Journal of Wilderness Medicine 2,330-350 (1991)

REVIEW ARTICLE

Equipment for airway warming in the treatment ofaccidental hypothermia

EVAN 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

Introduction

The 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) [3]. 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 is

0953-9859/91 $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 [5]. 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 water

Hospital humidifier

Electric 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 [2].The performance of many designs has been reviewed [10-13]. Unfortunately, 'humidi­fiers' do not always adequately humidify the inspired gas [14].

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 gener­ator limits the use of these designs. Even where electricity is available, the power require­ments of other equipment like radar or radios may be so great that there is insufficientspare generator capacity to power a humidifier.

Thermal parachute

Powered 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 [20] (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.

332

(a)

Portable

'Honda generator'

Weight: 20lb

Rewarming deviceWeight : 21bLength : 30cmThickness: 15 cm

(b)

---Electric cord(up to 40m)

Lloyd

Fig. 1. Thermal parachute. A. Diagrammatic representation. B. Original prototype

Airway warming equipment: a review

11

10

8

333

Fig. 2. The Heat-Treat consists of a breathing mask (1) attached to a one-way valve (2). Tempera­ture 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 water­trap (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. 43°C).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 high­temperature alarm and limiting device. A thermistor is located in the flexible hose (4) near the one­way flow valve (2). When the temperature exceeds 45°C (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, battery­operated, 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.

334 Lloyd

UVIC heat treat

In 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 [20]. 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 deliveringhumidified air at 45° C. Topping up the water reservoir dropped the inspired temper­ature to 30° C within 2 min [20]. The simple equipment was unstable and it was neces­sary to protect the gas flame from the wind. In bad weather, the fuel gas may freeze in thecylinder. This system could be very practical, since it can deliver air at a steady temper­ature and humidity [20]. It has a long duration limited only by the gas supply and theneed to refill the water container periodically. An early version of this type of equipmentwas used by the Washington Mountain Rescue Team to treat patients in the field. It is inuse in western Canada and by the Ski Patrol Association of Australia [20].

Solid oxygen

Though burning fuel is technically a chemical reaction which produces heat, there areother reactions which produce heat without fire. Sodium chlorate is packed into a cylin­der with fuel and a binder. When a lanyard is pulled, the fuel produces heat, and oxygenis produced by the destruction of the sodium chlorate. A portable version weighs 3.5 kgand can deliver pure oxygen at 4 I min-1 for 40 min or 8 I min-1 for 20 min. It is expen­sive to buy and refill [20] and although the oxygen is dry, it is sometimes too hot [20].Unfortunately, because of the relative heat capacities of oxygen and water, bubbling itthrough water for humidification would produce cold, humid oxygen. This system istherefore unsatisfactory.

Several other chemical reactions generate heat, but most of the chemicals are expen­sive, bulky, difficult to handle or control, and unlikely to be readily available. Since usingan exothermic chemical reaction as a method of heating water would be inefficient,further consideration is unnecessary.

C02 &°2gas inflow

--..........~

/ .reservoir.bag

Water.scamster Heidbrink

valve

,.....;.~~~~ patientI . ~normal gas In, low

tube closed

Fig. 3. Sample circuit for providing airway warming from an anaesthetic machine. The patient maybe connected by face mask or endotracheal tube as appropriate, and ventilation assisted if required.Instructions for use: the carbon dioxide is allowed in at a flow rate of 3 to 5 I min- I with oxygen at0.5 to II min-I. Once the heat in the Waters canister has reached the desired level, the carbon diox­ide is discontinued and the oxygen only continued.

335Airway warming equipment: a review

Utilizing a chemical reaction to produce heat and moisture

CARBON DIOXIDE/SODA LIME

The absorption of CO2 by soda lime is a chemical process of neutralization, producingheat and moisture. If a person breathes through a container of soda lime, the inspired airwill be heated and humidified. The temperature of the soda lime and the amount ofmoisture produced depend on the quantity of CO2 absorbed, rate of CO2 administration,environmental temperature and the thermal properties of the container in which thereaction takes place. Production of CO2 by an individual varies with metabolic rate. Atnormothermia, only enough CO2 is produced to raise the temperature of the soda limeslowly. If the system is being used to prevent someone from becoming hypothermic in acold stress situation, a slow rise in temperature of the soda lime is not disadvantageous.However, less CO2 is produced by a hypothermic person. The urgency of treatment oftenrequires that the temperature of the soda lime should reach therapeutic levels as soon aspossible. Care must be taken, because the soda lime temperature can reach 1030 C, withthe outflow gas transformed to steam at 990 C [1]. In practice, metered quantities of CO2released into the soda lime allow control of the temperature inside the soda limecontainer.

The system can be used either with an open, or non-return, circuit in which all exhaledgases are vented to the atmosphere, or with a closed circuit which recycles the exhaledgases.

6045

--- --- --- ---

T~mp~at~ of gas I~aving ~ters ccnst~r.

T~mperatur~ of inspif?d gas lIlder normal conditions.T~~ratur~ 01 ifl¥Jir~d gas IJfIder simulat~dhypoth~rmic ca'lditions.

15

"'----,,-----".' ---, ---

,/ - - -/

/'

60

55

50 I.... I

1-J I'- I

~ 45 I Ii2 I .

/

~ 1 I.l.u 40 1 •

~ ,.I~ 1/

35 ~I

(,30 t'

I'1I.

I I25

0 30

MINUTES

Fig. 4. Temperature graphs of gas temperatures achieved with original 'Lloyd' prototype. Sincethese figures were obtained using normothermic test subjects they cannot be directly transferred tothe hypothermic situation.

600 900 1200

RESPIRATIONS (mechanical)

Mean temp ambient+ 40'c for 2hrs

x x30sec 30sec

C02 CHARGE TIMES

Lloyd

Further, similarfigures available for

x full 2 hr test30sec

Approx breathing time,not elapsed time.Actual resp. rate 15'25/min

60 minutes

N.B. Increase in rate of heatingby C02 charges as timeprogresses

40 minutes

x30 sec

300

x30sce

20 minutes

336

TEMP.70c

60c

x60 sec

sac

40e

30e

20ca 60

Fig. 5. Graph showing gas delivered by 'Reviva' with increasing temperature (from P. Bell, personalcommunication)

In the closed circuit as much as possible of the heat and moisture produced by theequipment and the patient should be conserved, to minimize the weight of the equipment(Fig. 3). Tests showed that a single initial charge of CO2 could produce heat andmoisture with an inspired temperature above 45° C for 1 h (Fig. 4). In clinical use,however, the gas temperature dropped faster; after 1 h the temperature fell below 3r c.Nevertheless, additional CO2 added at intervals maintained the desired gas temperaturefor at least 8 h. For portable equipment, oxygen must be supplied from a cylinder. TheCO2 can be provided by using small soda syphon sparklets, each with a content of 5litres.

Use of the open circuit eliminates the weight of the oxygen cylinder and demandvalve, but necessitates use of a larger quantity of soda lime. Because the exhaled gases arevented to the atmosphere, the soda lime cools so rapidly that it requires recharging withadditional CO2 at frequent intervals (Fig. 5). Therefore, the non-return system is onlypractical with use of a CO2 cylinder.

The chemical reaction produces an initial surge of heat and moisture, which lasts 6-10min and then gradually declines. With the non-return system, the heat is rapidly trans­ported to the face mask, where the temperature can reach unacceptable levels with riskfor burning. One researcher twice sustained painful burns to the mouth from wet airissuing at 60-70° C [6]. This problem is not present with equipment of the closed circuittype, since CO2 is added at the distal end of the canister. When the patient breathes, theheat surge is absorbed by the proximal soda lime, heating the entire canister. In addition,the to-and-fro configuration allows heat in the soda lime to be transmitted more slowly tothe patient, which allows the heat surge to ebb before reaching the patient.

Airway warming equipment: a review 337

Fig. 6. Diagrammatic representation of the original 'Lloyd' prototype portable airway warmingequipment. Diagrammatic representation of portable rewarming apparatus. A Oxygen cylinder. BDemand reducing valve. C Corkette (Sparklets Corkmaster) - with the distal portion of the needleremoved and connected to the gas inflow limb of E. The oxygen supply is connected to the normaloutflow part of the assembly. D 6 litre reservoir bag. E Normal Waters valve assembly with theHeidebrink valve removed and the reservoir bag connected. The arrangement is necessary to trans­mit the inspiratory negative pressure to the demand valve B. F Soda lime. G Waters canister. HInsulation - expanded polystyrene in the box and air in the delivery tube. J Face mask or mouthpiece and nose clip as used by mine rescue or skin divers. K Double lumen delivery tube. The innerlumen is the respiratory tube while the outer is closed off and acts as insulation. The length of thetube is not critical since any build-up of CO2 due to the dead space would be beneficial in protectingthe heart from the effects of hypothermia. L Rubber tubing to give a flexible mounting for theCorkette C. Instructions for use: The oxygen cylinder (A) is opened and the face mask (J) ormouth piece strapped to the patient's face. The patient is allowed to breathe for 2 minutes till thereservoir bag (D) starts to fill (this is to ensure that an air-tight seal has been achieved round themask and to flush the circuit with oxygen). The corkette (C) is then activated till the sparklet cylin­der of CO2 is empty. After 5 minutes the corkette is reloaded with a fresh sparklet cylinder, andagain activated till empty. In severe conditions a third sparklet is required to reach adequate temper­atures. The apparatus is arranged with the canister (G) below the level of the face mask (J). There­after the only attention required is to ensure that the air-tight seal is maintained between the faceand the mask and check that the oxygen cylinder does not run out. After each hour a further spark­let cylinder should be activated to maintain the heat level of the canister. The standard Waters canis­ter used has a safe endurance of 6 hours if used as above. The filling of the Waters canister withsoda-lime should follow accepted anaesthetic practice, i.e. to maintain an even pressure on thegranules an ordinary nylon pot scourer is inserted before the end of the canister is screwed back intoplace. The soda lime should be replaced each time the apparatus is used.

The resistance to respiration caused by breathing through soda lime is affected by tidalvolume and respiratory rate [21]. In severe hypothermia, the respiratory rate and tidalvolume are both reduced, and respiratory resistance is likely to be low.

On the first successful ascent of Everest in 1953 [22], oxygen was carried for climbingat high altitudes. An open circuit or totally closed circuit configuration was available. Itwas noted that use of closed circuit oxygen supported a better climbing performancebecause of the heat and moisture generated [23]. However, the climbers became too hot,which led to the conclusion that open circuit oxygen was the better system [22].

338 Lloyd

Fig. 7. Lloyd airway warming equipment on Everest in 1975 with Khumbu icefall in the background

CLOSED CIRCUIT

'Lloyd' equipment

Large version. This configuration (Fig. 6) [24] weighs about 3 kg (7 lb). In the initialprototype, the large oxygen cylinder plus demand valve (Entonox with the pin indexoverridden by washers to allow use with an oxygen cylinder) added another 5 kg (12 lb).The first prototype was used to treat patients in hospital [1,2,24] and successfully to treata case in the field [25]. The design using a standard size of Waters canister produced therequired heat and humidity for periods of up to 8 h, but needed recharging with CO2 atintervals.

The thread on the corkmaster made changing the sparklet cylinders difficult even in awarm room. Since multiple sparklets were needed to heat the soda lime, it was not practi­cal for field use in cold situations with gloved hands. The method of activating the spark­let cylinders was therefore changed, with the sparklet loaded like a rifle cartridge intothe breach of a gun, and then depressed onto the needle by a winged bolt. This made themanoeuvre easier and manageable with gloved hands. This design was carried by thesuccessful British Everest Expedition of 1975 [26], since they were already carrying

Airway warming equipment: a review 339

Fig. 8. Light-weight 'Lloyd' portable airway warming equipment. (A) Oxygen cylinder.(B) First stage reducing valve and gauge. (C) Demand reducing valve with manual overrideto allow reservoir bag D to be filled if ventilation requires to be assisted. (D) Reservoir bag(2 1). (E) Corkette (Sparklets corkmaster), with the distal portion of the needle removedand inserted into the tail of the reservoir bag, spare sparklet alongside. (F) Soda lime. (G) PaediatricWaters canister. (H) Insulation, neoprene foam tubing. (J) Face mask. (K) Thermometer registeringmean air temperature at mask inflow. (L) Adaptor for refilling small oxygen cylinder from a largecylinder. Instructions for use: empty one sparklet cylinder into system by using the Corkette (E).Open the valve (B) on the oxygen cylinder (A). Apply the face mask (J) to the patient. If desiredthe reservoir bag (D) can be partially inflated by depressing the centre of the demand valve (C).Thereafter the system will work on demand. The thermometer (K) should be observed. This will risesteadily into the working temperature range and when the temperature again drops to 35'C, place anew sparklet cylinder in the corkette (E) and by depressing the lever allow CO2 to flow for 3 s. Thismay be repeated whenever the thermometer temperature drops to 35'C until the second sparkletcylinder is empty. Where possible the Waters canister (G) should be vertical rather than horizontalto reduce the risk of the gases channelling along the side of the canister. The gauge on the oxygencylinder should be checked regularly to ensure that it is not empty. After use the oxygen cylindershould be refilled from a large cylinder using the adaptor (L). The soda lime (F) should be replacedin the Waters canister (G) ensuring, by tapping and shaking, that the canister is completely full andthat no channelling of gases can occur along the side when the canister is not vertical.

340

\\

\

IF

B

Lloyd

--K--

Fig. 9. 'Reviva'. A. Air inlet which when removed allows soda lime container to be emptied andfilled. B. Key for the cylinder to allow a timed release of CO2, C. CO2 cylinder. D. Thermometer,measuring the temperature of the gas as it leaves the soda lime. E. Non-return valve. F. Boxcontaining 1.5 litres of soda lime. G. Mask. H. Breathing tube - uninsulated. K. CO2 delivery tube.L. Tubes for collecting warmed air.

F

Fig. 10. Guild deviQe. A. Air inlet. BV. Button valve for regulating the temperature of the gas as itreaches the patient. C. CO2 inlet. D. Reducing valve for the CO2 with a preset fixed flow of 15 Imin-I. E. CO2 delivery tube. F. COx cylinder. H. Heidbrink valve. M. Mask with thermometer. N.Neoprene insulation. R. Two ring system for CO2 discharge.

Airway warming equipment: a review 341

F

E

5

B

o

\c

Fig. 11. 'Little Dragon'. A. Air inlet. B. Breathing tube perforated at the distal end. C. CO2 inlet. D.Polyvinyl canister. E. end for filling canister with soda lime. F. Orifices closed by corks during trans­port. G. Opening covered by rescuer's finger and used to regulate the temperature of the inhaledgas. M. Mask. S. Soda lime. T. Thermometer.

oxygen with a demand supply system. Though it was not needed for treatment, it wastested at an altitude of 6100 m (20000 ft) (Fig. 7) and found to work as predicted.

Small version. The present equipment (Fig. 8) weighs 3 kg (7 lb) when ready to use,and utilizes the Waters canister designed for paediatric use [27]. This requires one spark­let cylinder to attain the required temperature; additional sparklets released in stages atintervals can maintain the temperature of the inspired gas in the recommended range of37-45° C for over 2 h. The smaller cylinder of oxygen contains sufficient oxygen for over2 h. Since respirations are depressed during hypothermia, the inspiratory effort of ahypothermic patient may be insufficient to operate the demand valve, even at its mini­mum setting. Therefore, even though the manual override of the demand valve enablesthe reservoir bag to be filled and ventilation to be assisted if necessary, it may be safer,lighter and cheaper to eliminate the demand valve and rely on a simple push button-typevalve, which would be activated to fill the reservoir bag as necessary.

OPEN CIRCUIT

Reviva

This (Fig. 9) was the first practical design. It weighs 5.3 kg (11.75 lb) and was used bythe designer in cases of hypothermia occurring in the Lake District hills. Oxygen can beadded through the circuit. Addition of an 'Ambu' bag allows ventilation to be assisted.To reduce the effects of heat surge, the delivery tube between the box and face mask is

342 Lloyd

not insulated. When tested in the field at 2000 m altitude with an air temperature of 0° C[20], the Reviva produced gas at 42-43° C and 100% relative humidity (measured byhygrometer) within 2 min. However, within 15 min the delivered air temperaturedropped to 25° C and a further CO2 charge was needed. In the version tested, themaximum duration of use was 40-45 min from the 1.5 litre of soda lime in the machine[20]. Unfortunately, since the thermometer only measures the temperature of the gasleaving the soda lime, it does not register until a person starts to breathe through theequipment. This led one group to press the CO2 release valve repeatedly, and blow steamfrom the air inlet orifice, before this flaw of the design was appreciated. This model is stillin use in the hills of the English Lake District, but is no longer in production.

'Guild' equipment

This design (Fig. 10) uses a double CO2 inlet to produce more efficient distribution ofheat in the large soda lime mass [6]. It weighs 4-5 kg (9-111b), but has never been takenbeyond the prototype stage.

Little Dragon

This device (Fig. 11) weighs 2.6 kg (5.75 Ib) when fully charged and ready for use. It isintended that the Little Dragon be placed inside the victim's sleeping bag for use. It canbe used with IPPV and with supplemented oxygen. The Little Dragon has been adoptedfor use in cave rescue work [28], because it fits into a standard container used by therescue teams.

Others

The CO2 soda lime reaction has been incorporated in a number of devices tested to aidthe survival of divers in a 'bell' which has lost its heat supply from the mother ship[29,30].

HYDROGEN COMBUSTION

Heat and moisture is produced by the reaction of oxygen with hydrogen. The reaction isassisted by a catalyst (a pellet of scintered porous alumina plated with platinum, as usedin smog control devices in cars) inserted into the gas stream just before the mouthpiece.For each 0.1 % hydrogen added to the breathing mixture, the temperature of the inspiredair will rise by approximately SO C, and 11°C if with a helium atmosphere. This assumescomplete combustion and no heat loss. Temperature increase is solely a function of thepercentage of hydrogen in the mixture, and is independent of the ambient pressure. The0.1 % hydrogen will add 4.3% relative humidity at 21°C at sea level. This change inrelative humidity is increased with lower temperatures and higher pressures.

Adapted for SCUBA use with 1% hydrogen (a quarter of the explosive percentage), itgives at 10 m (33 ft) depth of water (2 Atmospheres Absolute = 2 ATA), an inspiredtemperature of 49° C, i.e., a rise of 35° C above 10° C water temperature, with a relativehumidity of 17% at body temperature. Each 10 m (33 ft-l ATA) of incremental waterdepth increased the relative humidity by an additional 17%, until saturation is reached ata depth pf about 43 m (140 ft 5.5 ATA). with 0.5% hydrogen added to a heliumbreathing mixture, the temperature rise of the gas is 56° C. At a depth of 183 m (600 ft)(200 ATA) the inspired gas would be supersaturated at body temperature.

Each 0.1% hydrogen consumes 0.05% oxygen. In SCUBA use and in mountain

Airway warming equipment: a review 343

rescue, this would mean that the added 1% hydrogen consumes 0.5% oxygen, which is anegligible proportion of the 21 % oxygen present in compressed air. On the other hand,in deep diving using heliox, the situation is critical because with 0.5% added hydrogen,oxygen consumption would be 0.25%. To avoid oxygen toxicity, the safe inspired oxygenpercentage at 183 m (600 ft) is only 1 or 2%.

Though the risk of explosion with the use of hydrogen is only present during prepar­ation of the gas mixture, only persons with regular underwater work are likely to haveaccess to firms capable of producing the required mixtures.

This system with 1% hydrogen in air is being promoted for mountain rescue withdisposable cylinders [20]. When tested, the gas was delivered at 40° C and humidified,and one cylinder lasted 2 h. However, though the catalytic effect was immediate [20], ittook minutes to provide warm air to the patient; the humidity level at normal atmos­pheric pressure was doubtful, and would be lower at increased altitude. Since the systemuses a non-rebreathing circuit, it will use a large volume of gas, and even lightweightcylinders are heavy (about 6.8 kg 15 lbs) and bulky. It is therefore improbable that anycylinders are heavy (about 6.8 kg (15 lbs) and bulky. It is therefore improbable that anysurface rescue organisation would go to the expense and complicated storage facilities

Conserving heat loss

Boundary layer breathing

In the emergency hypothermia situation, the air is likely to be cold and, even though thenecessary to use this method. The value of this equipment is likely to be limited to under­water use.

Table 1. Results from a pilot study investigating mistakes made by operatives in a cold room. Partof the study was to try to identify safety equipment which should be kept in the cold room. Thesubjects climbed into an arctic type sleeping bag in the cold room when they became too miserableto continue with the experiment.

Subject T1 Time Temp T2 T3

1 33.1 15 -34 28.9 31.32 34.2 30 -31 28.3 29.63 33.4 30 -29.5 21.1 26.54 35.9 45 -32.6 28.6 30.15 33.6 60 -32.1 27.7 29.8

Tl = Abdominal wall temperature before entering the cold room.Temp = Air temperature of the cold room (C). There was an additional fan-induced windchillfactor.T2 = Abdominal wall temperature after a period in the cold room.T3 = Abdominal wall temperature after 15 min in sleeping bag.Time = Period in cold room before T2 was read. The variation was due to differences in the amountof protective clothing worn.

Note that the abdominal wall temperature remained high even in a very severe cold stress situ­ation, and rose when additional protection was provided.

344 Lloyd

ambient relative humidity may be high, relative humidity will be very low by the time theair reaches body temperature. The boundary layer (the layer of air under clothing next tothe skin) is warmer than ambient air (Table 1). It has increased humidity, from normalinsensible perspiration and possibly also from sweating, with the additional possibilitythat clothes are wet, which may have contributed to hypothermia. Breathing theboundary layer air would provide air which is warmer and more humid than the ambientair.

The concept of using the boundary layer for breathing to prewarm inspired air hasbeen utilized, and has enabled post-coronary rehabilitation patients to continue joggingeven in the very cold conditions of a Canadian winter [31].

Heat and moisture exchangers

Heat and moisture exchanges (HMEs) produce worthwhile conservation of moisture loss forthe respiratory tract, especially when the normal moisture conserving system of the naso­pharynx has been bypassed by tracheostomy or endotracheal intubation [32]. Perform­ance of different HMEs has been reviewed [33-39]. Heat and moisture exchanges takethree forms [40]:

(1) Condenser humidifiers (CH) condense water vapour in exhaled breath on thematerial in the CH and the relatively dry inspired gas picks up the moisture duringpassage through the CH. Since the CH usually has a high thermal conductivity,the temperature of the device remains relatively steady. Though condensor humidi­fiers improve heat and moisture conservation whatever the gas inhaled, theirefficiency is greatest when inhaled gas is humidified [37].

In one design, stainless steel mesh attached to a mask was used as a CH (Fig.12), a system which did not suffer from freezing [41].

Another design [27] utilized a Portex Humid-vent CH (Fig. 13), which would

1. The mask. (A)The metal wire nets(B)Rubber mask

Fig. 12. 'Soininen' mask

40 37

20 24

U0.

~

I--20

-40-47

-60 -60

Inside f4- The nets --+I Outside

2. Cross-section of the cartridge

A irway warming equipment: a review 345

Fig. 13. Respiratory insulator - exploded view. A. Mask. B. Portex condenser humidifier. C.Tubing. D. Neoprene insulating tubing

conserve about 50% of respiratory moisture loss [39]. As well as being the mostefficient of the three designs tested, it was the lightest, cheapest, smallest, and hadthe least respiratory resistance [39]. The equipment weighed 100 gm (3.5 oz), andutilised boundary layer breathing to prehumidify the inspired gas (Fig. 14).

(2) Hydrophobic condenser humidifier filters (HCHF) act like a super CH. Theelements warm quite markedly during the condensation phase in expiration, andcool as the moisture is evaporated during inspiration. The temperature changesimprove the efficiency of the device. The hydrophobic design means that no wateris absorbed or trapped in the device, but water is kept available. The only currentexample of HCMF is the Pall Ultipor BB50 (Fig. 15).

(3) Hygroscopic condenser humidifiers (HCH) are like CH with a hygroscopicmaterial impregnated into the condenser elements. The condenser component actslike a CH, but the hygroscopic element traps additional moisture in a loose bondwith the impregnated chemical. This moisture is released during passage of thedrier inspiratory gases, but the chemical release occurs without causing a drop intemperature. Examples of HCHs are Siemens-Elema servohumidifier, EngstromEdith and Cory Vent-aid (Fig. 15).

346 Lloyd

Fig. 14. Respiratory insulator in use. Note: the end of the tubing should be under the clothing andthe insulation should cover the condenser, humidifier and mask

In an assessment of current models the hygroscopic and hydrophobic designs were allmore efficient at conserving moisture and had a lower respiratory resistance than didearlier designs [38]. Though the hygroscopic humidifiers are undoubtedly efficient andthermally safe [42,43], fragments from the hygroscopic sponge of the Siemens-Elmaservo humidifier may break off and be inhaled [44]. The Engstrom Edith is less bulky,cheaper, and lighter than any of the other designs when a catheter mount is included[38].

Designs using the Edith (Fig. 16) and covered by a scarf or normal clothing would be ofvalue as a hypothermia preventive/early treatment for mountain users.

The Pall Ultipor, placed between the face mask and the escape valve in the Pneupacresuscitator, has been incorporated into the equipment used by the Search and Rescuehelicopters at Leuchars in Scotland.

In an emergency, even a scarf around the face or a polo necked sweater can act as acondenser humidifier [45], The Eskimos used tubes of wolf skin with the fur rolled in asrespiratory heat conservers.

Airway warming equipment: a review 347

Fig. 15. A selection of condenser humidifiers. A. Siemens-Elema servo humidifier. B. Pall UltiparBB50. C. Portex Humid - vent D. Engstrom Edith. E. Cory Vent - aid

Fig. 16. Engstrom Edith in using configuration

348 Lloyd

Expired air resuscitation

Expired air resuscitation provides warmed and humidified respiratory gas, and may bethe simplest and most portable method of supplying AW. It certainly has a long historicalbackground for this purpose [46]. Bristow et al. [47] quote an account of the resuscit­ation of a seaman who fell into the water near Spitzbergen in 1807. The seaman wasapparently dead but, after all the current conventional measures had failed to revive him,mouth to mouth ventilation was tried as a last resort and the seaman recovered. The CO2in the expired air might be of some additional benefit [48].

Summary

There are many methods of providing warm humidified gas for a hypothermic victim.Heat and moisture exchangers, though less efficient than active humidifiers, provideequipment which is cheap, light and simple. It should become part of the first aid kitcarried by climbers and other outdoor enthusiasts. The other devices which provide addi­tional heat and moisture are more suitable for rescue services, though the choice ofdesign will depend on the requirements and constraints faced by each individual team.

References

1. Lloyd, E.L. Accidental hypothermia treated by central rewarming via the airway. Br J Anaesth1973; 45, 41-8.

2. Lloyd, E.L. Hypothermia and Cold Stress. London: Croom Helm, 1986.3. Lloyd, E.L. Airway warming in the treatment of accidental hypothermia: a review. J. Wilder­

ness Med 1990; 1, 65-78.4. Miller, J.W., Danzl, D.F. and Thomas, D.M. Urban accidental hypothermia: 135 cases. Ann

Emerg Med 1980; 9,456-61.5. Lloyd, E.L. and MaCrae, W.R. Respiratory tract damage in bums, case reports and a review of

the literature. Br J Anaesth 1971; 43, 365-79.6. Guild, W.J. Rewarming via the airway for hypothermia in the field? J Roy Nav Med Ser 1978;

64,186-93.7. Hayward, J.S. and Steinman, A.M. Accidental hypothermia: an experimental study of inhala­

tion rewarming. Aviat Space Environ Med 1975; 46, 1236-40.8. Marcus, P. Laboratory comparison of techniques for rewarming hypothermic calualties. A viat

Space Environ Med 1978; 49, 692-7.9. Shanks, C.A and Marsh, H.M. Simple core rewarming in accidental hypothermia. Br J

Anaesth 1973; 45, 522-5.10. Chamney, AR. Humidification requirements and techniques. Anaesthesia 1969; 24, 602-16.11. Health Equipment Information. Evaluation of humidifiers for medical use: heated humidifiers.

Health Equipment Information No. 151. London: HMSO, 1986.12. Shanks, C.A Humidifiers and humidification. Br J Clin Equip 1977; 2, 21-5.13. Tontoschev, G., Lueder, M. and Benson, C. The efficiency of various humidifiers for respired

gas. Resuscitation 1980; 8, 167-79.14. Tamow-Mordi, W.O., fletcher, M., Sutton, P. and Wilkinson, A.R. Evidence of inadequate

humidification of inspired gas during artificial ventilation of newborn babies in the British isles.Lancet 1986; ii, 909-10.

15. Coniam, S.W. Accidental hypothermia. Anaesthesia 1979; 34, 250-6.16. Ledingham, I. McA and Mone, J.G. Treatment of accidental hypothermia: a prospective clini­

cal study. Br Med 11980; 1, 1102-5.

Airway warming equipment: a review 349

17. Shanks, e.A Humidification and loss of body heat during anaesthesia. 2. Effects on surgicalpatients. Br J. Anaesth; 1974; 46, 863-6.

18. Shanks, e.A Heat balance during surgery involving body cavities. Anaesth Intensive Care1975; 3, 114-17.

19. Todd, N.V. and Conn, A Near drowning and hypothermia mimicking severe closed headinjury. Br Med J 1986; 2, 594-5.

20. Legrand, V. Les rechauffeurs d'air. These pour Ie Doctorat en medicine. Lille, 1984.21. Adriani, J. and Rovenstine, E.A Experimental studies on carbon dioxide absorbers for

anesthesia. Anesthesiol. 1941; 2,1-19.22. Hunt, J. The Ascent of Everest. London: Companion Book Club, 1954.23. Bourdillon, T.D. Oxygen equipment. In: Hunt, J., The Ascent of Everest. Appx V. London:

The Companion Book Club, 1954.24. Lloyd, E.L., Conliffe, N., Orgel, H. and Walker, P. Accidental hypothermia: an apparatus for

central rewarming as a first-aid measure. Scot Med J 1972; 17, 83-91.25. Lloyd, E.L. and Frankland, J.C. Accidental hypothermia treated by central rewarming in the

field. Br Med 11974; 4; 717.26. Bonington, e. Everest the Hard Way. Appx. 10. Medicine. e.RA Clarke. London: Arrow

Books, 1977: 337.27. Lloyd, E.L. and Croxton, D. Equipment for the provision of airway warming (insulation) in the

treatment of accidental hypothermia. Resuscitation 1981; 9, 61-5.28. Frankland, J.e. Hypothermia in cavers. Trans Br Cav Res Ass 1984; 11, 154-9.29. Tonjum, S., Hamilton, RW., Brubakk, AO., Petersen, RE. and Youngblood, D.A Project

Polar Bear. Testing of Diver Thermal Protection in a Simulated "Lost Bell". Report 2-80Bergen: Norwegian Underwater Institute, 1980.

30. Lloyd, E.L. Diving and hypothermia. Br Med J 1979; 2, 668.31. Kavanagh, T. Heart Attack? Counter attack! Toronto: Van Nostrand Reinhold, 1976.32. Tsuda, T., Noguchi, H., Takumi, Y. and Aochi, O. Optimum humidification of air adminis­

tered to a tracheostomy in dogs. Scanning electron microscopy and surfactant studies. Br JAnaesth 1977; 49965-78.

33. Bethune, D.W. Hydrophobic versus hyproscopic heat and moisture exchangers. Anaesthesia1985; 40, 210-11.

34. Gedeon, A and Mebius, e. The hygroscopic condenser humidifier. Anaesthesia 1979; 34,1043-7.

35. Leigh, J.M. and White, M.G. A new condenser humidifier. Anaesthesia 1984; 39, 492-3.36. Ravenas, B. and Lindholm, C.E. The foam nose - a new disposable heat and moisture

exchanger. A comparison with other similar devices. Acta Anaesth Scand 1979; 23, 34-9.37. Shanks, C.A Clinical anaesthesia and the multiple-gauze condenser humidifier. Br J Anaesth

1974; 46, 773-7.38. Shelly, M., Bethune, D.W. and Latimer, RD. A comparison of five heat and moisture

exchangers. Anaesthesia 1986; 41, 527-32.39. Walker, AK.Y. and Bethune, D.W. A comparative study of condenser humidifiers. Anaesthe­

sia 1976; 31, 1086-93.40. Lloyd, G. Heat and moisture exchangers: principles of operation. Care of the Critically III

1987; 3,110-13.41. Soininen, L. and Lunden, J. A mask saving warmth of breathing air. An appliance for cold

climate. In: Harvald, B. and Hansen, J.P., eds Circumpolar Health 81. Copenhagen: NordicCouncil Arct. Med. Res., 1982: 576-7.

42. Gilston, A Hygroscopic condenser humidifiers: value for money? Anaesthesia 1984; 39,1030-1.

43. Smith, H.S.; Allen, R. Another hazard of heated water humidifiers. Anaesthesia 1986; 41,215-6.

350 Lloyd

44. James, P.O. and Gothard, J.W.W. Possible hazard from the inserts of condenser humidifiers.Anaesthesia 1984; 39, 70.

45. Guest, D. My night in a crevasse. Weekend Scotsman 1977, March 5, 1.46. Whitby, J.D. Treatment after exposure to cold. Lancet 1972; i, 377-8.47. Bristow, G., Smith, R., Lee, J., Auty, A. and Tweed, W.A. Cardiopulmonary arrest during

hypothermia due to exhaustion and exposure. Can MedAss J 1977; 117, 247-9.48. Morrison, T.B., Conn, M.L. and Hayward, J.S. Thermal increment provided by inhalation

rewarming from hypothermia. J Appl Physiol1979; 46,1061-5.

Appendix: Mannfacturers

Heating water

Hospital HumidifierThese can be obtained through local agents who supply hospitals.

Thermal ParachuteDr J. Foray, Hopital de Chamonix Mt Blanc, Chamonix, Haut Savoie, France.

Uvic Heat-TreatAlert Manufacturing, 1475 East Gorgia Street, Vancouver, BC, Canada V52 2A9.

CO2 Soda lime

Lloyd.The equipment can be self assembled from local components or the author orMr D. Croxton, Oxylitre (Edinburgh) Ltd., Riverside Works, Miller Row, Edinburgh,EH4 3RQ, Scotland.

RevivaPeter Bell, Engineering, The Slack, Ambleside, Cumbria, England.

Little DragonMike Mitchell, Heating Maintenance Ltd., School Lane Stavely, or Kendal, Cumbria,England.

Hydrogen combustion

Applinc Saving Breath Hypothermia System.Applinc, 125 West Union Street, Medina, Ohio 44256, USA.Ernesto Aviles, 7677 Ronson Road, San Diego, California, USA.

Heat and moisture exchangers

These should be obtained through local hospital suppliers.