Thermoregulation Flashcards
Describe the mechanisms by which heat is produced by the body and its transfer between the body and environment
Net flux of heat is a balnace between the body’s metabolic heat production and 4 mechanisms of heat loss being radiation, conduction, convection and evaporation. (Rad convicts con evangelicals)
Radiation-
Aka radiative heat exchange, anything above 0 kalvin will partake in this form of heat exchange.
It is proportional to the temperature of the person and the differential between the person and the environment.
This does not require a transfer medium to occur.
Makes up approximately 45% of heat loss in thermoneutral conditions.
Conduction-
Conduction is the transfer of heat (in the form of kinetic energy) via direct contact from an object of higher temperature to one of lower temperature.
This requires physical contact between 2 bodies to occur. Solids conduct heat better than gases and this does not occur in a vaccum.
In air this is limited but in immersion (i.e cold water) this is significant.
Fat is a poorer conduction medium vs muscle.
Convection-
Is the loss of heat by the same mechanism as conduction but due to a moving object.
This is the predominant for of heat loss in a human that is naked due to the wind.
Evaporation-
The loss of heat energy as the result of the latent heat of vaporisation of water, evaporation of 100ml of water will reduce the body temperature by approx 1 degree.
Describe radiation
Radiation-
Aka radiative heat exchange, anything above 0 kalvin will partake in this form of heat exchange.
It is proportional to the temperature of the person and the differential between the person and the environment.
This does not require a transfer medium to occur.
Makes up approximately 45% of heat loss in thermoneutral conditions.
Describe conduction and convection
Conduction-
Conduction is the transfer of heat (in the form of kinetic energy) via direct contact from an object of higher temperature to one of lower temperature.
This requires physical contact between 2 bodies to occur. Solids conduct heat better than gases and this does not occur in a vaccum.
In air this is limited but in immersion (i.e cold water) this is significant.
Fat is a poorer conduction medium vs muscle.
Convection-
Is the loss of heat by the same mechanism as conduction but due to a moving object.
This is the predominant for of heat loss in a human that is naked due to the wind.
Describe evaporation
Evaporation-
The loss of heat energy as the result of the latent heat of vaporisation of water, evaporation of 100ml of water will reduce the body temperature by approx 1 degree.
Describe the processes of body temperature sensation
Temperature sensors are present peripherally and centrally.
Central-
Central temperature sensors exist in several regions;
Abdominal visera
Spinal cord
Hypothalmus, with the anterior hypothalamus serving as the most important central thermoreceptor which is able to respond to both hypothermia and hyperthermia via altering the rate of depolarisations which elicits a array of neuronal and hormonal changes.
Brainstem.
Peripheral-
Free nerve endings, which alter rates of firing by orders of malgnitude in response to temperature change.
Divided into 2 seperate types;
1) Cold receptors-
Lie beneath the epidermis, excited by cold and inhuibited by warming. They are active between 10-40 degrees and have a static maxima at 25 degrees.
2) warm receptors
Lie deep to the dermis, excited by warming and inhibited by cooling. Active from 30-50 degrees and a static maxima at 44 degrees.
Temperature sensation runs from cutaneous receptors via the spinothalamic tract and from the medulla to the hypothalamus. Cortical input is recieved via the thalamocortical relay and primitive responses are effected via the mid brain.
What is the inter thershold range? How does it change under anaesthesia
Inter threshold range is the range of core temperatures that does not result in triggering a response.
Normally this is iin the range of 0.2 to 0.4 degrees.
Under anaesthesia this widens greatly to 4 degrees or more.
Describe the mechanisms of temperature regulation
Can be broken into CNS, CVS, MSK and metabolic responses.
Central nervous system-
Increase heat loss- act to remove clothing, sprawl/posture, seeking shade/cool shelter or reducing activity.
Reduce heat loss- add clothing, seek shelter, and huddling
Cardiovascular-
Increase heat loss- increase peripheral vasodilation and AV shunting, increase cardiac output to cutaneous tissues.
Reduce heat loss- vasoconstriction, peripheral circulatory shutdown.
Musculocutaneous
Increase heat loss- sweating
Reduce heat loss- piloerection (goosebumps) and shivering.
Metabolic-
Increase heat loss- nil
Reduce heat loss- increase basal metabolic rate and non shivering thermogenesis.
In the case of environmental temperature exceeding body temperature, what is the only way for the body to reduce its temperature?
Evaporative cooling via sweating. The efficacy of sweating is related to the relative humidity in the air with it being far less effective in humid environments.
What is piloerection? How does it relate to the regulation of thermal control?
Piloerection is the cutaneous hairs standing on end, as in goosbumps.
This acts to entrap a layer of warm air around the body to act as an insulator thereby reducing heat loss.
By which mechanisms do the body increase the basal metabolic rate?
1) shivering via simultaneous contraction of agonistic and antagonistic muscle groups
2) hormonal- via the thyroid and adrenaline release, this increases the metabolic activity at a cellular level.
3) brown fat- important in infants, this is sympathetically innervated with B3 receptors and when activated causes uncoupled oxidative phosphorylation meaning the electron transport chain produces heat instead of ATP.
How does general anaesthesia effect body temperature?
General anaesthesia will cause a drop of between 1 to 3 degrees in core temperature, which is described across 3 phases.
1) rapid reduction- a drop of 1-1.5 degrees which occurs in the first 30 minutes after induction, this is predominantly due to vasodilation due to the reduction in systemic vascular resistance and increased flow to the skin. This causes impairment to the thermoregulatory vasoconstrictive responses to heat loss.
2) gradual reduction
This stage is caused by the heat loss via radiation, conduction, convection and evaporation being greater than the heat production. The only response to heat loss in the paralysed and anaesthetised patient is non shivering thermogenesis (hormones and brown fat).
3) Plateau
Once the temperature falls far enough the impaired heat production means will reach equilibrium with heat loss.
What methods of maintaining body temperature are available during anaesthesia and sedation?
1) passive coverings- a blanket +/- warmed. Reduces heat loss via convection.
2) forced air warmers- bear hugger. Reduces loss via convection and radiation.
3) radiant heaters- humidicrib. Reduces heat loss via radiation. Heat transfer is related to the difference in absolute temperature and distance between objects, it is more effective in infants due to the relatively higher body surface area to mass ratio.
4) warmed IV fluids- reduce conductive heat loss from cold IV fluid intravascularly.
1L of room temp IVF will theoretically drop core temp by 0.2 degrees.
5) heat and moisture exchange filters- reduced evaporation.
What is the non electrical method in which temperature can be measured?
liquid expansion thermometry- i.e mercury thermometers, which consists of an graduated evacuated capillary of a very small volume attached to a reservoir of mercury with a constriction ring that prevents the gravity controlled leakage of mercury into the capillary. This functions by thermal expansion of the liquid mercury with increased kinetic energy when heated which causes the mercury to travel up the graduated capillary.
What are the electrical methods in which temperature can be measured?
1) resistance thermometer
2) Thermistor
3)Thermocouple
How does a resistance thermometer work?
A platinum wire increases electrical resistance with an increasing temperature
Therefore measuring a voltage drop across a wire will correspond with the temperature of the wire.
The change in resistance is linear across a temperature range.
Given the platinum content these are expensive.
