Abstract
Low-flow anaesthesia is considered beneficial for the patient and the environment, and it is cost reducing due to reduced anaesthetic gas consumption. An initial high-flow to saturate the circle system ( wash-in) is desirable from a clinical point of view. We measured the wash-in and wash-out times (time to saturate and to eliminate the anaesthetic agent, AA), for sevoflurane and desflurane, in a test-lung with fixed 3 MAC vaporizer setting at different fresh gas flow (FGF) and calculated the consumption of AA. We tried to find an optimal flow rate for speed and gas consumption, comparing two anaesthesia machines (AMs): Aisys and Flow-i. Time to reach 1 minimal alveolar concentration (MAC) (wash-in) decreased (p<0.05) at higher flow rates (1 - 2 - 4) but plateaued at 4-4.8 l/min. The consumption of AA was at its lowest around 4-4.8 l/min (optimal flow) for all but the Aisys /desflurane group. Wash-out times decreased as FGF increased, until reaching plateau at FGF of 4-6 l/min. Aisys had generally shorter wash-in times at flow rates < 4 l/min as well as lower consumption of AA. At higher flow rates there were little difference between the AMs. The "optimal FGF" for wash-out, elimination of gas from the test-lung and circle system, plateaued with no increase in speed beyond 6 l/min. A fresh gas flow of 4 l/min. seems "optimal" taking speed to reach a 1 MAC ET and gas consumption into account during wash-in with a fixed 3 MAC vaporizer setting, and increasing fresh gas flow beyond 6 l/min does not seem to confirm major benefit during wash-out.
Highlights
Low flow anaesthesia is associated with several benefits, reducing the heat loss caused by cold gases and improving humidification in the airways[1]
There was a plateau starting at fresh gas flow (FGF) 4–4.75/4.8 l/min, where increasing the FGF further up to 6 or 8 l/min did not shorten the time to reach 1 minimal alveolar concentration (MAC) significantly; similar patterns could be observed in all 4 groups
We found, in this test-lung setup, that it is possible to increase the anaesthetic agent concentration from 0 to 1 MAC value in around one minute by using a FGF of 4–4.8 l/min on both anaesthesia machines (AMs), but raising the FGF further did not result in shorter wash-in times
Summary
Low flow anaesthesia is associated with several benefits, reducing the heat loss caused by cold gases and improving humidification in the airways[1]. It is environmentally friendly, reducing the release of anaesthetic agents into the atmosphere, and lastly, it reduces costs. The aim of the present study was to measure the time required to reach stable end tidal 1 MAC anaesthetic (wash-in), to measure gas consumption during wash-in and the time needed to eliminate the anaesthetic agent (wash-out) for sevoflurane and desflurane, and to compare the impact of different fresh gas flows and anaesthesia machines
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