Thermal Stress

Threshold of dehydration at which physical and mental functions deteriorate

2% of body mass (~3% TBW) - recommended that fighter pilots not become dehydrated more than 1%

How efficient is metabolic work

About 25%, the remainder released as heat

Acclimatization vs thermal tolerance

Acclimatization reduces physiologic strain due to heat Thermal tolerance improves tissue resistance injury to a given heat strain Both depend upon intensity, duration, frequency, and number of heat exposures; serve to increase heat loss, reduce CV strain, improve fluid balance, reduce M

What factors influence heat exchange at the skin?

Air temperature Air humidity Wind speed Clothing Radiation (from/to ground)

Behavioral thermoregulation may include:

Altering physical activity Clothing selection Adjusting thermostats Seeking shade/sun/shelter

How can microclimate cooling mitigate hot cockpit environment?

At a typical metabolic rate of 100-250 W, uncooled work time in a hot dry cockpit of 70 min can be increased to 300 min by applying 100-150 W of microclimate cooling

At what levels does physical performance degrade in the cold?

At skin temperature of 20C, initial degradation in manual performance At 15C an abrupt performance degradation again occurs At 4C another drop in performance along with tactile sensitivity impairment occurs

Describe heat syncope

Minor heat illness Temporary circulatory failure due to pooling of blood in peripheral veins impeding diastolic filling (prolonged standing at attention) Immediate recovery of consciousness, but BP/HR may take hrs to completely recover

Factors that increase risk of serious heat illness

Not acclimatized Low fitness Dehydration High body fat

WBGT, outdoor and indoor, cockpit < 3000 feet

Outdoor WBGT = 0.7 Twb + 0.2 Tbg + 0.1 Tdb Indoor WBGT = 0.7 Twb + 0.3 Tbg WBGTgr = (WBGTcp - 0.333) / 1.183

Physiologic responses to cold exposure

Peripheral vasoconstriction Shivering Cold-induced diuresis (CID), self-limiting Cold-induced vasodilation (CIVD), transient or periodic

What does operational effectiveness in any environment depend on?

Proper assessment of the environmental threat Identification of increased susceptibility Implementation of controls Heat/cold injury recognition, mitigation and first aid

Importance of each heat exchange mechanism

R 60% loss E 22% loss K/C (air) 15% loss K (objects) 3% loss

Predominant mechanism of heat loss at low temperatures

Radiation and convection

Describe heat cramps

Minor heat illness Pain, spasms of abdomen/extremities esp with activity More common in ppl who sweat a lot (lose lytes)

What is the primary means of moving heat from the core to the skin?

Blood flow, a means of convection

3 reasons not to give alcohol in hypothermia

Blunts shivering response Impairs gluconeogenesis Causes peripheral vasodilation

FITS procedures, caution and danger zones

Caution (32-38C): Encourage hydration before and in flight Alert to heat stress symptoms Avoid exercise 4 hrs before takeoff Precook cockpits Assign alternate crew to preflight Keep sun out of transparencies Transport crew directly to AC Limit in-cockpit standby Dander (>38C): Cancel low altitude missions Keep sun out of transparencies (roofs, covers) Allow only 1 change of AC due to mechanical delay Optimize conditions for cooling, rehydration between sorties Support self-assessment and stand down for safety

Combined effects of heat stress and dehydration

Cognitive function altered Physical stamina reduced Reaction time slowed Error rate increased Cockpit management impaired Heat injury/illness risk increased

Effects of cold exposure

Cognitive function degraded Physical performance degraded Manual dexterity reduced Risk of injury increased

Multilayered protective ensemble in fighters may include:

Cotton underwear Fire-retardant coveralls AGS Parachute harness Boots Gloves Helmet +/- chemical defense layer

How and when does peripheral vasoconstriction begin?

Decrease blood flow to the shell (skin, SQ fat, skeletal muscles) effectively increasing insulation and reducing convection Starting at 33C (95F)

Hypothermia in water as warm as 70-75C depends upon:

Duration of exposure, depth of immersion, water temperature, body composition

Describe hypothermia and its tx

Early: slurred speech, loss of coordination (looks intoxicated) Then shivering accompanies rapid Tc fall (opportunity to rewarm) Remove wet clothes, insulate/protect; actively heat externally if no longer shivering

What is WCT index used for, and under what assumptions?

Estimate time to freezing of exposed facial skin while walking 3 MPH (1.3 m/s), taking into account wind speed and air temperature In wet conditions Ta used should be 10C lower

What can increase whole body metabolism?

Exercise, increasing M 15-20 times Shivering, increasing M 3-5 times

FITS eqn

Fighter Index of Thermal Stress FITS = 0.83 Twb + 0.35 Ta + 5.08 assuming Tbg is higher than Ta by 10C (clear skies)

Describe frostbite and tx

Freezing cold injury of exposed skin, even hands/feet Wooden sensation, significant pain upon rewarding Skin red, then waxy white Rapid rewarming (40C bath) unless refreezing likely

What is HELP? What other means to survive cold water?

Heat Escape Lessening Position Huddling Limit movement and surface area

HEM

Heat and moisture exchange module Minimize airway cooling by keeping inspired air warm and humid No value in reducing respiratory heat or moisture loss during cold exposure

What level of exposure will heat acclimatization require and how long will it last?

In the specific climate and activity level anticipated, it takes about 2 weeks of daily 100 min/d of progressive heat exposure and physical work Usually retained for a week, and 75% of the adaptation is lost by 3 weeks

Space suits and their purposes

LES, launch entry suit (polypropylene underwear, AGS, 2-layer nylon exterior, helmet, LCG) ACES, advanced crew escape suit They offer protection from changes in cabin temp, cold water immersion, pressure changes, ambient gas changes

How to optimize cold weather performance of skills

Learn the skill under ideal conditions Train under cold stress and while wearing cold gear

Heat storage eqn

S = M +/- W - E +/- R +/- C +/- K

Describe heat exhaustion

Serious heat illness CO unable to sustain skin blood flow for thermoregulation in addition to metabolic demands of exercise Syncope, altered mental status, HA, N/V/A, hypotension, tachycardia, cramps, hyperventilation

Describe heat stroke

Serious heat illness, emergency! Elevated Tc > 40C (104F) w/profound CNS dysfunction Delirium, convulsion, coma Transport to ED, actively (until 101F) and passively cool, keep skin moist/immersed

Physiologic thermoregulation may include:

Skin blood flow Sweating Metabolic heat production

No freezing cold injuries

Trench foot (0-15C) Chilblain (tender, itchy, painful even hrs after rewarming) Cold-induced bronchospasm (facial cooling, 25% athletes) Cold-induced asthma Cold urticaria (risk anaphylaxis) Snow blindness (wear sunglasses)

What are the components of the thermoregulatory proportional control system?

Vasodilation, sweating, skin blood flow that respond by changes to - Threshold (dependent upon set point) - Graded response (changes in sensitivity or slope) Modified by thermal factors (skin temperature) and nonthermal factors (dehydration, acclimatization, circadian rhythm, endocrine status)

Cold air vs cold water exposure

Water conducts heat 25 times better than air Cold air: localized cold injury, worse manual dexterity Cold water: hypothermia (Tc = 35C)