Thermoregulation Flashcards
Definition of thermoregulation
Balancing of heat production against heat loss
How is normal temperature determined
Set-point determined by hypothalamus
How is heat produced
Heat is a by-product of metabolism and so is determined by the metabolic rate
- mostly in the heart, liver, brain and skeletal muscule
Why should critically ill people be nursed in the thermoneutral zone
Heat production requires oxygen consumption - when people are critically unwell, all their oxygen reserves are being used to fight illness and supply the brain and heart with oxygen
What can the body do to elevate body temperature
Heat retention - cutaneous vasoconstriction - piloerection Increasing heat production - shivering - sympathetic excitaion - thyroxine stimulation - brown fat heating - behavioural modification e.g. putting on clothes
How does the body loose heat?
Core heat is conducted to the periphery and lost through the skin
What controls the rate of heat loss?
Insulation e.g. subcutaneous fat
Cutaneous blood flow - vasodilation
Thermoneutral environment - 1 degree heat loss/1L blood flow to the skin
How is heat lost from the skin?
Radiation - if ambient temperature is lower than that of the skin
Conduction - movement causes heated air molecules to move away from the skin and be replaced by cool air molecules
Evaporation - sweating (controlled by autonomic nervous system - stimulation of the anterior hypothalamus causing cholinergic sympathetic stimulation of sweat glands)
Summary of methods by which the body can reduce temperature
Sweating
Cutaneous vasodilation
Inhibition of shivering
Behavioural modification - e.g. removing clothes/finding shade
Causes of pyrexia
Toxins - infection or trauma (trauma causes release of cytokines that are involved in inflammation and repair)
Damage to thermoregulatory structures in the brain - via surgery or tumours
How do toxins cause pyrexia?
Causes the set-point of temperature in the hypothalamus to rise. Body goes into rigors to try and raise the temperature (vasoconstriction, sweating etc.)
Which toxins are most likely to affect the hypothalamus set-point?
IL-1 and IL-6
What is heat stroke?
When environmental factors prevent heat reducing mechanisms e.g. low air currents prevent conduction and high humidity prevents radiation)
What are the effects of hypothermia (<30 degrees C)
Below 30 degrees, the hypothalamus loses the ability to regulate temperature. Metabolism slows down and less heat is produced. Reduced GCS and come impedes activation of the CNS and shivering
How can certain drugs affect thermoregulation
Volatile anaesthetics and propofol cause vasodilation.
Fentynl and opiates depress the thermoregulatory centre in the brain
Neuromuscular blockades compromise shivering
Malignant hypothermis
