Hyperthermia is most commonly associated with heat stroke, although it can occur during the anaesthetic period as a result of numerous catalysts and factors (Pollock, 2009; Thomson et al, 2014). Hyperthermia refers to an increase in core body temperature above ‘normal’ parameters, and differs from pyrexia because of its lack of a physiological trigger (Pollock, 2009; Rigotti and De Vries, 2010). While pyrexia is the body's systemic response to pain, infection, or the inflammatory process, hyperthermia is often caused by external sources (Scales, 2023). It can arise as a result of over-eager peri-anaesthetic warming, or other triggers such as malignant hyperthermia and magnetic resonance imaging (MRI) scanners (Pollock, 2009; Rigotti and De Vries, 2010). Hypothermia is commonly associated with anaesthetic events, although hyperthermia can also occur with equally devastating effects (Pollock, 2009). Normal temperature ranges were outlined in part one of this series (Kershaw, 2023) (Table 1).
Table 1. Normal temperature ranges in small animals (Hall, 2021)
| Species | Temperature range (°C) | Site of temperature measurement |
|---|---|---|
| Dog | 37.2–39.2 | Rectal |
| Cat | 36.7–38.9 | Rectal |
| Rabbit | 37.4–39.6 | Rectal |
Hyperthermia pathophysiology
Commonly a result of an inability to dissipate heat, increased heat production/provision or a combination of both, hyperthermia can have significant deleterious impacts on the body (Thomson et al, 2014). The hypothalamus is the body's thermoregulatory centre, responsible for temperature control. Inhibition of normal hypothalamus function (such as through administration of general anaesthesia) can lead to an inability to thermoregulate (Scales, 2023).
Some disease processes, such as tetanus and prolonged seizure activity, can cause extreme muscle rigidity, leading to hyperthermia. Similarly, obesity, brachycephaly and breeds with particularly thick coats can have an increased susceptibility to hyperthermia (Pollock, 2009; Rigotti and De Vries, 2010; Scales, 2023). The use of rebreathing circuits can potentially exacerbate hyperthermia, as the exothermic reaction between the chemical absorption agent, and exhaled carbon dioxide will warm the anaesthetic gases the patient inhales (Pollock, 2009; Rigotti and De Vries, 2010) (Figure 1). Regardless of the cause, the physiological effects of hyperthermia are predictable and potentially fatal if left unmanaged (Stern, 2019).
Stress-induced hyperthermia
Stress-induced hyperthermia is commonly reported across numerous mammalian species including mice, dogs and sheep (Bouwknecht et al, 2000; Sanger et al, 2011; Thrift et al, 2017). The effects of sustained stress and hyperthermia are also well documented in wildlife and conservation fields with relation to capture myopathy (West et al, 2014). The cause of the stress trigger is likely to vary between individuals and species, so care should be taken to assess the situation at hand (Figure 2).
For example, a visit to the practice may not be considered a high-stress event to most patients, but in those with compromised respiratory systems (such as cases of laryngeal paralysis, tracheal collapse or brachycephalic anatomy), this slight increase in stress can lead to hyperthermia (Figure 3).
Hyperthermia prior to anaesthesia can lead to an increased incidence of hypothermia during and following the anaesthetic event when compared to normothermic patients, as heat will dissipate more quickly from the patient down a steep temperature gradient (Hart et al, 2011; Rauch et al, 2021). For this reason, patients who are pyrexic, hyperthermic or particularly stressed prior to anaesthesia should have their temperature carefully monitored to prevent onset of hypothermia (Hart et al, 2011). While it may seem counter-intuitive to prevent heat loss in anaesthetised, hyperthermic patients, passive warming aids (eg blankets) are often sufficient to reduce the speed of heat loss. As established in the previous article (Kershaw, 2023), hypothermia is better avoided than corrected, and so active warming methods should be initiated once the patient's temperature falls within normal range if the anaesthetic is to continue (Hart et al, 2011).
Malignant hyperthermia
Malignant hyperthermia is a rare genetic disorder of skeletal muscle documented in humans, pigs, cats and dogs (Pollock, 2009; Thompson et al, 2017). Some breeds of dog may be predisposed to the development of malignant hyperthermia, such as Greyhounds, Golden Retrievers and Border Collies (Pollock, 2009). While considered rare, malignant hyperthermia can cause muscle rigidity, hypercapnia, tachycardia, respiratory and metabolic acidosis, and may be fatal if left untreated (Pollock, 2009; Ellinas and Albrecht, 2020). First reported in humans in 1960 with an 80% mortality rate, this has been reduced to less than 5% and genetic testing is now available to identify susceptible individuals (Ellinas and Albrecht, 2019). In human medicine, it has been found that those susceptible to malignant hyperthermia may undergo numerous anaesthetics with no crises, and so predicting the occurrence of malignant hyperthermia events can be precarious, although there is some correlation between the use of halogenated anaesthetic gases and malignant hyperthermia incidences (Ellinas and Albrecht, 2019; Bahaidaran et al, 2023). Because of its rarity, literature relating to malignant hyperthermia in the veterinary field is sparse compared to that in the human medical field, although the pathophysiology is thought to be the same.
Malignant hyperthermia affects calcium release channels, leading to a hypermetabolic crisis that can cause muscle cell disintergration, hyperkalaemia, acidosis, myoglobinuria and an increase in serum creatinine kinase (Bahaidaran et al, 2023). These electrolyte derangements can result in renal failure, arrhythmias, disseminated intravascular coagulation and cardiac arrest, among other complications (Bahaidaran et al, 2023).
Management of malignant hyperthermia involves rapid identification of hyperthermia and the initiation of cooling mechanisms (discussed later in this article). Cessation of anaesthesia is recommended alongside administration of dantrolene sodium intravenously (Pollock, 2009; Rigotti and De Vries, 2010). Dantrolene's mode of action is to uncouple the heat-generating mechanism in muscle, preventing worsening of the condition (Kobayashi et al, 2009). Treatment for associated clinical signs (such as hyperkalaemia or cardiac arrhythmias) may be indicated (Bahaidaran et al, 2023).
Magnetic resonance imaging-related hyperthermia
MRI is an advanced imaging modality used in both human and veterinary medicine (Figure 4). It works by pulsing specific radiofrequencies into the patient's body to deliberately disrupt the alignment of atoms within various tissues (Smith, 2016). Through the use of receiver coils, the energy emitted from the atoms realigning is captured and amplified to form an image of the scanned area (Smith, 2016). Different tissues and pathologies will display differences in atom reactivity, which causes the various shades of black–grey visible in the image, aiding diagnosis (Smith, 2016). The energy generated by these radiofrequency pulsations is transformed into heat in the patient's tissues, which can lead to hyperthermia in large patients or those undergoing prolonged scans (Smith, 2016). Without the use of non-magnetic, MRI-safe thermometers, the patient's temperature when in the scan room will be a total mystery. Patients with obesity, who have very thick coats and/or are undergoing extensive imaging are most susceptible to hyperthermia following MRI (Rigotti and De Vries, 2010).
Inadvertent hyperthermia
While there is limited literature detailing the frequency of inadvertent hyperthermia in veterinary medicine, the consequences are equally as life threatening if left untreated. Once a patient has been moved into the recovery area, careful post-operative monitoring should continue to avoid accidental overheating when using active warming aids (Scales, 2023). Brachycephalic patients, those with abnormal respiratory pathology, those with obesity and any other patients considered at particular risk of hyperthermia should have active heating aids withdrawn shortly before achieving normothermia so as not to initiate panting, laryngeal swelling and potentially instigate hyperthermia (Scales and Clancy, 2020). Ensuring adequate staffing levels can also assist in the prevention of inadvertent hyperthermia, as a clinical member of staff will be assigned to monitoring patient recoveries, allowing early identification of hyperthermia signs. This is a key factor in improving patient safety, as patient mortality risk significantly increases in the recovery period, with almost half of all peri-anaesthetic deaths occurring within 3 hours of anaesthetic discontinuation (Brodbelt et al, 2007; Scales, 2023).
Impacts of hyperthermia
Because of an increase in blood–gas solubility and metabolic rate, hyperthermic patients often have a higher minimum alveolar concentration of anaesthetic gases (Clancy, 2023). However, increasing the volatile agent to compensate for hyperthermia will also increase dose-dependent side effects associated with that agent, including vasodilation, bradycardia and respiratory depression (Clancy, 2023).
Hyperthermia has been shown to prolong clotting times, exposing the patient to risk of coagulopathy, microthrombosis and necrosis of multiple major organ systems (Reynolds et al, 2008; Stern, 2019). Some studies show that these hyperthermia-induced haemostatic derangements are not corrected by returning the body to a normothermic state once the body has reached 42°C or above, meaning that the risk of clotting disorders and disseminated intravascular coagulation are not diminished just because the patient's hyperthermia is corrected (Bruchim et al, 2017).
In dogs experiencing heat stroke, neurological derangements have been reported. This significant clinical sign may not be noticeable under anaesthesia, highlighting the importance of temperature monitoring (Hall et al, 2020). Similarly, patients under anaesthesia cannot pant to reduce body temperature as this thermoregulatory ability is impaired, although an increased respiratory rate may be seen as a response to noxious stimuli (Grubb et al, 2020). Bacterial translocation from the intestinal tract into the blood stream has also been reported as a result of heat-related illness, potentially increasing the risk of systemic inflammatory response syndrome (SIRS), sepsis and multiple organ dysfunction (Pollock, 2009; Bruchim et al, 2017).
Cooling and prevention methods
Once identified, hyperthermia should be addressed and any secondary effects treated. Pollock (2009) discussed that hyperthermia should be corrected once the patient's temperature reaches at least 2°C above normal parameters. Much like hypothermia, hyperthermia is best prevented rather than treated, therefore it is recommended that action be taken prior to the 2°C above normal parameter threshold being reached. The action(s) taken may vary depending on the patient's level of consciousness, the nurse's access to the patient, the stage and type of procedure and the underlying cause (Scales, 2023).
Diligent temperature monitoring is the best hyperthermia prevention method, as active cooling in a normothermic patient is not recommended owing to the increased risk of hypothermia. Once an anaesthetised patient's temperature begins to reach the upper limit that is considered normal for their species, active heating aids should be removed (Pollock, 2009). The use of rebreathing circuits and low flow anaesthesia may contribute to heat retention, therefore increasing oxygen flow rates and considering switching to a non-rebreathing circuit can help to dissipate heat (Pollock, 2009; Scales, 2023). Vasodilatory drugs such as acepromazine may be prescribed by the veterinary surgeon, although care should be taken in cases of malignant hyperthermia (Pollock, 2009; Rigotti and De Vries, 2010).
Administration of fluid therapy at room temperature can help to effectively decrease a patient's temperature, though the administration of cold fluids is not recommended because of the risk of shock (Pollock, 2009; Clancy, 2023). If hyperthermia is recognised intraoperatively, room temperature fluids can be used to perform an abdominal or thoracic lavage, although this method relies on the use of a suction unit to remove the fluid, which is not something every practice has access to (Ellinas and Albrecht, 2020). Bathing the patient or hosing them with cool (not cold) water can also help to decrease their core body temperature, but this requires full access to the patient's body, potentially compromising sterility, so cannot be used during surgery (Hall and Carter, 2016). Wrapping or draping wet towels over the patient is not recommended, as these will trap heat between the covering and the patient (Hall and Carter, 2016).
Administration of an enema using room-temperature water is often easily carried out in anaesthetised patients via a Foley catheter, and because it is a highly vascularised area, it is rapidly effective when used in conjunction with other methods (West et al, 2014; Chaitanya and Rajesh, 2015).
Clipping hair away from large blood vessels, such as the femoral artery, jugular veins and abdominal surface, can prevent heat being retained in fur and allow better contact for cooling methods such as sponging with cool water (Hall and Carter, 2016). Encouragement of heat loss through methods such as evaporation can be facilitated by application of surgical spirit (>70% ethanol) onto the patient's digital pads, where sweat glands are located (Pollock, 2009; Rigotti and De Vries, 2010).
Conclusions
While hyperthermia is a rare complication of anaesthesia, veterinary nurses should be well versed in its identification, treatment and prevention. Hyperthermia can be catalysed and exacerbated by a plethora of factors, all of which can be prevented, managed and identified, by the veterinary nurse. Temperature management under anaesthesia can often be overlooked and its impacts underestimated, when it is in fact one of the most important considerations to ensure patient safety and comfort during their hospitalisation.
KEY POINTS
- Hyperthermia can occur during the anaesthetic period, although there is limited veterinary literature on its management and prevention.
- Hyperthermia can lead to metabolic acidosis, coagulopathies, systemic inflammatory response syndrome and cardiorespiratory arrest.
- Hyperthermia can be caused by overzealous perianaesthetic warming, magnetic resonance imaging, or malignant hyperthermia.
- Careful temperature monitoring throughout the anaesthetic period can help to avoid and prevent hyperthermic events.
- Brachycephalic patients or those with respiratory compromise are at particular risk of the impacts of hyperthermia.