Temperature Regulation

• Effects of hyperthermia

  • 39 - severe sweating and vasodilatation, breathlessness
  • 40 - vomiting, dehydration, weakness, dizziness
  • 41 - fainting, confusion, drowsiness
  • 42 - brain protein denaturing
  • 43 - death
• Effects of hypothermia

  • 35 - severe shivering, peripheral circulation shutdown
  • 33 - confusion, drowsiness, coagulation impairment
  • 31 - unconscious, major risk of arrhythmia (VF)
  • 28 - respiratory muscle failure
  • 26 - death (normally)
• Heat is a form of energy that can be

  • Transferred to other objects
  • Converted to and from other energy forms

  Temperature indicates the state of heat energy that an object possesses

  Almost all body metabolism is converted ultimately to heat
• Core temperature is normally maintained between 36.5 - 37.5 degrees

  Reflects the internal environment of the body, especially major organs

  Variation occurs with circadian rhythm and menstrual cycle

  Outside this range many physiological processes are impaired, predominantly because of reduced enzyme activity
• Hypothermia - temperature less than or equal to 35 degrees celsius
• Hyperthermia (aka pyrexia) - body temperature greater than 38 degrees celsius
• Body temperature represents the balance between heat production (and uptake) and heat loss)
• Thermoreceptors

  Cold receptors

  • Mostly peripheral in skin
  • Conducted via myelinated A fibres (like fast pain)

  Warm receptors

  • Mostly central in hypothalamus
  • A few peripherally in skin

  Extremes of temperature are identified by nociceptors and perceived as pain
• Hypothalamic control

  • Thermoregulation controlled by the anterior or preoptic area of hypothalamus
  • Extensive connections within CNS to effect responses including:
  • higher (cortical) centres
  • vasomotor centre - blood vessels
  • motor output - skeletal muscle
• Behavioural response mechanisms:

  • Voluntary actions to
  • increase muscle activity
  • general movement
  • change the body's immediate environment
  • seeking shelter/shade
  • adjusting clothing/cover
  • find heat (fire etc.)
• Physiological response mechanisms:

  • Involuntary efforts that influence the
  • rate of heat production
  • rate of heat loss
• Neurological physiological effects:

  SYMPATHETIC - vasomotor tone, sweating

  MOTOR - controlled activity, shivering
• Hormonal physiological effects:

  THYROID: thyroxine, T3

  ADRENAL MEDULLA: Adrenaline, NA

  Both increase cellular metabolism
• virtually all body heat comes from cellular metabolism. almost all metabolism is converted ultimately to heat. divided into basal production and activity-related.
• Basal metabolism = 1 MET (metabolic equivalent of task) with exercise graded as a multiple of this. Shivering increases heat production 4-5 fold, mostly mediated via muscle spindle activation.
• heat produced (and energy value) are linked to substrate used. In kcal/g carbohydrate and protein both have 4 and fat has 9. Oxygen is also needed for heat production - 1ml required to burn 5 calories.
• there are two main routes of heat loss: skin (90%) and lungs (10%). skin blood flow, regulated by sympathetic nervous system is main determinant of cutaneous heat loss.
• conduction - via direct contact with adjacent material

  convection - warming of the adjacent air which rises creating a heat-losing current
• Radiation - transfer through infra-red rays to a distant object at a lower temperature (radiated heat is gained by the body from objects at higher temperature)

  evaporation - latent heat of vaporisation is lost as sweat/respiratory humidity evaporates
• Sweating

  • from eccrine (aka merocrine) sweat glands
  • under control of sympathetic nervous system but cholinergic fibres
  • for sweat glands neurotransmitter is ACh not NA
• Sweating is the only way of losing heat in an environment that is >37 degrees celsius. It is ineffective in very high humidity. Composition varies:

  • [Na+] 20-100 mmol/L
  • falls as rate of sweat production increases
  • adapts with chronic high temp exposure to reduce Na+ loss
• Thermoneutral zone - environmental temperature band within which normothermia can be maintained:

  1. using basal energy production
  2. without evaporative heat loss
• Fever: elevated temperature from infection or inflammation
• Pyrogens: cytokines (IL-1, IL-6, TNFa) released by macrophages - hypothalamus to a higher value: act via PGE2
• Heat stroke

  • Rapid, extreme rise in temperature
  • usually exercise-related
  • may be exacerbated by drugs
  • Heat production outstrips ability to compensate
  • Hyperthermia impairs hypothalamus leading to failure of any response
  • Mortality rate high even with rapid intervention
• Neonates are at high risk of hypothermia because of their high body surface area to weight ratio. They are also unable to make behavioural changes. Brown fat is a specialised heat source used by neonates, located between the scapulae. Oxidative metabolism in mitochondria is uncoupled to phosphorylation and thus produces heat rather than ATP. Activated by thyroid hormones sensitising adipocytes to action of adrenaline.
• Near-drowning victims are at high risk of hypothermia because there is huge increase in heat loss by conduction.
• Drug overdose means high risk of hypothermia because there is reduced metabolism and heat production. They are also unable to make behavioural changes.
• Major trauma is a high risk for hypothermia because exposure increases heat loss (including evaporative). Replacement of blood loss with cold fluid. Hypothermia impairs blood clotting
• Accidental hypothermia:

  • Environmental heat loss and impaired response mechanisms:
  • Drugs (alcohol, overdose)
  • Hormonal (hypothyroid)
  • Trauma
  • Near-drowning
  • If extreme, may mimic death
  • Active warming whilst resuscitation continues
  • Not dead until warm and dead
• Induced Hypothermia:

  • We may want to cool a patient to reduce metabolic requirements
  • during surgery - brain, heart
  • following injury - brain
  • We can achieve this using
  • Surface cooling (ice bath/packs)
  • Cardio-pulmonary bypass
• Cold response

  • Warm clothing
  • Vasoconstriction
  • Thyroxine/T3 release
  • Catecholamine release
  • Activity shivering
• Hot response

  • Sweating
  • vasodilation
  • Exposure
  • Decreased muscle activity