Peri-operative hypothermia is known to be a common consequence of anaesthesia (Ng et al, 2003), however, there are various other common causes that should always be taken into consideration (Table 1). It is extremely important for the veterinary nurse who may be responsible for perioperative monitoring, to realize that due to its detrimental effects on the patient (Table 2), it is better to prevent hypothermia, than to have to treat it. The condition and means for avoiding it should therefore be thoroughly anticipated and understood.
| Pre-operative | Intra-operative | Post-operative |
|---|---|---|
| Clipping of surgical site | Effects of anaesthesia and drugs on the body | Lack of supervision (patients may thrash and fidget, therefore likely to push away blankets, warming devices etc.) |
| Failure to use a warming device, especially in the first 20 minutes of anaesthesia (Frank, 2001) | ||
| Wetting of surgical site (especially | Prolonged anaesthetic time | Low room temperature in recovery area |
| with cold skin preparations such as alcohol) | Additional medications administered | |
| Failure to pre-warm cages | Cold surface of operating table | |
| Low room temperature in preparation area | Low room temperature in theatre | |
| Absence of warming device during surgical preparation | Exposure of body tissues | |
| Food withheld. Can result in more heat loss due to reduced digestion and gut motility (Murison, 2011) | Administration of cold anaesthetic gases | |
| Administration of cold intravenous fluids | ||
| Predisposing factors | ||
| Increased surface area to mass ratio of small animals: small breed dogs, rabbits, rodents, small cats etc. is an important predisposing factor. Their small size means that they have less insulation and will suffer from more heat loss than a larger animal (Murison, 20011). Age: the thermoregulatory system in young or very old animals may be less developed or undergoing deterioration, resulting in less efficient response to cold compared with other animals (Murison, 2011). | ||
| System | Effect |
|---|---|
| Metabolism | Post-operative shivering increases total body oxygen consumption (average increase 40%, maximum100%) |
| Respiratory | Blunted ventilatory response to CO2 |
| Adrenergic | Activation of sympathetic nervous system 100–500% increase in norepinephrine. Little or no adrenomedullary or adrenocortical response (epinephrine and cortisol unchanged) |
| Cardiovascular | Systemic and pulmonary vasoconstriction |
| Coagulation | Impaired platelet function |
| Immune | Impaired neutrophil and macrophage function |
| Pharmacokinetics | Increased effect of neuromuscular blockers |
The purpose of this article is to focus on the indications, efficacy and risks of forced air warming (FAW) in the prevention of hypothermia by examining studies that have been carried out within the medical and veterinary fields. The benefits of using other warming methods alongside FAW will also discussed.
The efficacy of alternative methods of warming, use of intravenous fluids, methods of temperature measurement and cost effectiveness of FAW are outside the scope of this article.
Hypothermia
What is hypothermia?
An animal is hypothermic when the body temperature drops below normal values (Table 3) for that species (Boag and Nichols, 2008).
| Species | Normal body temperature (°C) |
|---|---|
| Dog | 38.3–38.7 |
| Cat | 38.0–38.5 |
| Rat | 37.6–38.6 |
| Rabbit | 38.5–40.0 |
| Guinea pig | 37.2–39.5 |
| Human | 36.5–37.5 |
Causes of peri-operative hypothermia
The word ‘peri-operative’ refers to the pre-operative, intra-operative and post-operative period in which the animal is undergoing a procedure. The pre-operative stage is where the animal is admitted, examined and prepared for surgery; the intra-operative stage is where the surgery is carried out and the post-operative stage is where the animal is recovering from surgery. There are many factors during this period which can predispose the animal to hypothermia (Table 1).
Failure to avoid the causes of hypothermia in the pre-operative stage will exacerbate susceptibility to hypothermia in the intra-operative stage and in turn will worsen susceptibility in the post-operative stage (personal exerience).
Effects of anaesthesia
Hypothermia occurs in up to 70% of patients that undergo anaesthesia for surgical procedures (Galvao et al, 2009).
Murison (2001) asserts that when a conscious animal is hypothermic, its body carries out a number of natural thermoregulatory responses, however, when under anaesthesia, these normal responses to hypothermia may not occur, or may be delayed (Table 4). Additional support to maintain or increase body temperature is therefore required.
| Increased muscle activity |
| Horripilation |
| Vasoconstriction |
| Behavioural changes |
Thermoregulation
Vasoconstriction and shivering are classed as two of the body’s major thermoregulatory defences against hypothermia. Both of these defences are impaired by the induction of anaesthesia which has a major effect on thermoregulation (Barash et al, 2011). This is due to the depression of the central nervous system caused by the induction of anaesthesia, resulting in the hypothalamus having a decreased sensitivity to the temperature drop (Murison, 2001). Anaesthesia induces generalized vasodilation within the body; this over-rides the physiological vasoconstriction which aids thermoregulation (Cabell et al, 1997).
Why is prevention important?
Over the past few decades in the medical profession, various studies and research have been carried out which have brought to light a large variety of adverse pathophysiological effects that peri-operative hypothermia can have on surgical patients (Kumar et al, 2005). A review of the consequences of hypothermia concluded that hypothermia during surgery can have a detrimental effect on wound healing and can also cause patients to be more susceptible to wound infection (Frank, 2001).
Another complication that can occur is prolonged recovery. One study looked at the effect of hypothermia on recovery from general anaesthesia in the dog (Pottie et al, 2007). The study, which included 69 healthy adult dogs undergoing routine neutering, showed an association between decreasing body temperature and prolonged recovery time (Figure 1). The results suggest that the lower the body temperature becomes, the longer it will take for a patient to recover.
Some of the other consequences of hypothermia include shivering (pre and post operatively) and excessive sympathetic nervous system stimulation, prolongation of coagulation time (causing excessive surgical bleeding), as well as prolonged drug metabolism (Ng et al, 2003) and even morbid cardiac events (Buggy and Crossley, 2000).
It is important to also consider the mental and physical comfort of patients; they cannot always communicate the severity of their discomfort. Frank (2001) established that peri-operative hypothermia in humans causing uncontrollable shivering and an intense cold sensation is often remembered by patients as their worst discomfort throughout the whole procedure. A human study has also shown that lower temperature during surgery can lead to increased infection contraction after the surgical procedure (Galvao, 2010). It is important therefore, that effort is made to prevent animals from suffering the experience of peri-operative hypothermia.
FAW systems
There are many different types of FAW system, made by various manufacturers, but they all function in a similar way. A commonly used FAW system in veterinary practice is ‘The Bair Hugger’. The blanket (Figure 2) is made of tissue paper and plastic, which is bonded into tubular channels. The heating unit (Figure 3) consists of a blower to force the air towards the patient, where it inflates the blanket and exits through slits to exert core warming. A heating element and a micro-processor-based temperature control device are contained within the unit (Marders, 2002).
The systems are easy to set up and usually come with a wide variety of blanket sizes allowing the selection of the most appropriate for the size of the patient.
Indications
FAW is used both in human and veterinary hospitals, not only for the prevention of peri-operative hypothermia, but also for temperature control of hospitalized patients. The method can be used for short procedures, but is particularly useful for procedures that may require a longer anaesthetic time. FAW systems can also be used to warm procedural and recovery areas for small animals such as rabbits, guinea pigs and rodents (Rembert et al, 2004).
Efficacy
A randomized controlled study compared the use of a FAW system with passive measures (Lindwall et al, 1998). Twenty eight human adult patients scheduled for elective operations in which the expected duration of anaesthesia exceeded 3 hours, were randomized into a control group given standard passive treatment (room temperature controlled at 22°C, insulation with double layers of terry cloth and operation drapes covering the whole body), and an experimental group which underwent the same passive treatment but also had a supplementary FAW system.
Although the study had a small sample size, the study was randomized and controlled and therefore the results can be considered significant. From start to finish, the experimental group core temperatures were higher than the control group, therefore demonstrating the efficacy of using FAW (Figure 4).
A more recent study by Ng et al (2003) included a sample size of 300 human patients who underwent unilateral total knee replacement and were randomized equally into three groups:
The study showed the effectiveness of the FAW compared with the other groups; there was a significantly smaller decrease in body temperatures from the induction room to the recovery room in the FAW group compared with the other groups (Figure 5). The study, therefore, demonstrates the advantage of using a FAW over using blankets.
Combining methods
Traditional thermal support (e.g. blankets, hot water bottles etc.) is still widely used in practice today and it should not be forgotten how useful, cheap and risk free these methods can be. Furthermore, it is important to recognize the advantages of using these traditional methods alongside FAW.
A trial evaluated the efficacy of three peri-operative warming protocols to improve control of body temperature in anaesthetized dogs (Cabell et al, 1997). Thirty two client-owned dogs admitted for either dental or orthopaedic procedures were arranged into three groups:
The study concluded that dogs in the peripheral warming group maintained significantly higher core body temperatures than dogs in either trunk warming group throughout the 2.5 hour study period, suggesting that it is more effective to warm the extremities rather than the trunk.
This study may inspire further research in this area and provides a good example of why combining another method with FAW is beneficial. The design of most FAW systems is such that even if a large blanket is selected for a large patient, it is still likely that extremities may be exposed. It is important therefore that additional insulation methods are used, such as bubble wrap, in order to protect the peripheral areas.
A study by Rembert et al (2004) showed not only how effective FAW was in warming rodent microenvironments in comparison with other methods, but also, how much more effective it was in combination with a plastic surgical drape. The surgical drape was being used solely for the purpose of asepsis; in this case however, an additional advantage was proven.
The study compared the use of a FAW system to traditional thermal support (which included an infra-red heat emitter and a circulating water blanket) for rodent microenvironments (i.e. procedural area temperature in the first experiment and the recovery cage temperature in a second experiment).
Both experiments resulted in a statistically significant temperature change in the areas warmed with FAW. The study by Rembert et al (2004) showed how much warmer the microenvironment becomes with FAW compared with the other methods, as well as how much more effective the FAW was when accompanied by a plastic drape:
Risks
Although FAW systems are relatively easy to set up, there is always a risk that the person responsible for doing this may fail to double check the hose is attached to the blanket correctly. This is referred to as ‘free hosing’ (Marders, 2002) and due to the resulting hot air blowing directly onto the patient, the consequences can be severe.
Two events due to free hosing have been described. In the first event, the free hosing resulted in third degree burns to the lower extremities. In the second event, the patient suffered a burn to the lower leg which resulted in skin necrosis and eventually led to an above the knee amputation (Marders, 2002). The events described by Marders occurred in human nursing; literature review has found no evidence of free hosing relating to the veterinary field.
The risks related to FAW can be easily avoided as long as the operators take the time to familiarize themselves with the correct set up and safety precautions.
Analyzing findings
There were 21 papers examined during this research.Out of these papers, seven compared FAW with other methods: three of these concluded FAW as more effective than other methods (Ng et al, 2003; Rembert et al, 2004; Leung et al, 2007); three concluded FAW as being just as effective as other methods (Linwall et al, 1998; Machon et al, 1999; Ng et al, 2006) and one was a meta-analysis of 23 papers concluding warm water garments as more effective than any other method (Galvao et al, 2010). Five of these were papers were human trials. There has been very little research performed on temperature monitoring in the veterinary field, hence the majority of studies referred to in this report were human related.
The remaining 14 papers were all reviews or studies on the condition of peri-operative hypothermia and its consequences, nine of these were human related.
Conclusion
FAW is effective in aiding the prevention of peri-operative hypothermia. Traditional methods of thermal support should not be underestimated and the benefits of using these methods alongside a FAW device are significant.
The risks associated with the use of FAW are severe but rarely reported and easily avoided as long as staff familiarize themselves with the correct methods for set up and adhere to safety guidelines.
Extensive research has been carried out regarding the use of FAW, as well as peri-operative hypothermia and its consequences in the human medical industry.
There are other methods of warming such as warm water circulating garments that outperform FAW systems according to human studies; this does not mean that FAW is not effective and further research with animals is required to gain better knowledge in this area.