W5- Thermoregulation and pyrexia

what are the 6 main processes responsible for heat production in the body?

-metabolism

-shivering

-brown fat

-vasoconstriction

-piloerection

-neuroendocrine

how does metabolism increase temperature?

heat is generated a a by product of metabolism for regulation of body temperature

why does metabolism generate heat?

no pathway is 100% efficient, so heat is lost at each stage

how can basal heat production in the body be supplemented?

by heat produced in metabolic processes

why is shivering predominantly truncal?

heat is generated locally and is maintained in vital organs/central core

(shivering can appear to be in arms and legs, but only due to mechanical exaggeration)

why is shivering only used to produce heat in the short term?

metabolically expensive- uses a lot of O2 to produce heat

(eg patient who is already unwell and hypoxic may shiver and use up O2 which is already in short supply)

when do we shiver?

short term solution for heat production which allows us to buy extra time

(cannot be maintained in long term due to use of O2)

what is the purpose of brown fat in heat production?

brown fat= normal adipose tissue which appears brown due to being rich in mitochondria

metabolises fat very inefficiently, leading to heat production

how is brown fat thought to be able to be used for weight loss?

-B3 receptor mediated (breakdown of fat is triggered by B3 adrenergic receptors to produce heat)

-thought to be able to target B3 receptors in body tissue to increase metabolic rate in localised areas

-allows fat to be turned into heat = weight loss

why is brown fat of little importance for heat production in adults?

only seen in animals and neonates

what are the two mechanisms that alter regional blood flow?

vasodilation

vasoconstriction

what is the result of vasoconstriction in terms of body temperature?

decreased blood flow to skin = decreased heat loss

what is the clinical appearance of vasoconstriction?

pale- blood is diverted centrally to maintain heat

what is the result of vasodilation in terms of body temperature?

increased blood flow to skin = increased heat loss

what is the clinical appearance of vasodilation?

flushed- blood close to the surface of the skin for heat loss

what controls skin blood flow (vasoconstriction and vasodilation)?

sympathetically mediated (under control of autonomic nervous system)

which vessels are involved in vasoconstriction and vasodilation?

both small and large vessels (any vessels which have a degree of smooth muscle for vasodilation and vasoconstriction)

what is paradoxical increased blood flow causing excess heat loss in prolonged cold exposure?

-very cold= reduce blood supply to skin and peripheral tissues (vasoconstriction) to reduce heat loss

-can cause tissue ischaemia due to lack of O2, causing accumulation of byproducts of anaerobic metabolism in tissue

-toxic byproducts trigger vasodilation in attempt to remove build up and improve O2

-vasodilation results in heat loss

why is there are paradoxical increased blood flow in prolonged cold exposure?

body prioritises O2 delivery over temperature control in prolonged cold exposure

(vasodilation to remove toxic byproducts and improve O2, which results in paradoxical heat loss)

what is piloerection?

hair 'stands on end'

what causes piloerection?

erector pili muscles contract under sympathetic stimulation

erector pili pulls on the base of the hair root, causing the hair to stand on end

what is the mechanism of piloerection in terms of thermoregulation?

creates a pocket of warm air between the hair and the skin

(acts as an insulator to prevent heat loss)

why is piloerection of minimal efficiency in humans?

not very hairy

(more of an insulating effect in animals)

what is the short term neuroendocrine response for heat production?

increased catecholamines (adrenaline, noradrenaline) to increase metabolic rate and contractility of muscles

= heat generated as byproduct

what is the long term neuroendocrine response for heat production?

thyroxine- controls metabolic rate over a long period of time

(hyperthyroidism/hypothyroidism causes dysregulation in temperature control)

why does hypothyroidism cause increased risk of hypothermia?

lack of thyroxine (controls metabolic rate over long period of time, controlling heat production as a result)

what are the main mechanisms of heat loss?

-conduction and convection (60%)

-radiation (25%)

-evaporation (sweating, breathing out humidified air) = 15%

(and other body fluids- eg diarrhoea is losing fluid at 37C)

what is sweat?

sweat= ultrafiltrate of plasma

how is sweat produced?

-filter plasma to leave proteins in blood

-filtrate enters sweat gland in the dermis

-forms a mixture of electrolytes, Na+ and urea, which are concentrated

-some Na+ is absorbed as the filtrate ascends towards the epidermis (body retains Na+ for functions)

how is the sweat gland, duct and pore involved in the sweating response?

-sweat gland- primary secretion is mainly protein-free filtrate (under sympathetic control)

-sweat duct- absorption of Na+ and Cl-

-sweat pore- by the time fluid reaches the skin, Na+ has decreased and the fluid is more concentrated in urea

what causes body odour?

urea in sweat

why does acclimatisation to heat cause us to sweat more?

increased sweating = increased heat loss = better toleration of heat

why are salt tablets given with excessive sweating?

sodium losses can be significant in sweat

how does panting cause heat loss?

-small breath of dry air is inhaled

-air is exhaled as humidified, warm air

(shallow breath means air is not entering lungs, so not breathing out too much CO2)

what is involved in homeostasis?

keeping biological systems stable so they can function

(biological systems operate under tight range of conditions)

what occurs in body temperature exceeds 45C?

enzymes denature and are irreversibly damaged

(not much leeway from optimum at 37C before encountering irreversible problems with enzyme function)

at what body temperature do we have 'conscious intelligence'?

35C-40C

(outside of this range = decrease in conscious brain function)

what are the 3 things involved in thermoregulation for the tight control of body temperature?

-receptors- monitor changes in temperature

-coordinator- looks at what the temperature should be and what it is

-effectors- cause change in temperature

what are thermoreceptors?

specialised outgrowths of neurons (nerve cells) which are endings on the nervous tissue

function= recognise changes in temperature (too hot/cold depending on their type)

what is an example of conscious and subconscious recognition of change in temperature by thermoreceptors?

conscious= touching something too hot

subconscious= recording temperature around the body at all times (not aware of it)

what are some examples of where thermoreceptors are located?

-skin (cutaneous thermoreceptors)

-GI tract

-great veins

-hypothalamus

what allows the passage of information about changes in temperature from thermoreceptors to the CNS?

A-delta and C fibres

what are A-delta and C fibres?

groups of nerves which carry thermoregulatory signals and pain signals

why can pain relief be tested using a cold spray?

tests if A-delta and C fibres (responsible for passage of thermoregulatory and pain signals to CNS) have been blocked

what is the coordinator in thermoregulation?

anterior hypothalamus

what is the 'set point' in thermoregulation?

optimum temperature which has been set by the hypothalamus

(temperature which we want to be at)

what is the set point body temperature

37C

what mechanisms are activated if our body temperature goes below the set point (37C)?

below set point = too cold

-evaporative heat loss (sweating) is inhibited

-excess heat is produced

what mechanisms are activated if our body temperature goes above the set point (37C)?

above set point = too hot

-heat production is decreased

-evaporative heat loss occurs (sweating)

why is heat production never reduced to 0, even when we are above the set point?

all metabolic processes generate heat

what can cause the hypothalamus to alter the set point?

disease

(leads to clinical pathology)

what are the two systems working in conjunction in terms of effectors to cause a change in temperature?

-specific heat increasing mechanisms

-specific heat loss mechanisms

what is diurnal variation in terms of body temperature?

change in body temperature throughout the day and night

(lowest before waking up, related to sleep-wake cycle)

what is intersex variation in terms of body temperature?

differences are observed in body temperature between men and women

(eg +0.5C at ovulation)

how do voluntary actions affect maintenance of body temperature?

we alter our behaviours to maintain body temperature at a conscious level

(eg wearing more layers when it is cold)

why do babies and elderly often need to have their temperature regulated for them?

cannot alter their behaviours to maintain body temperature

-elderly- weak, frail, confused (may not be able to make decisions to regulate temperature)

-babies- temperature is maintained by incubation

what is an example of how 'normal' temperature depends on the site at which it is measured?

core = 0.5C higher than skin

why is measuring temperature on skin unreliable?

-body aims to maintain heat in core central areas (core organs such as intra-abdominal cavity and brain)

-maintaining 37C in skin and peripheries is less important

(+0.5C in core compared to skin)

what are 5 ways to measure temperature?

-thermometer

-forehead temperature monitor

-tympanic membrane temperature measurement

-oesophageal temperature probe

-rectal temperature probe

what are the properties of a mercury thermometer?

mercury thermometer placed under the tongue, left until it stabilised and then read

why did mercury thermometers stop being used?

-dangers of mercury in mouth

-difficult to leave for long periods of time while it stabilises (esp in children)

-was artificially elevated/depressed by what had been in the mouth previously

what are the properties of the forehead temperature monitor?

-relies on infrared measurement of skin temperature

-quick and well tolerated

-HOWEVER- can get differences in readings (not most accurate) and only measures temperature of skin, which can be variable from core temperature

what are the properties of the tympanic membrane temperature measurement?

-thermometer is placed in ear, wait 5 secs and obtain reading

-uses infra-red

-accurate if well positioned at the tympanic membrane

-HOWEVER- can be altered by the presence of debris

what are the properties of the oesophageal temperature probe?

-thermistor is an electrical device entering through nose, down back of oropharynx and into oesophagus

-accurate reading of body temp due to being in body core

-used in temperature recording during surgery/ICU (continually monitors changes over time)

-HOWEVER- difficult to do in patients who are awake

what is the rectal temperature probe?

-thermistor inserted into anus and rectum

-used in ICU/hypothermic patients who are drowsy/unconscious

-gives accurate reading of core temperature

what are two causes of abnormalities in temperature regulation?

-behaviours which resulted in temperature not being able to be maintained

-disease process acting on hypothalamus to alter the set point (body is regulating itself towards a temperature that is pathological)

what are two consequences of altered set point?

hypothermia

hyperthermia

what is the clinical relevance of hypothermia and hyperthermia?

hypothermia- more common in terms of significant pathology

hyperthermia- more common in mild forms

what is the body temperature in hypothermia?

core temperature <35C

(severe is <28C= unconscious)

what are the three main causes of hypothermia?

-behavioural- could have been prevented if patient was able to (eg put on coat)

-hypothyroidism- due to lack of thyroxine production, which is involved in regulating heat production

-dermatological- heat loss from skin if it is very vasodilated (eg widespread redness of skin or secondary infection caused by eczema or psoriasis)

what are the clinical features of hypothermia and hyperthermia caused by?

loss of enzymes and brain function

what are the clinical features of hypothermia?

-confusion

-coma

-cardiovascular damage

what is the spiral of deterioration caused by confusion in hypothermia?

coldness = confusion = behavioural decisions which will not help to increase body temperature

why is coma a clinical feature of hypothermia?

due to CNS depression

why is cardiovascular collapse a clinical feature of hypothermia?

due to total body shut down

what are the main features of cardiovascular collapse as a result of hypothermia?

prone to cardiac arrhythmias and ventricular fibrillation from small/insignificant stimuli

how is normal heart rhythm regained in hypothermic patients?

electric shock

(resistant when cold, need to be warmed up first)

what are the properties of the ECG of a hypothermic patient?

J-wave- small wave after QRS complex

irregular- due to cardiac arrhythmias

how do aluminium-type blankets reduce heat loss in hypothermic patients?

decrease radiation, conduction and convection losses

why do athletes lose heat very quickly when they stop running?

running= sweating, vasodilated due to increased heat production

finished= heat production stops but still sweating and hyperventilating, causing excessive heat loss

what is the ABC for hypothermic patients?

airway- needs to be protected due to low consciousness level

breathing- ensure breathing sufficiently for O2 in and CO2 out

cardiovascular stability- ensure HR and BP are normalised

what can occur as a result of uncontrolled warming of hypothermic patients?

-cardiovascular instability

-prone to arrhythmias and plummeting BP

when is controlled warming of patients often used?

operating theatres- everyone with operation over 20mins due to heat loss with anaesthesia

what are some examples of controlled warming methods?

-warm air blanket which blows air at 35C-40C

-close-fitting inflatable blanket above patient

-cardiopulmonary bypass machine (severe cases such as cardiac arrest)

how is the cardiopulmonary bypass machine used in severe cases of hypothermia?

-patient plugged into machine and gives them cardiac output (don't have to do chest compressions)

-increases body temperature in a slow and controlled way

what are the body temperatures of hyperthermia?

>38C

(>40 is life threatening due to effects on brain and consciousness levels)

what are the 4 main causes of hyperthermia?

-exertional- using energy in exercise, but very little is lost as heat

-situational- combination of factors (eg over-exerting on hot day, not dressed appropriately, lack of fluids)

-drugs- eg 'pan-activation' of muscle activity causes huge generation of heat (like pathological version of shivering)

-pyrexia- set point is pathologically increased so body temperature is increased

what are 4 clinical consequences of hyperthermia?

-confusion

-seizures

-coma

-cardiovascular collapse

what is pyrexia?

fever

what causes pyrexia?

usually due to altered set point in hypothalamus

(body tries to regulate temperature to new set point)

what is the pyrogenic response when the set point is suddenly raised to a high value due to pathogen altering the hypothalamus?

-pyrogens produced

-body tries to maintain heat due to body temperature being below the new set point

-involves vasoconstriction, piloerection, adrenaline secretion, shivering

what is the pyrogenic response when the set point is suddenly lowered to a low value due to pathogen altering the hypothalamus?

-body tries to lose heat due to the body temperature being above the new 'set point'

-involves vasodilation and sweating

what are 5 examples of causes of pyrexia of unknown origin (PUO)?

-infections- virus, bacteria etc

-blood transfusion/iatrogenic pyrogens

-inflammation

-malignancy

-hypothalamic insults

how does blood transfusion/iatrogenic pyrogens cause pyrexia of unknown origin?

-triggered by introduction o foreign substances from transfusion, drug, implant etc

-generates substances which alter setpoint

how does malignancy cause pyrexia of unknown origin?

-cells are dividing much more rapidly than normal and are much more metabolically active = generate excess heat

-paraneoplastic pnuemoniae caused by tumours can also alter setpoint

how can hypothalamic insults cause pyrexia of unknown origin?

-trauma, tumour, vascular event (eg stroke) which causes hypothalamic function to decrease

-causes variable setpoints due to control of body temperature being altered

-causes intermittent swings in temperature

what are pyrogens?

substances that cause fever

what kind of pathways are involved in pyrogenesis?

many different prostaglandin pathways lead to the production of pyrogens

(pyrogens alter body signalling and control body temperature)

what occurs in pyrogenesis?

-cytokines- early mediators of immune/inflammatory response (eg IL-1, IL-6)

-cytokines are involved in activation of arachidonic acid pathway for prostaglandin synthesis

-prostaglandins/prostaglandin-like substances have variety of effects at a cellular level to generate heat and alter hypothalamic set point

-this causes heat conservation and leads to fever

what are the 2 main types of enzymes involved in the prostaglandin pathways involved in pyrogenesis?

-phospholipase A- breaks down cell membrane phospholipids into arachidonic acid

-COX1/2- break down arachidonic acid into prostaglandins