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Journal of Wilderness Medicine 2,330-350 (1991)
Equipment for airway warming in the treatment ofaccidental hypothermiaEVAN L. LLOYD
Department ofAnaesthetics, Princess Margaret Rose Hospital, University of Edinburgh, Fairmilehead,Edinburgh EHIO 7ED, UK
Airway warming as a method of treating accidental hypothermia produces its main benefit throughreducing or preventing the respiratory loss of heat and moisture, though, with some devices, theremay be a small additional heat input. Warm water vapour can be added to the inspired gas. Thepower to heat the water can be provided by electricity or by gas. An alternative method utilizes achemical reaction to produce heat and moisture, which is transported by the inspired gases passingthrough the reaction bed. The most widely used reaction is that between carbon dioxide and sodalime, and practical designs use either an open (non-return) or closed breathing system.
Heat and moisture exchangers (HMEs) vary in efficiency, but all are less efficient than the othermethods, since they merely reduce the individual's respiratory heat and moisture loss. HMEsprobably have a role as first aid equipmen~ to be carried by individuals or groups going into the wild.The more efficient equipment, which provides positive heat and moisture input, is more suitable forrescue services. All the currently available models have practical advantages and disadvantages, andeach rescue service will have to decide which design best suits its particular needs.
Key words: accidental hypothermia, ~irway warming
IntroductionThe technical requirement for airway warming (AW) is to produce a warm, moist gas forthe hypothermia victim to inspire in order to prevent the normal loss of heat andmoisture which occurs during breathing and, if possible, provide some heat input.
Airway warming accelerates the rate of rewarming of a hypothermic victim [1-3] verysignificantly (p < 0.001) . Airway warming also has specific effects improving cardiacrhythm, cardiovascular function, cerebral function (particularly respiratory drive andlevel of consciousness), and pulmonary function (clearing cold induced pulmonaryopacities and improving gas exchange) [2,3]. Airway warming also appears to reducemortality in patients with immersion (acute) hypothermia and exhaustion hypothermia[2-4]. In subchronic hypothermia found in the elderly and with malnutrition, AW willincrease mortality because of its effects on fluid shifts, unless used with intermittentpositive pressure ventilation and intensive care monitoring [2,3].
Because of the relatively small quantities of heat contributed by using AW, it shouldonly be considered as an adjunct to other measures of insulation to prevent heat lossfrom the body surface. Suitable equipment will therefore have to fulfill certain criteria:
(1) It must be safe even in inexperienced hands, since medical people are unlikely tobe available when the equipment is required. The equipment may lie unused on ashelf for many years.
(2) It must be practical and convenient to use: For example, in the mountains it is0953-9859/91 $03.00 + .12 1991 Chapman & Hall
Airway warming equipment: a review 331
useless to have equipment which requires a high level of manual dexterity ortactile sense, since climbers will then have to use bare fingers and risk frostbite.
(3) It must be relatively inexpensive to own and operate, since most rescue servicesare chronically short of funds, and there are always other items of equipmentwhich will have a higher priority.
(4) Since AW equipment will always be additional to basic rescue equipment, it willbe carried on many occasions when it is not needed. It will therefore have to belight and compact, though the detailed needs of the different rescue services vary.
(5) The equipment must meet the requirements of any individual rescue service orenvironment where treatment is likely to be given. For example, the RoyalNational Lifeboat Service currently does not allow oxygen to be carried on itsboats because of the fire risk.
The respiratory tract and lungs are very resistant to damage by heat . Nevertheless,prolonged ventilation with humidified air at 80 C produced reversible laryngeal scaldingin one patient [1-3]. The maximum tolerable inhaled air temperature for prolongedperiods is recommended not to exceed 45 C [2,6-9]. Airway warming can be achievedby (1) heating water and adding the hot water vapour to the inspired gas stream, (2)utilizing a chemical reaction to introduce heat and moisture into the inspired gas, or (3)conserving heat loss from breathing.
Heating waterHospital humidifierElectric powered humidifiers are ideal for hospital use, since the thermostat can beadjusted to the. required temperature. In some of the designs, the temperature sensorcontrolling the thermostat is located at the patient end of the delivery tubing, making thesystem potentially very accurate. This equipment is readily available and can be usedwhere there is a power supply. Provided the humidifier is of the water bath type, and nota nebulizer, the risk of infection is minimal and overhydration is almost impossible .The performance of many designs has been reviewed [10-13]. Unfortunately, 'humidi-fiers' do not always adequately humidify the inspired gas .
In hospital, patients with hypothermia have been successfully treated using electricpowered water bath humidifiers [1,4,9,15-19]. In the field, a modified Bennett cascadehumidifier is being used by rescue services that operate along the Oregon coast.However, electric-powered humidifiers have the limitation of requiring a power source.Although some rescue teams carry portable generators, the weight and cost of the gener-ator limits the use of these designs. Even where electricity is available, the power require-ments of other equipment like radar or radios may be so great that there is insufficientspare generator capacity to power a humidifier.
Thermal parachutePowered by a petrol-driven generator, this design, with an electric heater, humidifier, andblower, was specially developed by Dr J. Foray for use in crevasse rescue on Mt Blancnear Chamonix, France  (Fig. 1). Air is delivered through a specially insulated tubeat 45 C with 100% humidity for 2 h before the water reservoir needs refilling. Thethermal parachute is lowered to a victim trapped in a crevasse, and the person canbreathe through a mouthpiece, face mask, or head-covering hood.
Portable'Honda generator'Weight: 20lb
Rewarming deviceWeight : 21bLength : 30cmThickness: 15 cm
---Electric cord(up to 40m)
Fig. 1. Thermal parachute. A. Diagrammatic representation. B. Original prototype
Airway warming equipment: a review
Fig. 2. The Heat-Treat consists of a breathing mask (1) attached to a one-way valve (2). Tempera-ture of the air breathed by the patient is monitored by a dial thermometer (3). Air is transported tothe valve and mask by a flexible hose (4) attached to a temperature-regulating valve (5) via a water-trap (6) for condensed water vapour. This manually-controlled valve mixes ambient air and steamfrom the steam generator (7) to provide water-saturated air of the desired temperature (e.g. 43C).A constant flow potential is provided by this valve for all possible control positions. The steamgenerator is heated by propane from an adjacent cylinder (8) via shutoff (9) and fine control (10)valves. Heat given off by the generator is directed around the propane tank by a shroud (11) toensure vaporization of the propane in extremely cold environments. Also illustrated are the opening(12) in the generator for lighting the burner; the opening (13) for adding water to the generator; thetransparent gauge (14) for indicating water level; the two adjustable stablizers (15); and the carryinghandle (16). Ambient air enters the steam chamber via channels in the upper part of the generator,which results in pre-heating of entering air due to the high temperature of the metal exposed to theexhaust gases. This maintains the high temperature of the steam despite cold ambient air supply.Although the temperature regulating valve (5) is designed for simple and effective operation, it ispossible that operator error could cause excessive temperature of the inhalate. As a safety accessory,an electronic control valve can be installed adjacent to the manual valve (5) and operates as a high-temperature alarm and limiting device. A thermistor is located in the flexible hose (4) near the one-way flow valve (2). When the temperature exceeds 45C (or a setting near this, according to choice),the opening (17) allows cool ambient air into the air passage. This opening (17) is provided by asolenoid valve (18) activated by a removable connector (19) from the rechargeable, battery-operated, control box (20). During activation of this limiting device, audible and visible alarms (21)signal the need for manual adjustment of the temperature-regulating valve (5). If such adjustment isdelayed, the electronic valve will provide control of inhalate temperature within a satisfactory rangeby intermittent activation. A button (22) permits pre-testing of the function of this accessory device.
UVIC heat treatIn this design (Fig. 2) steam, which may be generated from an ordinary cooking pot witha hood or from a special gas or electric-powered steam generator, is injected into adelivery tube which must run up to the patient so that any water condensation runs backdown to the water-trap and not into the patient . When a simple version using acooking pot and gas burning stove [weighing 2.3kg] was tested at 2000 m altitude withan air temperature of 0 C, the equipment took 15 min to reach the stage of delivering