24. Disturbances in body temperature regulation – hypo- and hypertermia.

DISTURBANCES IN BODY TEMPERATURE REGULATION - HYPO- AND HYPERTHERMIA

Hypothermia

• ⭐ Hypothermia - a core body temperature below 36°c usually due to environmental exposure
• There is an increased risk of hypothermia in:
• Older people - diminished temperature sensation and subcutaneous fat
• Very young people - diminished mobility and communication and increased surface area/mass ratio

Causes of hypothermia

• Hypothermia occurs when body heat loss exceeds body heat production
• Most common during cold weather or immersion in cold water
• Wet clothing and wind increases risk
• Conditions that cause loss of consciousness, immobility, or both (trauma, hypoglycemia, stroke, alcohol intoxication) are common predisposing factors

Pathophysiology of hypothermia

• The body maintains a stable core temperature through balancing heat production and heat loss
• Heat loss occurs via several mechanisms, the most significant being radiation under cold and dry conditions (55-65%)
• Conductive and convective heat loss account for about 15% of additional heat loss, and are the most common causes of accidental hypothermia
• Conduction heat loss is particularly significant in drowning/immersion accidents as thermal conductivity of water is up to 30 times that of air
• Respiration and evaporation account for the remainder
• The hypothalamus controls thermoregulation via increased heat conservation (peripheral vasoconstriction and behavioural responses) and heat production (shivering and increasing levels of thyroxine and epinephrine)

Physiological effects

• Thermoregulation ceases below about 30°c, to which the body must then depend on an external heat source for rewarming
• Hypothermia slows all physiological functions including:
• Cardiovascular system
• Decreased depolarization of cardiac pacemaker cells → bradycardia
• Mean arterial pressure and cardiac output decrease
• Immersion in cold water can trigger the diving reflex, which involves vasoconstriction in visceral muscles; blood is shunted to essential organs (e.g. heart, brain)
• Renal system
• Renal cell dysfunction and decreased levels of ADH lead to production of a large volume of dilute urine (cold diuresis)
• Diuresis plus fluid leakage into the interstitial tissues causes hypovolemia
• Vasoconstriction, which occurs with hypothermia, may mask hypovolemia, which then manifests as sudden shock or cardiac arrest during rewarming (rewarming collapse) when peripheral vessels dilate
• CNS
• Hypothermia progressively depresses the CNS, decreasing CNS metabolism
• At core temperature less than 33°c, brain electrical activity becomes abnormal

Symptoms of hyperthermia

1. Mild hypothermia

• Shivering
• Hypertension
• Tachycardia
• Tachypnea
• Vasoconstriction

2. Moderate hypothermia

• More violent shivering
• Muscle miscoordination
• Slow and laboured movements
• Confusion
• Pale skin

3. Severe hypothermia

• Heart rate, respiratory rate, and blood pressure all decrease
• Difficult speaking, sluggish thinking, and amnesia
• Exposed skin becomes blue and puffy
• Poor muscle coordination, walking becomes almost impossible