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The Big Chill - Temperature Management in Sedated and Anaesthetised Patients

What is hypothermia?

Hypothermia is reduced body temperature that happens when a body dissipates more heat than it generates. In humans, it is defined as a body core temperature below 35.0°C (95.0°F)1. In animals, the line is less clear and there is a gradation, but it has generally been accepted that hypothermia starts at a core temperature of <36°C for dogs and cats. However, recent studies are showing that the effects of hypothermia may start at even higher temperatures2

Clinical relevance

In this article we will explore accidental hypothermia that may occur in the process of performing procedures within veterinary practice

In recent years, in specially selected cases, and when performed in a very deliberate and precise manner, the induction of hypothermia has been used to aid patient recovery by controlling the physiological and biochemical changes in the body. This article does not explore this scenario, as it is one that requires specialist knowledge and equipment.

The effects of hypothermia are very far reaching, from the moment that the premedication is given to the extended recovery period.

Premedicant drugs designed to decrease anxiety in patients and to make them more amenable to handling have several effects that make animals prone to hypothermia. 

  • They decrease muscular activity, leading to less heat production. Patients are also generally starved, resulting in less heat being generated from digestion. 
  • They have direct effects on the hypothalamus lowering the hypothermic threshold. 
  • They may lead to peripheral vasodilation potentiating heat loss via the surface– see mechanisms of cooling. 
  • The normal protective behaviours of the patient may also be lessened and they may place themselves in hypothermia inducing situations e.g. off the bedding on to the metal cage floor, or their water bowl. 

It is therefore important to ensure that patients are not over premedicated. A lot of confusion occurs around the distinction between a sedation level sufficient for anaesthetic induction and the level of sedation that is used without general anaesthesia, the benefits of premedicant sedation are seen at far lower doses than those commonly required in animals where general anaesthesia is not used (sedation only).

Drugs used for induction and maintenance of anaesthesia may also potentiate the hypothermic effects of premedicant drugs.

Patients should also be monitored through the whole experience, right from premedication through to complete recovery. Hypothermia is a contributing factor to prolonged recovery and possibly the high rates of pre-operative mortality recorded in the post operative period.3

What are the effects of hypothermia?

1. Decreased blood flow

Initially this is more pronounced in the peripheral organs (eg. muscle and gut) but if hypothermia gets more severe, the vital organs (like hear, brain and kidneys) become compromised.


Figure 1.-Blood flow to the organs reduces with progressive hypothermia. The shaded area is total flow. The heart, liver and brain maintain a higher level of flow in comparison with the kidneys to 27°C. However, at 25°C to 20°C renal and myocardial blood flows are sustained at 20% to 25% of normothermic levels whereas cerebral and hepatic blood flow show greater decreases. (Reproduced from Clinical Hypothermia by E. Blair. Copyright [1964, McGraw-Hill]

This can result in a number of clinical problems:

  • Difficulty reading pulse oximeter with low peripheral blood flow - the pulse oximeter has difficulty reading and may not read accurately (this has contributed to the belief that pulse oximeter machines are not reliable).
  • Reduced kidney function - drug clearance may be delayed, toxic effects of some drugs may be enhanced and the effects of hypoxia on the kidney tubules may be enhanced.
  • Blood pressure changes, initially increases  (catecholamine release and increased vasomotor tone) followed by a decrease as the baroreceptors lose sensitivity.

2. Decreased chemical reactions 

Van't Hoff-Arrhenius's law states that the rate of chemical reaction is doubled for each 10°C rise in temperature or halved for each 10°C fall in temperature, this results in:

  • Decreased oxygen demand - a 6 %fall in oxygen consumption per degree-Celsius fall in body temperature, this is what can be used to help neural or cardiac patients by deliberately inducing controlled hypothermia.
  • Decreased Minimum Alveolar concentration (MAC)- this appears to be a direct effect on the CNS decreasing the amount of anaesthetic that is required (5% per °C for Isoflurane in children5) this means that patients may be too deep with the concurrent cardiac and respiratory depression.
  • Decreased metabolism of drugs6:
      • Increased half life. 
    • Impaired Immune system 7– even mild hypothermia can affect the functioning on the immune system at a biochemical level:
      • Increased healing time
      • Greater risk of infection

Also remember that these effects will combine. A recent study found that hypothermic patients had recoveries not only twice as long but also, much less predictable (more than 4 times as variable as far as length of recovery) than those whose temperatures were normal3.

From the above information, it is clear that hypothermia (other than if it is deliberately and carefully induced for very specific cases) is a bad thing, so what can we do about it?

To understand that, we need to understand how something cools down and how something warms up.

Mechanisms of Cooling

There are 4 ways patients can lose heat convection, radiation, conduction and evaporation, in anaesthetised dogs convection and radiation result in 80% of heat loss4 but in certain circumstances the other two can become very important.

Convection

What is it?

Heat lost to air and its movement away from the body

Where does it occur?

A natural result of animals being warmer than their environment.

Worse in a cold environment or where there is air movement

How do we stop it?

Insulate animals, especially the exposed parts like paws, from the air e.g. with blankets, bubble wrap or foil blankets

If you minimise the difference in temperature between the air and the patient, convection is kept to a minimum, this is the best way to prevent heat loss, especially in smaller animals so make sure your theatre and prep room are warm (some exotics specialists run them at 25°C).

Radiation

What is it?

Heat lost by infrared radiation to environment

Where does it occur?

A natural result of animals being warm

How do we stop it?

The silver coating of Foil blankets is specially designed to reflect infrared radiation.

Conduction 

What is it?

Heat loss due to direct contact with a colder object

Where does it occur?

A metal table or kennel 

Cold water flowing over an animal e.g. the dental machine or flush a site

Administration of cold fluids

Cold gas breathed in by the patient (see also evaporation)

How do we stop it?

Insulate patients from cold surfaces e.g. with bedding or bubble wrap

Ensure that water from dental machine or saline from flushed sites is not excessive and drains away from the animal.

Warm all fluids used to body temperature (a baby bottle warmer in theatre/Prep can be very useful in this regard)

Use rebreathing circuits where appropriate or heated breathing systems, use appropriate oxygen flow rates

Evaporation

What is it?

heat loss due to loss of fluid from mucosa/tissue or if patient is wet

Where does it occur?

Surgical sites, especially open abdominal or thoracic surgery

Wet patients

Patient breathing in dry air

How do we stop it?

Wherever possible use rebreathing circuits, or heat/moisture recovery devices, use appropriate oxygen flow rates

Keep patients as dry as possible (e.g. minimise fluid use while prepping, make sure fluid drains away from dental sites or from flushing. Remove water from kennels on premedication)

Preform keyhole surgery where possible and minimise size of open sites

Mechanisms of Warming

You may not be surprised to see 2 of the same headings but this time we will discuss how to use them in a positive manner

Radiation

Infrared heat lamps use this method to heat patients, this can be useful but care must be taken not to overheat and burn the skin

Conduction

This is the most common type of active warming and comes in several forms

Air – Bair huggers blow warmed air all round the patient, gently and efficiently warming providing heat. 

Objects – Hot water bottles, “hot hands” heat pads etc all use this principle to heat area of the patient, they tend to have limited use in larger patients as the surface areas that can be heated tend to be small, this has the added disadvantage that by trying to heat animals from a small surface area the risk of burns increases. Never put a patient in direct contact with a hot object/device, always place something in between, a towel or blanket for example.

Fluids – While it is always a good idea to warm fluid to body temperature before giving, it is more about preventing hypothermia than warming an already cold patient, there have been some recent papers using peritoneal lavage with warm fluid to warm patients8 but this should be considered a last resort.

Bearing all these effects in mind, the best way to prevent hypothermia is to detect it early so that there is less to correct, a rectal or preferably oesophageal probe is often part of a multiparameter monitor and will provide continuous monitoring to have a happy, healthy and warm patient.

Article by
James Tennant

Technical Adviser, Jurox UK

Originally published: Thursday, 16th August 2018

References

  1. Brown, DJ; Brugger, H; Boyd, J; Paal, P (Nov 15, 2012). "Accidental hypothermia". The New England Journal of Medicine. 367 (20): 1930–8.
  2. Pottie R.G. et Al Effect of hypothermia on recovery from general anaesthesia in the dog. Australian Veterinary Journal Volume 85, No 4, April 2007
  3. Brodbelt DC et al, Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF) Veterinary Anaesthesia and Analgesia, 2008, 35, 365–373 
  4. Franklin et Al 2012 JAHAA
  5. Mingzheng et al , The Effect of Hypothermia on Isoflurane MAC in Children,  Anesthesiology 3 2001, Vol.94, 429-432. 
  6. Leslie K et al ,Mild Hypothermia Alters Propofol Pharmacokinetics and Increases the Duration of Action of Atracurium, Anesthesia & Analgesia: May 1995 - Volume 80 - Issue 5 - p 1007-1014
  7. R. Armstrong DVM, Stacy & Brian K. Roberts DVM, DACVECC & Michael Aronsohn VMD, DACVS. (2005). Perioperative hypothermia. Journal of Veterinary Emergency and Critical Care. 15. 32 - 37. 10.1111/j.1476-4431.2005.04033.x.
  8. D. C. Barnes et al, Effect of peritoneal lavage solution temperature on body temperature in anaesthetised cats and small dogs, 

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