Temperature Regulation Flashcards
How is heat lost and produced
HEAT LOSS
- Radiation
- Convection
- Evaporation (Respiration and sweating)
- Conduction
- Loss in urine and faeces
HEAT PRODUCTION
- BMR
- Exercise
- Shivering
- Brown fat thermogenesis
What is feedforward temperature control
Seeing that its snowing and putting on a coat. Preventing temp change from occuring
Describe temperature sensors in the body
Temperature gated ion channels call TRANSIENT RECEPTOR POTENTIAL (TRP) channels
WARMTH
TRPV1, TRPV2, TRPV3, TRPV4
(TRPV1 is activated by capsaicin - chemical responsible for the ‘hotness’ in chilli)
COLD
TRPM8
TRPA1
Describe negative feedback temperature control
SENSORS
- Peripheral and core temperature gated ion channels = Transient receptor potential channels (TRP) send information to the hypothalamus
CONTROL CENTER
- Hypothalamus analysis incoming signals and checks them against a set-point and controls the efferent response
EFFECTORS
- Anterior hypothalamus –> temp losing mechanisms i.e. sweating and skin vasodilatation
- Posterior hypothalamus –> temp conservation mechanisms i.e. Vasoconstriction, Shivering, Non-shivering thermogenesis, behavioural changes (clothes/heating)
Which aspect of temperature regulation do the anterior and posterior hypothalamus control respectively?
- Anterior hypothalamus –> temp losing mechanisms i.e. sweating and skin vasodilatation
- Posterior hypothalamus –> temp conservation mechanisms i.e. Vasoconstriction, Shivering, Non-shivering thermogenesis, behavioural changes (clothes/heating)
Can BMR increase in response to cold
No
By what magnitude can shivering increase heat production
Sixfold
How does general anaesthesia disturb the normal thermoregulatory mechanisms
Mild hypothermia (34.0 - 36.5) is common during GA.
- SET POINT reduction
Normal hypothalamic set points for vasoconstriction (T < 36.5) and shivering (36.0) are reduced by 2 - 3 degrees C under general anaesthesia - VASODILATION (therefore no vasoconstriction)
- MUSCLE PARALYSIS prevent shivering
- BEHAVIOURAL component impaired
Describe the pattern of reduction of core body temperature during anaesthesia
3 Phases:
- REDISTRIBUTION
- VD + Reduced set point VC –> redistribution of blood and therefore heat to the periphery
- 1 - 2 degree drop over 30 - 45 mins - LINEAR decline
- 1 degree C drop over 2 hours
- Radiation (40%) / Convection (30%) / Evaporation (20%) / Respiration (8% evap H2O + 2% heating of air)
- depends on type of surgery –> highest open abdominal surgery - PLATEAU
- Peripheral VC becomes activated
- heat loss = heat production
- If GA + Neuraxial –> continued linear decline and no plateau phase
Describe and depict the pattern of reduction in core body temperature associated with:
- Neuraxial blockade
- GA
- GA + neuraxial blockade
More significant heat loss with GA vs. neuraxial. All three phases present
No plateau phase if GA combined with Neuraxial and redistribution + linear phases are faster
Describe the cardiovascular effects of hypothermia
- Bradycardia and J waves
- Arrhythmias –> T < 28 deg C –> spont VF (Titanic)
- OHDC P50 left (reduced O2 offloading tissues)
- Increased Blood viscosity –> Increased LV work
All above increase risk of MI
Describe the effects of hypothermia on the CNS
- Confusion / irritability (mild hypothermia)
2. At core Temp of 20 deg C the EEG may be consistent with brain death
Describe the metabolic effects of hypothermia
- BMR drops 6% every 1 deg C
- Hyperglycaemia (reduced glucose uptake by cells)
- Slowed enzymes –> drug metabolism –> prolonged action muscle relaxants
What are the effects of hypothermia on the kidneys
“cold diuresis”
Suppression of ADH secretion
What is the effect of hypothermia on the haematological system
Platelet and clotting dysfunction (minor)
