthetic (VA) consumption is necessary to perform agent sparing dosing measures and for billing purposes. The gold standard technique for this purpose is to measure the weight difference of the vaporizer that occurs during VA delivery. Because suitable equipment is rarely available and weighing the vaporizer is only possible in prospectively planned fashion, a calculation method from recorded fresh gas flow (FGF) and VA courses is a viable alternative. The objective was to evaluate the accuracy of VA calculation from fresh gas composition vs. the gold standard of weighing the vaporizer before and after anesthesia.

In this prospective laboratory investigation, we compared the formula-based calculations of VA consumption to the measured vaporizer weight differences before and after 10 sevoflurane and 10 desflurane anesthesia. We assessed the average difference and spread of values between the calculated and the measured values.

The calculated sevoflurane consumption overestimated the measured values by 3.0 ± 2.9 ml (6.2%). The calculated desflurane consumption overestimated the measured values by 3.5 ± 6.7 ml (5.0%). All pairs of values from both agents were within ± 1.96 standard deviations.

The calculated VA consumption for sevoflurane and desflurane are sufficiently accurate to estimate the economic impact of VA delivery during inhalational anesthesia. This method enables the assessment of VA consumption both ways: (1) retrospectively from sufficiently detailed and accurate anesthesia recordings, as well as (2) by using this method in a prospective setting.