Heat and Temperature

Heat and Temperature

Take home messages

  • Your patient will start losing heat the second they're anaesthetised
  • More than 30 minutes and you're going to need active warming measures
  • Anaesthesia resets the hypothalamic set point to around 34°C

Chill out

"Patient temperature?"

"Hang on ..." - beep - "Thirty-six seven - we don't need a warmer do we?"

This is one of those little things that winds me up way more than it should. I get really picky about making sure my patients are warm enough, because it's just not that well understood, and the consequences can be really bad for the patient.

Two definitions to start us off

  • Heat is the kinetic energy of a substance's molecules
  • Temperature is a thermal status of a substance, and whether it is likely to transfer heat energy to, or receive heat energy from, its surroundings

A hot frying pan placed onto an iceberg will melt the ice it touches, because the pan is at a higher temperature than the iceberg, and therefore will transfer heat energy to the ice it touches.

However overall, the enormous size of the iceberg as a whole will mean it has more heat energy than the pan - because even though each molecule has less energy, there are so many more molecules that the total energy is higher. 

Your patient starts losing heat the second they go under.

Your body works really rather hard to maintain a steady core temperature, which is no mean feat when you realise it's literally fighting one of the fundamental tenets of the universe:

  • The second law of thermodynamics

Which brings us neatly to our first OSCE question:

What are the four laws of thermodynamics?

Zero-th law

  • If two systems X and Y are both in thermal equilibrium with a third system, Z, then X and Y must be in equilibrium with each other

First Law

  • In a closed, perfect system, the total amount of internal energy remains constant

Second Law

  • In a natural thermodynamic process, entropy increases
  • (stuff gets colder unless you do something about it)

Third Law

  • Entropy tends to a constant as temperature decreases to absolute zero

How does the body generate heat?

The body is trying to keep itself at around 36.5-37.5°C and to do this it has four main tricks up its sleeve:

  • Basal metabolic rate
  • Heat generated by digestion
  • Muscle activity
  • Heat seeking behaviour (e.g. putting on clothes)
All four of these are disrupted by anaesthesia.

The second the patient is anaesthetised, the body loses heat in a triphasic pattern. Firstly, there is a redistribution phase as heat is transmitted from the core to the periphery, due to loss of vasomotor tone.

Secondly, there is a steady drop in core temperature as heat is lost to the surrounding environment.

Finally, the temperature plateaus at the new set point.

You can see the graph in this useful tweet:

What are the main routes of heat loss in the anaesthetised patient?

There are five routes through which heat energy can be lost to the surrounding environment in an anaesthetised patient.


  • 40%
  • Heat loss by infrared radiation to the environment, mainly determined by the difference in temperature between patient and environment


  • 30%
  • Heat loss to the air in contact with the skin which is then carried away and replaced by cold air


  • 15%
  • Latent heat of vaporisation from mucous membranes and exposed body surfaces
  • Ever wondered why a hand dryer always feels cold for a few seconds before it starts to warm up - it's the latent heat of vaporisation in action - all that heat energy needed to evaporate the water is being taken from your skin
  • This is going to be much greater in a laparotomy than a laparoscopy


  • 10%
  • 8% due to the energy required to humidify the air entering the respiratory tract
  • 2% to warm the air
  • This is why we have heat and moisture exchange (HME) filters


  • 5%
  • Loss of heat by direct contact
  • IV fluids
  • Operating table

Why is heat loss an issue?

As you might expect of an organism that depends on tightly controlled homeostatic conditions for optimal protein and enzyme function - hypothermia has several deleterious effects:

What are the effects of hypothermia?

  • Impaired cognition and delirium
  • Arrhythmias and reduced cardiac myocyte function
  • Vasoconstriction, increased systemic vascular resistance and hypoperfusion to skin
  • Delayed drug clearance
  • Wound infection and delayed healing
  • Oxygen dissociation curve shifts left
  • Bleeding and coagulopathy
  • Increased blood viscosity and DVT risk
  • Discomfort
  • Hyperkalaemia and hyperglycaemia
  • Shivering and increased energy expenditure

Delayed wake up

Cold patients wake up more slowly, in part because of slowed metabolism of drugs.

A drop of 3°C increases plasma concentration of propofol by 30%

Prolonged paralysis

Vecuronium lasts twice as long in patients with a core temperature of 35°C or lower.

How does the body modulate heat loss?

Generally speaking the (alive) human body is at a higher temperature than its surroundings, meaning there will be continuous loss of heat energy regardless of what it does.

However it can slow down the rate of heat loss:

  • Reduced distribution of blood to the skin - vasoconstriction
  • Countercurrent exchange - warm arterial blood transfers heat to veins returning to the core
  • Heat seeking behaviour - putting on more clothes or moving to a warmer environment

The body can then try to generate more heat.

Much like the heavily contested thermostat in the family home, the hypothalamus has a ‘set point’ for core temperature.

To help it do this, it has peripheral and central temperature receptors distributed throughout the body.

  • Cold receptors fire between 10 and 36°C
  • Warm receptors fire between 30 and 45°C
  • Neurons rapidly adapt to any temperature change between 20-40°C
  • The anterior hypothalamus coordinates the incoming information, and the posterior hypothalamus compares it to the set point or threshold temperature, whereupon it triggers increased heat production if needed

How does the posterior hypothalamus increase heat production?

This zone detects a drop in temperature (via peripheral cold sensors) and triggers:

  • Vasoconstriction - at around 36.5°C
  • Shivering - usually at around 36.0°C
  • Chemical thermogenesis (a.k.a. non-shivering thermogenesis)

The sympathetic system can essentially uncouple oxidative phosphorylation to allow for increased heat production without generating too much ATP. This mainly occurs in brown fat, which is more abundant in neonates and children than adults.

The new set point

Anaesthetic agents have all sorts of wonderful wacky effects, only some of which we genuinely understand.

One thing that seems consistent is the effect on the hypothalamic set point - i.e. the temperature to which the hypothalamus will aim to keep core body temperature - anaesthetic agents seem to lower this to around 34°C

It's sort of the opposite to the pyrexia seen in bacteraemic sepsis, where bacterial pyrogens cause IL-1 release, which increases the desired set point, to conserve heat and produce a fever.

Anaesthesia desensitises the body's thermoregulatory systems, meaning these responses don't kick in until a much lower temperature, and if you've paralysed the patient's NMJ, they're not going to shiver at all.

Not only does anaesthesia reduce vasomotor tone, and increase distribution of heat to the peripheries where it can be lost more easily, it then doubles down by reducing metabolic rate as well, meaning less heat is generated in the first place.

Oh, and your patient can't exactly pull on a sweater.

What about regional anaesthesia?

When you inject a spinal anaesthetic, or an epidural, you essentially give the patient a chemical sympathectomy, removing their vasomotor tone and facilitating redistribution of warm blood to the skin.

One of the signs of an effective epidural is warm feet.

This effect depends on the site of local anaesthetic administration - clearly the smaller your affected area or block, the less effect the anaesthetic is going to have on the body's core temperature - an ankle block isn't going to have the same impact as a spinal.

Weirdly, epidural analgesia in labour can actually increase the patient’s temperature, and this is thought to be due to loss of active vasodilatation which is parasympathetically mediated, combined with the increased metabolic rate associated with the exertion of labour itself.

How we can keep the patient warm

We can do three things in our quest to avoid the dreaded perioperative hypothermia:

  • Try and reduce heat loss in the first place
  • Warm the patient's peripheral compartment
  • Warm the patient's central compartment


A cold orthopaedic operating theatre with laminar flow will result in dramatic convective heat loss, which can be reduced by a blanket. Interestingly, multiple blankets doesn't help much over and above a single layer.

Fluid warmers

A simple way to reduce heat loss is to avoid administering anything cold to the patient, especially if you're giving large volumes of fluid. Technically speaking, if you're rapidly infusing enormous volumes then this can actively warm the patient, however in normal scenarios a fluid warmer simply reduces conductive heat loss to the IV fluids.

A litre of cold IV fluid, or one cold pack of red cells, can reduce the core temperature by up to 0.25°C

What methods are there of keeping patients warm?

Ambient temperature

  • Target ambient temperature is 22°C–24°C and humidity 50-60%

Blankets and mattresses

  • Cheap and easy to use
  • Rely on patient's own heat generation 

Forced air warmers (FAWs)

  • Use convection to prevent and treat hypothermia
  • In children and neonates it is best done by placing the patient on the blanket and forcing the warm air up around the baby
  • Complications include burns and pressure ulcers

Electrical blankets and heated pads

  • Can interfere with other monitoring equipment

Water-filled mattresses/blankets

Fluid warming devices

  • These should be used if more than 500ml of fluid are to be given, and can be divided into pre-warming and warming during administration
  • Counter-current heat exchange systems (good for low-flow, e.g. in neonates)

Invasive methods

  • Warmed irrigation of cavities (e.g. peritoneal) and organs (e.g. stomach, bladder)
  • ECMO and CPB (cardiopulmonary bypass)

A bit more about HME filters

These cheap, single use little antimicrobial filters are part of the furniture in your breathing system, because they're very effective. They can achieve a relative humidity of 60-70%, and a temperature of between 28°C and 35°C.

How they work:

  • 15mm and 22mm ports ensure correct-way-round connection
  • Some also have a little port for end tidal gas sampling
  • The filter is 0.2 microns and hygrophobic, made of cellulose, paper, or some other sort of microscopic fibre
  • The patient exhales warm, humid gas through the filter
  • Warm water condenses onto the filter
  • The patient then inhales fresh gas, which picks the warm vapour up on the way through the filter

A couple of drawbacks 

  • It takes around 20 minutes for the HME filter to reach maximum efficiency
  • They can get plugged up with mucus and secretions
  • They should be changed after 24 hours
  • Each filter will have a tidal volume written on the side - the correct type needs to be selected for the patient to ensure the extra dead space won't impact on ventilation
  • Inspiratory and expiratory flow rates affect how long the gas spends moving through the filter, and therefore how long the filter has to warm and humidify the gas mixture
  • A larger filter is more effective but has a greater deadspace volume


  • PC_BK_10 Heat: including temperature, absolute zero
  • PC_BK_11 Heat transfer and loss: conduction, convection, radiation, evaporation
  • PC_BK_12 Temperature measurement: including Hg, alcohol, infrared, thermistor, thermocouple, Bourdon gauge, liquid crystal. Anatomical sites used for measurement
  • PC_BK_13 Latent heats, triple point of water
  • PC_BK_14 Patient warming systems: principles
  • PC_BK_15 Warming equipment for intravenous fluids: principles

Useful Resources

References and Further Reading

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