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




Central ProcessorAnterior hypothalamus:

sensitive to heat/warm blood, effects cooling

Posterior hypothalamus: sensitive to cold afferent from peripheral receptors,

effects warming




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.


Regional: Regional vasodilation. Decreased physical activity. Cold OT Drop in temperature not as exaggerated as GA

and GA+RA.


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.