Download - Thermoregulation
ThermoregulationJunior Sharp 13 June 2014
Why is thermoregulation
important in humans?
Why is thermoregulation
important in humans?Enzymatic reactions in the body are temperature
sensitive.Optimal function between 35 – 41 C.
Rate of reaction drops 2.5x per 10 C drop in temperature. Enzymes denature above 45 C.
Concepts and DefinitionsTemperature:
measure of the average kinetic energy associated with the disordered motion of atoms and molecules.
Heat: state of energy an object has in relation to the
kinetic energy of its molecules or atoms.
Concepts and DefinitionsSecond law of thermodynamics:
Energy will disperse from a concentrated form to a dilute form if it is not hindered from doing so.
Energy will move from high to low states. Thus heat moves from high temperature to low
temperature.
Concepts and Definitions Interthreshold range:
Range of core BODY temperature over which no autonomic thermoregulatory response occur.
Also can be considered “range of core temperature that the body is most economical with energy” since no energy is expended to increase or decrease temperature.
Narrow range: 36.5 – 37.5 C (In some references) but 36.8 to 37.2 C (according to Peter Kam).
Classic Diagram Interthreshold Range/Zone
HP = Heat Production HL = Heat Loss Gain = Degree or extent of response
Concepts and DefinitionsThermoneutral zone:
Range of ENVIRONMENTAL temperature in which the metabolic rate is minimal.
Also can be considered “range of environmental temperature that the body is most economical with energy” since no energy is expended to increase or decrease temperature.
Range wider and more variable. Environmental acclimatization.
World Cup in Brazil!
Classic DiagramThermoneutral Zone
LCT = Lower comfort temperature UCT = Upper comfort temperature
Applied DiagramThermoneutral Zone
Mechanism of Heat LossThermal Radiation (40%)
Convection (30%)
Evaporation (15%)
Respiration (10%)
Conduction (5%)
Mechanism of Heat LossThermal Radiation (40%)
Largest mechanism of heat loss. A form of electromagnetic radiation Occurs without the presence of a medium.
Reflective blankets for hypothermic patients or patients with burns.
Mechanism of Heat LossConvection (30%)
Refers to the movement of molecules away from a warm object as a consequence of their reduced density as they gain heat and expand.
This creates convection currents, which transfer heat away from the object.
A medium is needed for convection.
Covering the patient with a blanket to reduce convection.
Forced air warmer to introduce warm convection currents.
Mechanism of Heat LossEvaporation (15%)
Latent heat of vaporization. When a liquid converts to a gas, it needs to gain
energy. This energy in the form of heat is taken from the
patient.
Wrapping neonates with non-permeable films.
Mechanism of Heat LossRespiration (10%)
A form of evaporative heat loss.
Can’t stop the patient breathing, can you? So use of HME in circuits.
Mechanism of Heat LossConduction (5%)
Occurs between two objects in direct contact where a temperature gradient exists between them.
Use of warming mattresses, warmed blankets.
ThermoregulationEssentially a feedback loop involving an:
Afferent System (sensor)
Central Processor
Efferent System (effector)
Thermoregulation Essentially a feedback loop involving an:
Afferent System (sensor) Peripheral
Skin cold receptors: CMR-1, via A delta and C fibres, fire maximally at 25-30 deg C
Skin warm receptors: VR1 and VRL-1, via C fibres, fire maximally at 45-50 deg C
Central Receptors in spinal cord, afferents travel lateral
spinothalamic tracts to medulla to hypothalamus.
Central Processor
Efferent System (effector)
ThermoregulationEssentially a feedback loop involving an:
Afferent System (sensor)
Central ProcessorAnterior hypothalamus:
sensitive to heat/warm blood, effects cooling
Posterior hypothalamus: sensitive to cold afferent from peripheral receptors,
effects warming
Efferent System (effector)
ThermoregulationEssentially a feedback loop involving an:
Afferent System (sensor)
Central Processor
Efferent System (effector)Peripheral
Skin Sweat glands Muscle - Shivering thermogenesis Brown fat, muscle – Non shivering thermogenesis
Central Cerebrum: behavioral changes.
Hyperthermia ResponsePeripheral
SkinVasodilation/increase cutaneous blood flow via
arteriovenous shunts. Increase radiation, convection, conduction.
Sweat glands Increase sweating
Increase evaporation
Shivering and Non shivering thermogenesis inhibited
Hyperthermia ResponseCentral
Cerebrum: behavioral changes.Lethargy: decrease metabolic rate. Increase fluid intakeFanningRemoving excessive clothingLooking for shade
Hypothermic ResponsePeripheral
SkinVasoconstriction/decrease cutaneous blood flow via
arteriovenous shunts. Decrease radiation, convection, conduction.
Sweat glandsDecreases sweating
Minimize evaporation
Hypothermic ResponsePeripheral
Shivering: Uncoordinated large muscle group contraction. Metabolic heat generated from mechanical work.
Non shivering thermogenesis:Oxidative phosphorylation in brown fat and muscle. Actived by beta 3 sympathetic activity. Metabolic heat production WITHOUT mechanical
work.
Hypothermic ResponseCentral
Cerebrum: behavioral changes.Wearing extra clothes. Increasing physical activity. Seeking heat. Consuming hot drinks.
Effects of Anaesthesia on Thermoregulation
3 Phases of Heat LossPhase 1:
Rapid reduction in core temperature of 1.0–1.5°C within the first 30–45 min.
Attributable to vasodilatation and other effects of general anaesthesia.
Phase 2: More gradual, linear reduction in core temperature
of a further 1°C over the next 2–3 h of anaesthesia. Attributable to heat loss by radiation, convection
and evaporation exceeding heat gain which is determined by the metabolicrate
3 Phases of Heat LossPhase 3:
‘Plateau’ phase where heat loss is matched by metabolic heat production.
Occurs when anaesthetised patients become sufficiently hypothermic to reach the altered threshold for vasoconstriction which restricts the core-to- peripheral heat gradient.
Effects of Anaesthesia on Thermoregulation
Regional: Regional vasodilation. Decreased physical activity. Cold OT Drop in temperature not as exaggerated as GA
and GA+RA.
Effects of Anaesthesia on Thermoregulation
General Anaesthesia: Interthreshold range increases (34.5 to 38.3 C):
Lower threshold to cold by 2.5 C Increase threshold to heat by 1.3 CBody does not respond over wide range and
assumes the environmental temperature. Unconscious, hence removing central
temperature responses. Paralyzed: unable to shiver. Vasodilation with drugs/volatiles: increase heat
loss. Cold OT/environment/Surgical exposure
Anaesthesia and Hypothermia
Hypothermia defined by NICE: 36 C
Associated:
Effects of HypothermiaNeurological System
Linear depression in cerebral metabolism, amnesia, apathy, impaired judgement and maladaptive behaviour. Progressive deterioration until coma at 30°C.
Progressive hyporeflexia. Shivering is replaced by muscular rigidity at about 33°C. Rigor mortis-like appearance at 24°C+/- involuntary flapping tremor
Reduced CMRO2 with reduced CBF. Flat EEG at ~20°C
Effects of HypothermiaCardiovascular System
Initial tachycardia and vasoconstriction with mild hypothermia - increased CO (via rate, contractility and stroke volume). Sympathetically driven.
Progressive decrease in CO thereafter (halved at 28°C).
ArrhythmiasProgressive bradycardia and increasing degrees of
heart block with increasing degrees of hypothermia.1st degree heart block common <33°C, CHB <20°C.Atrial fibrillation common <34°C, asystole (about 20°C) Increased ventricular irritability makes ventricular
fibrillation a risk from about 28°C.
Effects of HypothermiaRespiratory System
Initial tachypnoea in mild hypothermia, followed by a reduction in minute volume and reduced oxygen consumption.
Bronchospasm and bronchorrhoea. Protective airway reflexes are reduced
predisposing to aspiration pneumonia. Loss of aensitivity to pCO2 stimulation below
34°C; hypoxic drive is maintained to deeper levels of hypothermia.
Respiratory drive ceases at about 24°C
Effects of HypothermiaGastrointestinal system
Decreased GI motility <34°C, Ileus <28°C. Spontaneous gastric ulceratio nAcute pancreatitis Impaired liver function
Effects of HypothermiaNeuromuscular changes
Increased preshivering muscle tone, followed by shivering induced thermogenesis and ataxia in mild hypothermia.
Followed by hyporeflexia, diminished shivering induced thermogenesis and rigidity.
In severe hypothermia, there is decreased nerve conduction velocity and peripheral areflexia.
Effects of HypothermiaRenal
Initial cold induced diuresis (shunting of blood from peripheries to centre)
Followed by decreased RBF and GFR due to decreased cardiac output
Impaired renal function
Effects of HypothermiaMetabolic:
Hyperglycaemia due to multiple causes (decreased insulin release due to corticosteroids and direct cooling on islet of Langerhan cells, increased insulin resistance of peripheral tissues, and increased GNG)
Decreased metabolic rate ~50% by 28°C
Effects of HypothermiaHaematological System
Impaired oxygen unloadingHaemoglobin has an increased affinity for oxygen
leading to decreased oxygen availability, however oxygen solubility is increased (Henry's Law) though this effect is not clinically relevant -. This is balanced to some degree by the resultant lactic acidosis.
In severe hypothermia, acidosis is frequently profound, so that there is an overall right-shift to the ODC.
Increase in blood viscosity. Inhibition of coagulation factor activity and
platelet function (clinically significant at <34°C)
Preventing Hypothermia Adjusting ambient temperature
Airway heating/humidification
Warmed IV fluids
Passive insulation Blanket Wrap
Active warming Forced air mattress – works Circulating water mattress – minimally effective. Overhead infra red warming devices.
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