StandardsReducing vapour costs with “Free-Ox” machine
Using the Humphrey ADE-circle anaesthetic breathing system.
With the dramatic reduction in oxygen costs using an oxygen concentrator, the other significant cost is the vapour used. Isoflurane has become the standard (replacing halothane/fluothane because of its safer properties), but sevoflurane is the next generation offering very rapid induction of anaesthesia and wake-up times with much the same benefits. However both isoflurane and sevoflurane are expensive. While the concentration required to anaesthetize a patient cannot be reduced (much as petrol in a car cannot be diluted to make it go further), reducing wastage by increasing efficiency and reducing flows have been the goal behind much research. The Humphrey ADE-circle system (Fig 5) has been designed by the author to significantly reduce fresh gas flows by up to 70% compared to other breathing systems. Since 1982 when it was first introduced into human anaesthesia, it has a proven track record having been used in an estimated 20 million anaesthetics worldwide both in human and veterinary practice.
Fig 5: The Humphrey ADE-circle system. For animals under 7kg it is used without the soda lime canister; for larger animals the canister is attached by two lock nuts. The unique exhaust valve always saves anaesthetic gas within the system, and only opens when the reservoir bag is full. Minimal flow techniques of just 500 ml/min for animals up to 50kg save on isoflurane or sevoflurane usage. (see vet section on website www.anaequip.com).
Techniques now used with the ADE system allow minimal flows of just 500 ml/min to be used for animals up to 50kg. For animals up to 7kg the ADE-circle system is used without the soda lime canister with flows set at 70-100 ml/kg/min such that a 5 kg cat would require just 350--500 mls/min. For heavier animals from 7-100kg re-cycling through the soda lime canister is employed; after induction maintenance flows can be reduced to 10 ml/kg/min – again just 300 ml/min for a 30kg dog and 500 ml/min for a 50 kg dog! Such low flows are mainly possible due to the design of the system itself, but are also due to a unique exhaust valve. This valve is designed not to vent gases from the system until the reservoir bag is full, ready for the next breath. With or without the soda lime canister, only unwanted alveolar gas is eliminated from the system, making it very much more efficient than any other system. A full description of its function is available in the vet section on the website given in the reference section. Since 500 ml/min is well below the fresh gas flow used in many veterinary clinics (observed at anything between 1-3 l/min and sometimes more) this reduction will more than halve vapour costs for almost all vet clinics.