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

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%

Graph

Phase 1

·   Distribution ∝ (T_core- T_periphery) 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

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)

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

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

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

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