Temperature
Definitions
Heat
Total amount of kinetic energy of the molecules of a substance.
Temperature
Average level of kinetic energy. (Iceberg = more heat energy; hot cup of tea = greater temperature.)
SI unit is the Kelvin.
Kelvin and centigrade have the same magnitude.
1 degree C = 273.15K.
Heat capacity
Amount of heat required to ↑ the temp of a substance by 1K.
Specific if 1kg of substance – J/kg/K.
Specific latent heat
Amount of heat required to convert 1kg of a substance from 1 phase to another at a given temp.
Measurement
Non-electrical:
Liquid thermometer
Liquid expands as temp ↑. Mercury or alcohol (low temp)
Bimetalic strip
Coiled and connected to a pointer
Gas expansion thermometers
Bourdon gauge calibrated for temp – constant volume hydrogen thermometer gold standard
Liquid crystal
Electrical
Thermistor
Metal oxide semiconductor.
Resistance falls exponentially with ↑ temp.
Robust and small.
Thermocouple
Seebeck effect.
Voltage produced at junction of 2 different conductors, the magnitude of which is proportional to temperature.
Measuring and reference junction maintained at different temps.
Resistance thermometer
Resistance linearly proportional to temp.
Infra-red
Wavelength of emited IR varies with temperature.
Generates an electrical output in the sensor proportional to the temp.
Tympanic membrane.
Fast.
Poor accuracy (slightly better than axillary).
Heat loss
Mechanisms
3 phases
pastedGraphic
Redistribution Phase
This accounts for the largest drop in core temperature of the three stages. Vasodilatation causes redistribution of heat from core to periphery. Body heat content remains unchanged.
Reduction of 1-1.5 within 30-45mins
Linear Phase
This begins at the start of surgery as the patient is exposed to cold cleaning fluids and cool air flow in the theatre. Heat loss exceeds heat production and most surgery does not extend past the linear phase.
More gradual reduction in core temp of 1 over next 2-3h
Radiation contributes the most (50%) and is proportional to environment/core, temperature difference to the power of four.
Convection contributes up to 30% and is proportional to air velocity and area exposed
Conduction contributes up to 5% and is proportional to the difference in surface temperatures
Evaporation contributes up to 15% and occurs from cleaning fluids, skin, respiratory and wound. A laparotomy can contribute up to 50% of the total.
Respiratory contributes 10% (8% evaporation of water; 2% heating of air) and is enhanced by the cooling effect of cold anaesthetic gases.
Cold intravenous fluids – conduction/convection
Plateau Phase
Once core temperature falls below the thermoregulatory threshold, peripheral vasoconstriction increases and acts to limit the heat loss from the core department. When core heat production = heat loss to the peripheral compartment, core temperature reaches a plateau. Patients with an autonomic neuropathy (diabetics) have impaired sympathetic vasoconstriction and are unable to establish a core plateau in phase 3. Combined general and regional anaesthesia will have a similar effect as the regional anaesthetic (spinal/epidural) will prevent vasoconstriction in the legs; i.e. failure to establish a core plateau.
Effects of mild hypothermia
Evidence based (for ref see http://www.biomedcentral.com/content/pdf/1471-2253-5-7.pdf )
CVS
Arrhythmias
Myocardial ischaemia
↓ CO
Shivering -↑ cardiac and systemic energy demand → ↑ O2 consumption and CO2 production and an ↑ in cardiac work
↓ O2 delivery (L shift, ↓CO, ↑viscosity)
CNS
Confusion <35
Progressive deterioration in mental function to the point at which EEG will show no activity
Haem
Impaired coagulation
↑ viscosity
Host defence
Impaired immunity → ↑ surgical wound infection
Metabolic
O2 consumption ↓
Enhanced anaesthetic drug effects (↑ NMB action)
↓ metabolic rate and enzyme reactions
Other
↑ recovery room and hospital stays
Temperature regulation
Receptors
Central – hypothalamus, brainstem, SC
Peripheral – abdo viscera, skin, skeletal muscle. Aδ and C fibres
Controller
Hypothalamus – efferents via symp
Effectors
Symp - vessels, sweat glands, piloerectors, shivering
Behavioural
Endocrine – T3, adrenaline
Rewarming
Passive (prevention of heat loss)
Remove wet clothes
Blankets
Temp ↑ 0.5/h
Active
Peripheral
FAW
Hot water bottles
Note peripheral vasodilatation can lead to further heat loss and CVS collapse
Central
Warm IV fluids (max 40)
Warmed inspired gases (max 45)
Warm irrigation (max 40) - NG, bladder, pleural cavity
Intravascular warming catheter (warms blood as it goes past)
Haemofiltration (2h to get from 28-34)
Cardiopulmonary bypass (by far the most effective – 15mins)