Author: Philip Judge BVSc, MVS, PG Cert Vet Stud, MACVSc (Emergency and Critical Care, Medicine of Dogs)
Inadvertent perioperative hypothermia is a relatively common occurrence with both general and regional anesthesia and can have significant adverse impact on patients’ outcome1,2. While guidelines for perioperative temperature management have been proposed, there are no specific guidelines regarding the best site or best modality of temperature monitoring and management intraoperatively.
Almost all our small animal patients will develop hypothermia under anaesthesia,3,4. One study of 1525 dogs showed that 83.6% presented with hypothermia3. Another study in cats found that 96.7% of cats suffered hypothermia whilst under anaesthesia4.
Hypothermia can have devastating effects for our patients. It can affect the cardiovascular system, depressing heart conductivity, which lowers heart rate and cardiac output that in turn leads to a decrease in arterial blood pressure. Patients that are cold are also at a greater risk of developing cardiac arrhythmias, prolonged blood clotting times, and hypoventilation5.
The Physiology of Thermoregulation and the Effect of Anaesthesia
Temperature of the core compartment of the body (head and thorax) is normally around 38°C. The periphery is typically 2°C–4°C cooler than the core. Thermoregulation is the mechanism by which the hypothalamus regulates body temperature at a stable level. Thermoregulation is based on multiple signals coming from nearly every type of tissue1,2.
The hypothalamus is the primary central nervous system structure regulating temperature. It integrates thermal input from tissues (via afferent nerves) and activates effector mechanisms (via efferent nerves), which normalize temperature by altering metabolic heat production and environmental heat loss. Each effector mechanism has its own threshold, and there is orderly progression of responses, with the more energy efficient responses such as vasoconstriction occurring before metabolically costly responses such as shivering are activated. It is important to note that efferent responses include both behavioral and autonomic regulation. Behavioral regulation is the most powerful mechanism of patient warming – but requires conscious perception of body temperature – something that is lost under general anaesthesia1,2.
Because behavioural regulation of body temperature ceases under general anaesthesia, the patient must rely on autonomic defenses (blood pressure maintenance, vasoconstriction, etc.) and external thermal management for thermoregulation. However, as with behavioural responses, autonomic responses are markedly depressed or impaired under anaesthesia. Table 1 shows the effects of various anaesthetic agents on thermoregulatory control. Note that local anaesthetics will reduce vasoconstriction in the regions to which they are applied also and can accelerate heat loss5.
|Anaesthetic Agent||Anaesthetic Agent|
Hypothermia during general anaesthesia occurs from a combination of factors, including1,2,6:
- Reduced metabolic rate – up to 20-30%
- Exposure to a cold environment
- Convective heat loss into a cold atmosphere
- Conductive heat loss onto cold surfaces
- Evaporative loss from body tissues and cavities opened to the air, and via anaesthetic circuit
- Heat loss through infrared radiation
As described above, anaesthesia not only contributes to heat loss via the aforementioned mechanisms, but also decreases or suppresses behavioural and physiological mechanisms that would normally be employed to preserve body heat, or generate body heat, e.g., shivering, vasoconstriction, movement, etc1,2.
The following table details potential causes of hypothermia during anaesthesia. An understanding of these factors can contribute to the development of an effective management plan to reduce or prevent hypothermia from developing in any given patient1,2.
|Pre-operative Environment||– Duration of anaesthesia|
– Cold surgical site preparation solutions
– Alcohol application to patient skin
|Theatre Environment||– Operating theatre temperature|
– Exposure of body surface
– Cold surgical irrigation fluids
– Cold intravenous fluids and blood products
|Anaesthesia||– Vasodilatation – redistribution of body heat from core to periphery|
– Inhalant anaesthetics – impair thermoregulatory vasoconstriction
– Muscle relaxants – impair shivering
– Intravenous agents – (midazolam, sufentanyl, dexmedetomidine) lower shivering threshold
– Regional analgesia – vasodilatation occurs in local anaesthetic-affected tissues
|Surgical Technique||– Exposure of body tissues to sub-physiological temperatures|
– Duration of surgery
The Incidence of Hypothermia
In humans, the incidence of hypothermia in anaesthesia varies between 6 and 90% – depending on the surgical population1. Regardless, the risk of hypothermia increases with:
- Extremes of age
- Prolonged surgery
- Lower pre-operative temperature
- Severe trauma
- Intra-operative blood loss
Again, this is valuable knowledge, in that it provides information that can aid in development of strategies to minimise anaesthesia-induced hypothermia
The Effects of Hypothermia
Hypothermia has numerous effects on the body, including2:
- Prolonging the effects of anaesthetic drugs
- Impaired coagulation and platelet activity
- Increased requirement for transfusions
- Increased wound infection rate
- Increased risk of decubital ulcers
- Increased circulating catecholamine levels
- Impaired microvascular circulation
- Cardiac arrhythmias
Additionally, in the post-anaesthetic period, shivering can increase tissue oxygen demand by up to 100% over basal requirements, along with numerous other effects, including increased wound pain and intra-cranial pressures. Prevention of shivering in the post-anaesthetic period, therefore, can have positive impacts on patient recovery2.
The following table summarises the adverse effects of hypothermia2
|Temperature (degrees Celsius)||Adverse Effects|
|32-37||Shivering (impaired in anaesthesia)|
Peripheral vasoconstriction (impaired in anaesthesia)
Prolonged clotting times
Impaired platelet function
|<34||ECG: Increased P-R; Wide QRS; Increased Q-T interval|
|31-32||Depressed consciousness, lethargy and coma|
Marked decrease in metabolism (“hibernation”)
|28-31||Cardiac arrhythmias (atrial fibrillation etc.)|
Prevention and Management of Anaesthetic-Induced Hypothermia
There have been numerous guidelines published in human medicine regarding recommendations for monitoring, and providing active patient warming1. The guidelines, which are readily applied to most small animal veterinary patients, fall into three (3) time periods during the anaesthetic period: The pre-operative period, the anaesthetic period, and the anaesthetic recovery period (up to 24 hrs following anaesthesia).
- Pre-anaesthetic patient warming: skin surface warming before induction of anaesthesia increases peripheral tissue temperature, and total body heat content, without significantly increasing core temperature. This increase in peripheral tissue temperature reduces the gradient for temperature loss between core and peripheral tissues, maintaining core temperature for longer. Skin surface warming for as little as 30 minutes, and up to 60 minutes prior to induction of anaesthesia has been shown to reduce the extent of hypothermia, and is considered both safe and effective in several studies1,2,7.
- The anaesthetic period1,2,8.9.
- Airway heating and humidification: there are several techniques for warming and humidifying anaesthetic gases, including:
- Low-flow anaesthesia: low-flow anaesthesia utilizes closed, or semi-closed exhalation valves on the anaesthetic circuit to recycle warmed, humidified, dead-space anaesthetic gas. This reduces the requirement for high fresh-gas flow rates and can reduce evaporative heat loss from the respiratory tract. Fresh gas flow rates recommended are 30ml/kg for rebreathing circuits, and 200ml/kg for non-rebreathing circuits. Higher flow rates are not only wasteful, but are colder.
- Airway heating and humidifiers: several devices have been developed to warm and humidify inspired anaesthetic gases. These are of greatest benefit in very small patients on non-rebreathing circuits, with high fresh-gas flows.
- Warming intravenous fluids: administration of cold/room-air intravenous fluids causes significant heat loss. One litre of crystalloid solution at room temperature administered to a human, can reduce mean body temperature by 0.25 degrees Celsius. Fluid warming is the only method of patient warming that produces direct core warming, and is recommended for all intra-operative infusions, and lavage solutions. Fluids should be warmed to 37°C; or warmed using an in-line fluid warmer unit for intravenous infusions2.
- Cutaneous warming: Cutaneous warming can result in significant reductions in body heat loss in the anaesthetic period. A combination of warming methods applied has a greater effect on body temperature maintenance, than one method alone, according to several studies2.
- Operating room temperature: is the most significant factor in determining cutaneous heat losses via radiation, convection and evaporation. Maintaining an operative room (OR) temperature of 25-27°C will lead to minimal heat loss, but is reportedly uncomfortable to staff working in the OR. Most recommendations in the human field support an OR temperature of 23°C, and 50% humidity as a compromise between ideal patient requirement, and OR staff comfort.
- Passive warming: passive warming is insufficient alone to provide temperature support, but can aid in slowing heat loss from the patient, by as much as 30%. Blankets, sleeping bags, space blankets and plastic wraps (bubble wrap) may all be used.
- Active warming: Devices such as water recirculating blankets, radiant heaters, and warm-air body heaters have all been used. Air warming devices such as Bair Huggers are the most commonly used, tested and recommended devices.
- Airway heating and humidification: there are several techniques for warming and humidifying anaesthetic gases, including:
- Post-anaesthesia warming therapy9.
- External warming devices have low efficacy, due to patient peripheral vasoconstriction, and take a long time to achieve effect.
- Fluid warmers are the most effective warming method during the post-anaesthetic period, and can be used in conjunction with external warming devices.
|Fluid warming cabinets/incubators||– Cheap|
|– Fluid cools with low fluid rates||– Fluid bags must be incubated for 12 hrs to ensure uniform warming|
|Counter-current warmers||– No loss of heat even with low flow rates|
– Efficient delivery of warm fluid to patient
– Air embolism
– Dilutional electrolyte disturbances if leaks within coaxial system
|– Non-sterile warm water in countercurrent coaxial system. Leakage of non-sterile fluid possible.|
|Blood warmer||– Haemolysis|
– Electrolyte disturbances
|– Maximum operating temperature is 43°C – possible to cause haemolysis and protein denaturation at this temperature|
|Dry-heat exchange device||– Controlled rate of heating|
– Flow-rate adjusted heating
– No risk of fluid line contamination
|– Widely used with good success|
– Uses aluminum plates to warm fluid close to patient
Enter your email address to download a useful infographic for your practice, on how to avoid hypothermia:
- Díaz M, Becker DE. Thermoregulation: physiological and clinical considerations during sedation and general anesthesia. Anesthesia progress. 2010 Mar 1;57(1):25-33.
- Armstrong SR, Roberts BK, Aronsohn M. Perioperative hypothermia. Journal of Veterinary Emergency and Critical Care. 2005 Mar;15(1):32-7.
- Redondo JI, Suesta P, Serra I, Soler C, Soler G, Gil L, Gómez‐Villamandos RJ. Retrospective study of the prevalence of postanaesthetic hypothermia in dogs. Veterinary Record. 2012 Oct;171(15):374
- Redondo JI, Suesta P, Gil L, Soler G, Serra I, Soler C. Retrospective study of the prevalence of postanaesthetic hypothermia in cats. Veterinary Record. 2012 Feb;170(8):206
- Lenhardt R. The effect of anesthesia on body temperature control. Front Biosci (Schol Ed). 2010 Jun 1;2(June 1):1145-54.
- Tanaka M, Nagasaki G, Nishikawa T. Moderate hypothermia depresses arterial baroreflex control of heart rate during, and delays its recovery after, general anesthesia in humans. The Journal of the American Society of Anesthesiologists. 2001 Jul 1;95(1):51-5.
- Rufiange M, Leung VS, Simpson K, Pang DS. Pre-warming before general anesthesia with isoflurane delays the onset of hypothermia in rats. PloS one. 2020 Mar 3;15(3):e0219722.
- Clark-Price SC, Dossin O, Jones KR, Otto AN, Weng HY. Comparison of three different methods to prevent heat loss in healthy dogs undergoing 90 minutes of general anesthesia. Veterinary anaesthesia and analgesia. 2013 May 1;40(3):280-4.
- Bindu B, Bindra A, Rath G. Temperature management under general anesthesia: Compulsion or option. Journal of anaesthesiology, clinical pharmacology. 2017 Jul;33(3):306.
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