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A new technique to generate hypothermia aims to produce cooling by adiabatic gas expansion for brain protection in out-of-hospital cardiac arrest (OHCA) and stroke. CO2 appears as the most effective gas to produce a significant amount of cooling from a high pressure gas container. A regulator reducing the pressure to 435 psi was used in all of our previous experiments. The purpose of this work was to compare the cooling produced by expansion of CO2 with and without regulator.
The thermodynamics of cooling were studied from experiments in which a given amount of 200ml of water was cooled by emptying a prototype 0.7 l bottle. The bottle was filled with CO2 at the saturated vapor pressure (841 psi), under normal conditions of temperature and pressure. When a regulator including its output pressure was calibrated at 435 psi, the exit of the polyamide injector was adapted to obtain the same flow of CO2.
The extraction of calories was, 1232 ±133 (mean±SD, n=11) for regulated pressure and 1151 ±263 (mean±SD, n=16) for non-regulated CO2 administration (P = 0.37). Therefore, a similar amount of cooling (lower for non-regulated probably because of higher forced convection) is produced by the two methods with or without a regulator. The next advance has been to put the bottle on a tripod above the head of the patient and to inject liquid instead of gaseous CO2 into the mouth. However, this is only possible with a very small exit hole which needs clock mechanics technology. Carbonic snow is produced at the exit immediately transform in very cool gaseous CO2. This produces even further reduction in the complexity and the price of the equipment for emergency cooling, consisting of a gas container, a valve, a rigid tube in the mouth and an injector surrounded by roll bars and jet deflector to prevent contact of the injector tip with mucosa. This device can be covered by a blind cannula to reject CO2 outside of the respiratory tract (if there is no return of spontaneous respiration). Liquid CO2 can produce 2.5 more gas than gaseous CO2 making the bottle even more efficient.
An injector with a reduced exit hole is able to produce significant cooling without the need for a regulator. This final feature of the emergency cooling device reduces further the price of this equipment, allowing it to be disseminated in many places and even in the home. We propose the name of the “GF (tripod) Bottle” to identify this equipment.