Hypothermia - Extreme Physiology
- Created by: anniecritchlow
- Created on: 22-05-19 15:22
Hypothermia
Cold air is a worse stressor than heat.
It can lead to:
1. Peripheral cooling - incapacitation or cold injury
2. Deep body cooling - hypothermia
Hypothermia - when body temperature reaches below 35 degrees
Effects of hypothermia
Unresponsive
Decrease in minute ventilation rate
Temperature regulation impaired
Tachycardia or bradycardia
Amnesia
Below 32 degrees shivering stops
Eventually coma and death
Categories
1. Mild - early onset, easy to recover from
2. Moderate - significant
3. Severe - death zone, losing reflexes and responses
Major responses
Increased heat production (shivering, non-shivering thermogenesis)
Decreased heat loss (peripheral vasoconstriction)
Behavioural responses (clothing, shelter and huddling)
Predisposing factors to hypothermia
Low ambient temperature
High wind speed
Rain
Fatigue/exhaustion
Immersion
Inadequate clothing
Shock following traumatic injury
Body size and composition
Age - particular old and young
Starvation
Wind Chill
Air movement increases heat loss via convection
Therefore - risk of hypothermia and cold injury increases with wind speed
When -29 degrees, with a wind chill it can be as cold as -44 degrees in which skin will freeze within 1-2 minutes
Mild hypothermia
33-35 degrees
Thermoregulatory responses are normal:
1. Intense cold sensation
2. Peripheral vasoconstriction
3. Violent shivering
Shivering - changing tensions in the muscle of antagonistic pairs in attempt to generate heat, that when they go out of sync produce ocillations
Moderate hypothermia
30-33 degrees
Thermoregulatory responses begin to fail and spontaneous recovery becomes unlikely
1. Shivering gradually decreases
2. Joints become stiff, muscles rigid
3. Progressive reduction in metabolism and VO2
4. CNS cooling leads to dulling, irrational behaviour and eventually unconciousness
5. Cardiac cooling leads to decreasing heart rate and declining cardiac output
Ezymes arent working as well within the tissues so processes are slow
Semi-conciousness occurs
Severe hypothermia
Below 30 degrees
The effects of moderate hypothermia become intensified:
1. Heart rate continues to decline as core temperature falls
2. Heart becomes prone to arrythmias
3. Cooling and dehydration increases blood viscocity
4. Respiration becomes slow and shallow
5. Ventricular fibrillation is the most common cause of death (about 27 degrees)
Rewarming
Active (above 34 degrees)
For fully conscious victims
Hot bath
Hot showers
Passive rewarming (below 34 degrees)
For unconcious victims
Sleeping bag and wrapping in blankets
Slow rate of rewarming to reduce risk of collapse
Extraneous heating may be necessary
Hot water bottle/electric blanket
Frostbite
If temperature of exposed peripheral tissues falls below -0.55 degrees, tissue fluid may freeze
Mild - only skin freezes
Severe - deeper tissues freeze
Damage to affected cells results from mechanical action of ice crystals and from cell dehydration
Permeability of small blood vessels is increased by freezing - on thawing there is a loss of fluid from the circulation into the ISF
This may lead to sludging of RBC in the mitochondrion, which reduces or stops local blood flow, leading to gangrene and possible loss of fingers, toes etc
Non-freezing cold injury
Tissue temperatures between 17 and -0.55 degrees lasting for a protracting period can result in NFCI
Feet are most commonly affected
Cold, wet feet lose heat rapidly which induces intense vasoconstriction
Local hypoxia and accumulation of toxic metabolites results in tissue death, with deeper tissues often affected
Blisters, ulcers and gangrene may develop and amputation of toes or even the foot may be necessary
Cold water immersion
Thermal conductivity of water is 27x greater than air
Rate of heat loss from resting subject in water is 2-5 x greater than in air
Although risk of hypothermia following accidental immersion in cold water is high, the victim must first survive the initial shock response and avoid drowning in first 30 mins
Cold shock
CSR lasts for about 1 minute after sudden immersion into cold water
First response to rapid skin cooling is involuntary gasp
Followed by hyperventilation which may lead to loss of consciousness
Peripheral vasoconstriction leads to increases afterload on heart
Main causes of death during this phase are drowning, cardiac arrest and cardiac arrythmias
Cold incapacitation
Cold water incapacitation occurs within 5-15 minutes in cold water
Vasoconstriction decreases blood flow to the extremities allowing the periphery to cool rapidly
Muscle and nerve fibres lose function when cold
Within critical time frame you will lose meaningful movement in hand and feet and then arms and legs for floatation
Other importanr life-saving/survival activites become difficult and then impossible
Hypothermia in water
It takes at least 30 mins for an adult to become even midly hypothermic following cold water immersion
Survival times are influenced by many factors:
1. Subcutaneous fat levels
2. SA:V ratio
3. Magnitude of shivering response
4. Activity levels and posture
5. Clothing (amount and type)
Circus-Rescue collapse
Collapse can happen just before, during or after rescue
The symptoms can range anywhere from fainting to death
Several factors involved:
Imminent rescue results in a mental relaxation, so levels of stress hormones decrease
Blood pressure can fall and muscular relaxation occurs
This can cause collapse, and in extreme cases, death as a result of cardiac arrest
Cardiac function is significantly affected by the way the victim is handled and removed from water
Transport casualty horizontally/slightly head down
Human cold response
Acute cold
Localised cold has direct effects on smooth muscle:
Cells disturb mitochondrial function and increase release of free radicals
This activates Rho kinase - a signalling molecule that can promote adrenergic receptors
Can also inhibit myosin light chain phospotase (MLCP) - which deactivates myosin light chain
This in conjunction with the adrenergic receptors causes vasoconstriction
Relaxation is switched off so contraction occurs
Sensing cold
Recognised by trip receptors which signals back to dorsal root and trigeminal ganglion sensory neurons
This enhances metabolic heat production and vasoconstriction
Vasoconstriction is mediated by noradrenaline release (60-70%) and neuropeptide Y (20-30%)
Phases of Response
1. Reflex vasoconstriction
2. Local cooling
3. Response to core temperature reduction
Hunting reflex
Cold induced vasodilation - CIVD
Periodic fluctations in vasoconstriction/dilation to keep tissue alive
Centrally mediated and suppressed when core temperature falls
Enhances with exercise training
Shivering
Metabolic enhancement:
Increased muscle tone
Recruitment of brown adipose tissue
Involuntary rhythmic contractions initiated by skin temperature fall then enhanced by core fall
Maximal at core temp of about 35 degrees but ceases at about 31 degrees
Whole body adaptation to cold?
1. Decreased metabolic response to cold (shivering)
2. Increased thermal comfort
3. Faster fall in deep body temperature
Most evident at rest in the cold
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