aus heatstroke lessons learnt
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Lessons Learnt from a Heat Stroke Death
BRIG Stephan Rudzki (Rtd)MBBS, Grad Dip Sport Sc, MPH, PhD , FACSEPConsultant Medical Advisor Directorate of Army Health
BackgroundSoldier died of heatstroke in Nov 2004 while engaging in strenuous physical activity during the Northern Territory wet season.Early signs of heatstroke not identified.Initial axillary temp recorded as 41.3oC (106o F).Treated with ice packs and evacuated by ambulance to Darwin.On arrival at Royal Darwin Hospital, 2 hours post collapse, rectal temperature of 41.2oC (106o F) recorded just prior to fatal respiratory arrest.
Lesson Areas identified Prevention
Prevention: AcclimatisationStudents had 6 weeks training in air-conditioned classrooms prior to the field phase.The course program did not allow for regular Physical Training.It was assumed that soldiers living in the tropics were acclimatised.Work-rest tables assume acclimatised soldiers.
To achieve acclimatisationIndividuals require ongoing exposure to:
At least 100 minutes per day of heat conditions similar to those in which they will work. The best and most effective acclimatisation is achieved with a single 100 minute exposure, but two 50 minute exposures will also be effective; and
graduated levels of increasing physical exertion.
Exercising in the morning to acclimatise for work in the heat of the day is a recipe for failed acclimatisation.
To sustain acclimatisationIndividuals require:
a minimum of three work sessions per week in the heat of the day. These sessions should last for at least 100 minutes in total. This work may be broken into two 50 minute sessions, and
the work intensity should reflect the type of activity to be undertaken. Conducting sessions at low intensity when work will be of high intensity will not achieve adequate sustainment.
Prevention: Lack of understanding the heat threatFailure to appreciate that heat (and body temperature) was the key hazard led to failure to manage the risk of injury effectively.
Staff believed that simply ensuring adequate hydration would effectively manage the risks associated with heat exposure.
Water was freely available and all soldiers were encouraged to drink frequently. Numerous briefings throughout the training reinforced this message.There was no understanding that you could become a heat casualty even if well hydrated.A full radiator can still boil !
Prevention: Lack of understanding the heat threat
DehydrationHumans are the only mammal to have salt in their sweat.
Thirst is driven by the concentration of sodium [Na+] in the plasma.
Animals just lose water and increase their plasma [Na+] concentration. They then drink to end their thirst, which occurs when their [Na+] concentration returns to normal.
Humans lose salt and water, so [Na+] can change little, despite the loss of significant fluid.
The thirst reflex in humans does not kick in until a person is 2-3% dehydrated.
If you just drink WATER to thirst you will always be around 2% dehydrated after exercise.
Extra water you drink will not stay in the plasma unless consumed with sufficient salt [Na+].
If you wish to avoid dehydration after activity You MUST replace your salt losses.
1942 - 2nd AIF Guidelines
Salt and SweatRoman solders were paid in salt, hence the term worth his salt.Individual sweat losses can vary by up to 10 fold Bigger individuals generate more heat and sweat more.You should only consume the water & salt you need to replace what you have lost.If you are urinating > 5 times/day you are well hydrated.Fixed hydration schedules are not recommended, and create the risk of hyponatremia (low blood salt).
Effect of Sodium Intake on Rehydration
>50 mmol/l salt intake restores Plasma VolumeWater alone does not restore Plasma Volume
Increased salt intake increases thirst. (thats why bars serve you chips & nuts).
Why does increased salt intake cause high blood pressure? because it leads to fluid retention and increased plasma volume.
Salt should come from your food, but many use sports drinks. This is a very expensive (and inadequate) way to obtain salt!
Prevention: Manage Risk of Heat Injury Under previous policy a WGBT >32C (89.6 F) was considered extreme heat stress.
Revised WBGT tables consider 32-36C (89.6 96.8 F) as high and >36C (96.8 F) extreme.
New work-rest cycles impose a requirement for standardised risk management when HSI are in the high or extreme range.
Work Rest Tables
Very Heavy Work
Standardised Risk managementQuality is defined as the absence of variation.We had significant variation in how the risk of heat was managed.The policy sought to defined the required level of risk management and place that requirement on CommandersThe next slide shows the required level of risk management.
Prevention: Education of Soldiers Soldiers taught to monitor the physical signs of dehydration and the symptoms of heat injury in themselves and others.DehydratedPee Clear Twice a day
Diagnostic ConfusionNo appreciation of the difference between self-limiting heat illnesses such as heat exhaustion and life threatening heat injuries such as heatstroke.
No appreciation of temperature difference between the axillary and rectal methods.
Heat ExhaustionHeat exhaustion is caused by strain on the cardiovascular system as the unacclimatised body tries to lose heat.
Main method of losing heat is an increase of blood flow to the skin (hot and flushed).
This reduces blood flow to the organs (core) resulting in low blood pressure, rapid pulse and fainting (exhaustion).
A properly conducted acclimatisation program will reduce the risk of heat exhaustion.
Core Temp almost never exceeds 40C (104 F) Pulse rate >100 / min is a measure of cardiovascular strain.
Heat StrokeHeat Stroke is a medical emergency defined byCore temp > 40.6C (105 F) andMental disturbanceMore complex issues at play , e.g. endotoxin. Persistent elevated core temperature leads to cellular damage (injury).
Core body temperature is always at least 1C higher than oral or axillary temperature.
Early Heatstroke symptoms:confusion, disorientation, aggressive or inappropriate behaviour
Late Heatstroke symptoms:coma and death.
Anyone with mental disturbance in a hot climate has Heatstroke until proven otherwise.
Heatstroke severity is a function of the actual core temperature and the duration the body stays at that temperature.
A core temperature above 42.2C (108 F) will kill within an hour if effective cooling is not undertaken.
Treatment The key to the treatment of heatstroke is to use the most rapid and effective means of cooling available.
TreatmentEffectiveness of different cooling methods was not understood. Use of ice packs applied to the groin, axilla and neck were utilised as standard practice despite the literature showing this is the slowest method of reducing core temperature.The key to the treatment of heatstroke is to use the most rapid and effective means of cooling available.
CoolingCooling occurs through:
Radiation (direct heat loss)
Convection (air flow over the skin)
Evaporation (of sweat )
High humidity reduces the efficiency and effectiveness of evaporative cooling significantly.
Clothing reduces cooling efficiency by restricting air flow over the skin and increasing the air humidity near the skin.
Keeping Cool: 30th Bde 2nd AIF New Guinea 1943
1942 - 2nd AIF Guidelines
1942 - 2nd AIF Guidelines
Cooling comparison from the Literature
Cooling Method Cooling RateIce Water Immersion 2-4 oC 0.16oC/minEvaporative Cooling 0.08-0.14oC/minChilled (4oC) NS 40mls/kg 0.083oC/minIce packs 0.027oC/minCold Air Blanket 0.005oC/min
Cooling MethodsLE Armstrong (Ed). Exertional heat Illnesses. Human Kinetics 2003
Cool water as effective as iced waterYou dont need iced water to achieve rapid cooling.Immersion in any water leads to rapid heat loss in minutesThe logistics of water at 20-25C is much simpler than water at 4C and while slower, the difference is not clinically significant as seen on the next slide.
Chilled IV SalineUsed in AMI patients post resuscitation to reduce cerebral damage
Potential for use in situations where water immersion is not available or cannot be practicably used.
Cooling rates reportedRajek (2000) 40mls/kg0.083oC/minBernard (2003) 30mls/kg 0.053oC/min
Ice packs (current gold standard ARC)Bernard (1997,2002) 0.015oC/min
Field Cooling 2 L of IV Sa