2015A15 Compare and contrast the effects of general and neuraxial anaesthesia on thermoregulation.

 

List:

·       Intro

·       Overall effect of anaesthesia

·       Sensor

·       Controller

·       Behavioural effectors

·       Autonomic effectors: heat conservation/gain

·       Autonomic effectors: heat loss

 

Intro:

Thermo-regulation

·  Tight control of temp ~37°C

·  Heat loss = heat gain except over short period

·  Maintains function of enzymes and ion channels, metabolic rate, blood Viscosity

·  System = sensors + integrator + effectors

·  Dose-dependent impairment of thermoregulation

·  Hypothermia (<36°C) common peri-operatively

Modes of heat loss

·  Radiation 40%

·  Convection 30%

·  Evaporation 15%

·  Respiration 10%

·  Conduction 5%

 

Overall effect of anaesthesia:

Graph

Phase 1

·  Distribution ∝ (Tcore- Tperiphery) at onset of anaesthesia

Phase 2

·  Heat loss > heat production

Phase 3

·  Vasoconstriction constrains heat to the core

·  Peripheral temp continues to fall

·  Note heat loss rate may still exceed heat production rate

Recovery phase

·  Core temp normalizes over 2-5 hours post-op

 

Sensors:

Physiology

·  Ambient temp: skin

o Warm receptors: Ruffini endings via C fibres (active >30°C)

o Cold receptors: Krause bulbs via Aδ fibres (active <25°C)

·  Core temp: spinal cord, deep thorax and abdomen

·  Afferents via spinothalamic tracts

General anaesthesia

·  ↓All afferents

Neuraxial aneasthesia

·  ↓Afferents below block level (loss of heat afferents may be the cause of shivering)

 

Integrator and controller = hypothalamus:

Physiology

·  Receives afferents

·  Regulates set point (single target temperature)

·  Regulates inter-threshold range (range of core temp over which no thermoregulatory response occurs; normally 36.8-37°C)

·  Controls behavioural and autonomic effectors

General anaesthesia

·  ↑Inter-threshold range from 36.8-37.0 (0.2°C) to 34.3 – 38.3 (4°C)

Neuraxial anaesthesia

·  ↑Inter-threshold range ∝ number of segments blocked

·  Max reduction in lower threshold 0.6°C

·  Loss of tonic thermal input from spinal cord receptor)

·  Preserved cold perception, can ask for help

 

Behavioural effectors:

Heat conservation

·  Go to warm place -> ↓temp gradient

·  Put on clothes -> insulation -> ↓convection

·  Curl up -> ↓surface area

Heat loss

·  Rest (↓heat production)

·  Hide from sun (↓radiation gain)

·  Take off clothes (↑convection)

·  Pant (↑evaporation) – unimportant in man

·  Lie on a cold surface (↑conduction)

General anaesthesia

·  Ablated responses

Neuraxial anaesthesia

·  Impulses preserved

·  Ability to respond impaired

 

Autonomic effectors: heat preservation:

Vasoconstriction

·  ↓Skin blood flow and volume -> ↓skin temp -> ↓gradient -> ↓radiation, convection, conduction

Shivering

·  ↑Metabolic rate 2-3x -> ↑heat production

Brown fat metabolism

·  ↑MR 2-3x -> ↑heat production

·  Only significant in neonates

Calorigenic hormones

·  Effect of catecholamines etc. on skeletal muscle, liver

General anaesthesia

·  All impaired

·  Rapid distribution of heat from core to periphery by vasodilation

Neuraxial anaesthesia

·  Responses ablated below block level, even when threshold reached

 

Effectors: heat loss

Vasodilation

·  ↑Skin blood flow and volume -> ↑skin temp -> ↑gradient -> ↑radiation, conduction, convection

Sweating

·  Evaporation -> loss of latent heat

General anaesthesia

·  Both impaired

Neuraxial anaesthesia

·  Both impaired below block level