The previous article discussed human studies which indicated a link between perioperative hypothermia and surgical site infection (SSI) rates (Kurz et al, 1996; Seamon et al, 2012). Having established this link, particularly in those patients who are at risk, it is necessary to look into methods to avoid perioperative hypothermia and assess which methods are the most effective at maintaining normothermia.
There are a number of methods that have been used and evaluated in human patients and this is also an area where there is a growing amount of research within the veterinary field.
General anaesthesia and hypothermia
In animal physiology it is known that a reduction in core temperature of 1–1.5°C occurs shortly after the induction of general anaesthesia due to the re-distribution of heat from the core to the periphery (Armstrong et al, 2005). Reasons for this occurring are twofold, first, general anaesthesia reduces the patient's temperature threshold, meaning that a lower temperature is required before the thermoregulatory centre in the hypothalamus in the brain is activated to cause vasoconstriction; second, anaesthetic agents cause vasodilation (Armstrong et al, 2005).
Preoperative patient warming
Preoperative patient warming may help, by increasing a patient's core temperature preoperatively so that this initial drop in temperature does not immediately put the patient into the hypothermic category; mild hypothermia in veterinary patients is described by Armstrong et al (2005) as between 32 and 37°c. Preoperative warming in animals is carried out from premedication until the induction of general anaesthesia; the animal is more likely to tolerate it better once they have been sedated.
Forbes et al (2009) carried out an extensive literature review of human patients and came to the conclusion that forced air warming (FAW) should be used to warm patients preoperatively. There is currently no literature available into preoperative warming of veterinary patients. In human medicine, studies on preoperative warming have established significant results. A randomised control study by Horn et al (2012) tested the peri and post-operative temperatures of four groups of human patients. The first group had no active warming pre operatively, the second had 10 minutes of active warming, the third had 20 minutes and the fourth 30 minutes. 15 minutes into the procedure core temperature in the first group dropped to approximately 35.5°C and below whereas the other groups dropped slightly but none below 36°C. There was a correlation between the length of preoperative warming and the drop in temperature and it was found that even as little as 10 minutes of preoperative warming can stop perioperative temperature falling below 36°C. They also found that when the first group became hypothermic, using active warming once the procedure had started did not reverse or prevent further hypothermia.
Melling et al (2001) carried out a randomised study into preoperative warming comparing no preoperative warming to both systemic and local warming. They found that both systemic and local warming over the proposed surgery site led to lower SSI rates and 30 minutes of pre operative warming reduced infections from 14% to 5%. They did not find any significant difference between local and systemic warming suggesting that either one of these methods may be effective.
A study by Nicholson (2013) found no significant difference in post-operative temperatures in patients who were preoperatively warmed passively with a wool blanket or actively with FAW. This study was similar to the ones previously discussed regarding type of study, however, sample size in this study was much smaller at only 66 patients whereas Melling et al (2001) and Horn et al (2012) had sample sizes of 200 and 421 respectively. Although the relevant sample size power analysis calculations were carried out for the Nicholson (2013) study it could be argued that a small sample size is not representative of the general population (LiBiondo Wood and Haber, 2006).
Practically the use of preoperative warming in veterinary patients has limitations, such as those posed by having to keep the animal still. It could therefore only realistically be done in recumbent patients or those that have been premedicated (Tan et al, 2004). Davies (2012) suggests keeping patients that have had premedication in a warm environment. This can be done by placing small patients in a prewarmed incubator or increasing the ambient temperature in the room they are in (Figure 1).
Perioperative patient warming
There are numerous methods that can be used in the perioperative phase to maintain normothermia in patients including passive warming through the use of blankets and bubble wrap, and active warming with FAW, heat mats, resistive polymer warming (‘hot dog’ blankets) and fluid warmers. A study by Tan et al (2004) which compared four different intraoperative warming methods in dogs found that the most effective method of maintaining normothermia was to have the patient on a heat mat with radiant heat supplied via a heat lamp 50 cm away from the patient's head and forelimbs and heated saline bags placed alongside the trunk of the patient. In reality this would be a very impractical method of maintaining normothermia perioperatively as the heat lamp would likely provide a warm ambient temperature which may be uncomfortable for the surgeon and it would potentially disturb the positioning of the patient to remove the fluid bags to reheat them when needed. Heated fluid bags also have the risk of causing localised burning if used incorrectly. The use of a number of different warming methods, however, could be adopted for some cases (Henderson, 2012). The second most effective means of maintaining normothermia was FAW, which in human literature is the most effective way of maintaining normothermia (Wu, 2013).
FAW
FAW is the most popular method of maintaining normothermia in the perioperative period and has been proven to be effective in a number of clinical trials (Association of Perioperative Registered Nurses, 2007). A human study suggested they may be a potential source of post-operative SSI (Huang et al, 2003). The same study actually concluded that there was no increase in bacterial contamination in the operating theatre when using FAW. An extensive literature review on humans by Kellam et al (2013) found that the evidence was inconclusive and more reliable studies are required to confirm whether or not FAW is a potential source of SSI.
A number of studies indicate that the use of FAW in orthopaedic surgeries increases the number of bacteria over the surgical site. According to McGovern et al (2011) when surgery is carried out in theatres with ultra clean ventilation the use of FAW can disrupt air currents and move contaminated air towards the surgical site of patients. In addition, Dasari et al (2012) found a similar pattern in laminar flow theatres. Both of these studies however used mannequins in a simulated theatre set up rather than a number of real patients, but it does highlight a potential problem when using FAW in operating theatres with both laminar flow and ultraclean ventilation. A similar study using a single human patient by Legg et al (2012) found more bacteria-carrying particles near the surgical site when using FAW compared with resistive polymer warming. This study on a single patient is not enough to be able to draw any conclusions but the same author published a study looking into air currents in the operating theatre and found 2 174 000 particles/m2 over the surgical site when using FAW compared with 1000 particles/m2 when using resistive polymer warming (Legg and Hamer 2013). Many of these studies only use mannequins or single patients but the fact that there are a number of similar studies with the same conclusions shows that there is a high likelihood that there is a higher chance of bacterial contamination with FAW than resistive polymer warming.
In a comparison of the efficiency of the two methods by Brandt and Kimberger (2010) FAW and resistive polymer warming were found to be similar in their effects on perioperative warming suggesting that it would be more appropriate to use resistive polymer warming on orthopaedic patients, especially those undertaking joint arthroplasty. In the veterinary operating theatre the risk of stray hairs also needs to be considered. Patients should be clipped and their hair vacuumed away before entering the operating theatre, however stray hairs may still enter the theatre environment and get caught up in the air flow, which according to human studies using FAW (McGovern et al, 2011) may get drawn towards the surgical site.
There are risks associated with the use of active warming devices such as fire and burn risks (Wu, 2013), so any registered veterinary nurses who may be using these should be familiar with their safe use. There also have cost implications and so may not be appropriate for smaller practices where they may not prove cost effective.
Reflective blankets
The use of a reflective blanket for temperature management in animals under 10 kg has been documented (Tunsmeyer et al, 2009) (Figure 2). The objective of the study was to use the blanket with warmed gel pads and compare this to the sole use of gel pads. It was found that after 40 minutes there was a significant difference in mean body temperatures between the two groups. The sole use of a reflective blanket to maintain body temperature when under general anaesthesia in veterinary patients has proved inferior to other methods in a study by Haskins (1981), however, with the use of heated gel pads in the Tunsmeyer et al (2009) study the reflective blanket maintained normothermia more effectively by increasing the temperature inside the reflective blanket. Exclusions were made in this study so that each sample was similar in variables, however, there were only 20 patients in each of the two groups meaning it was not very representative of the actual population. The time during which the animals were being prepared for surgery was not included and the baseline temperature was taken from the beginning of surgery. It does not specify the patient preparation protocol or whether the patients were prepared by the same person. The patients would have had varying sizes of surgical areas meaning some may have had more hair clipped away and a larger area to be cleaned. This could have had an effect on the baseline temperature which was recorded for the patient at the beginning of surgery. Although the reflective blankets are cost effective they would only be suitable for surgery on the extremities.
Recommendations for further study
Overall, compared with the human field the subject of hypothermia and its effects on SSIs in veterinary patients has very little evidence-based research to support it. Generally this is a recommendation for further study, perhaps looking into each different type of surgical wound such as clean, clean contaminated, contaminated and dirty. Beal at al (2000) examined clean wounds in their study into perioperative hypothermia finding no correlation between perioperative hypothermia and SSIs. Human studies, for example that carried out by Kurz et al (1996), which found perioperative hypothermia to be a contributor to SSIs, looked at clean contaminated wounds and so it would be interesting to see if an equivalent veterinary study was carried out what results it would yield and whether it would find a correlation between the two variables.
Human studies have found strong correlations between preoperative warming and peri and post-operative temperatures. There are no similar studies in the veterinary field, maybe due to the practicalities of getting animals to lie still while they are warmed. Studies could be carried out into warming the patients once they have had premedication compared with those that have not been warmed, and a comparison could be carried out of their temperature when they reach theatre as well as their temperature post operatively. A positive result would certainly help to change the attitudes of many RVNs who do not currently adopt preoperative warming methods.
Any veterinary practice or hospital which keeps records of surgical complications including infections could retrospectively look at the preoperative temperatures of their patients to assess whether hypothermia could have contributed to the complication, however, there may be many other factors also so these would also have to be assessed at the same time to see whether certain patients may have been more prone to an SSI in the first place.
Recommendations for practice
When dealing with surgical patients, RVNs should be able to identify those who may be more at risk of developing an SSI and plan their care accordingly. All perioperative patients should be kept warm in order to increase patient comfort and therefore patient welfare, however, there will be some patients where it will be more critical than others that they maintain normothermia. There is a small evidence base available in literature highlighting high risk areas such as endocrinopathy, clean contaminated surgeries, surgery of a long duration and high body condition score (Eugster et al, 2004; Nicholson et al, 2013). RVNs should be looking at human literature to broaden the evidence base which is currently available while being critical of the evidence.
Human literature states that in the preoperative stage temperature can drop shortly after induction by up to 1–1.5°C. With this in mind, preoperative warming and warming of the patient in the preparation time is very important. Practically, pre warming of veterinary patients is very difficult until the animal is sedated and so once the patient has received premedication warming can begin, and the patient should be kept warm during this period with FAW. Horn et al (2012) found that as little as 10 minutes of preoperative warming was effective and so there is no reason why this cannot be effectively carried out with veterinary patients in practice before induction of anaesthesia. Patients should then be induced on a veterinary bed which is on top of a heat mat and while they are being prepared the registered veterinary nurse should make efforts to keep the animal covered as much as is possible with bubble wrap, blankets or FAW blankets. The use of a number of methods to maintain normothermia may be adapted depending on the type of surgical procedure the patient is having and how the patient is going to be positioned in the preparation stage (Henderson, 2012)
Looking into the experimental work into FAW and SSIs in humans, it is recommended that an alternative warming protocol be used for orthopaedic patients when they are in theatre, especially those receiving joint replacements. Resistive polymer warming has been found to be a viable alternative for these patients, showing similar results in maintaining body temperature to FAW.
With the knowledge that radiation and convection account for up to 85% of heat loss in perioperative patients, the use of a reflective blanket with warm gel or wheat bags inside may be of great use in patients with whom this would be practical such as surgery on the distal limbs and the head or ears for example dental work and total ear canal ablations.
Conclusion
In order to maintain comfort and potentially reduce the risk of SSIs, patients should be kept warm throughout the perioperative period. Although evidence for preoperative warming is inconclusive the registered veterinary nurse could implement heating measures as soon as the patient has been premedicated (Davies, 2012). The preoperative environment could be made warmer and if the animal was very small and such facilities were present, the patient could be placed in an incubator.
The RVN should assess whether the patient may be at risk for an SSI and also should take into account the surgery they are having and plan normothermia controls appropriately. It has been shown that the use of FAW blankets may increase bacterial counts over surgical sites in orthopaedic surgeries therefore an alternative patient warming method for use in joint arthroplasties such as resistive polymer warming could be considered.