Heat production in the body occurs secondary to metabolism. The brain and major organs in the trunk generate the majority of metabolic heat due to the high number of chemical and physical processes taking place in these parts (Sessler, 2000). Normally, excess heat is dissipated into the environment through the respiratory tract and across the skin to maintain thermal homeostasis. The body is split into two thermal compartments: the core compartment consisting of the trunk and head; and the peripheral tissue compartment comprising the extremities (Armstrong et al, 2005). Core temperature remains consistent while peripheral temperature tends to vary (Armstrong et al, 2005). Heat moves slowly from the core to the periphery by conduction and convection. Conductive heat distribution occurs via adjacent tissues and depends on the characteristics of the tissue, for example fat insulates better than muscle thus slowing heat transfer. Convective heat distribution takes place via blood flow from the core to periphery and is influenced by peripheral blood flow, counter current heat exchange between arteries and veins, and the core-to-peripheral temperature gradient (Sessler, 2000).
Thermoreceptors
Thermoreceptors are found centrally and peripherally. Peripheral receptors are located in the skin and central receptors are located in the hypothalamus, spinal cord, brain stem, abdominal organs and skeletal muscles (Robertson, 2008). Receptors for both cold and warm exist; when stimulated cold receptors initiate reflexes designed to increase body temperature including shivering and increasing cellular metabolism to maximize heat production, and peripheral vasoconstriction to reduce heat loss. The posterior hypothalamus acts as a ‘thermostat’ by using the autonomic nervous system to integrate thermal input and control effector organs (Sessler, 1997; Robertson, 2008).
Defnition of hypothermia
Hypothermia is defined as a low body temperature (Blood et al, 2007). The canine and feline normal temperature range is between 37.8–39.2°C (Armstrong, 2005). A temperature of 32–37°C constitutes mild hypothermia with moderate hypothermia occurring at temperatures between 28 and 32°C, and severe hypothermia occurring at temperatures below 28°C (Armstrong et al, 2005).
Factors influencing body temperature
In the healthy unanaesthetized patient, body temperature will be maintained even when the temperature is lower than body temperature 37.8°C; however hypothermia resulting from anaesthesia is routine and this is thought to be due to an anaesthetic-induced impairment of thermoregulatory responses (Sessler, 1997). Anaesthetics depress the central nervous system, leading to decreased sensitivity of the hypothalamus to changes in body temperature and with this the animal also loses the ability to move voluntarily or stimulate muscle activity to generate heat. Shivering is a common effect of hypothermia. This involuntary action is the body's method of trying to increase body temperature however this is imparied by the depressant effects of anaestheisa (Seymour and Gled, 1999).
The withholding of food preoperatively leads to a reduction in the heat producing chemical and physical processes that take place in the digestive system following food intake.
The smaller the animal the larger the surface area to volume ratio, therefore animals under 5 kg are more susceptible to hypothermia due to more surface from which heat can be lost. The age of the animal also plays an important role in heat regulation with younger animals having less body fat for insulation and thermoregulatory mechanisms that are less well developed. In contrast, thermoregu-latory mechanisms may be deteriorating in older animals (Murison, 2001).
The three phases of perioperative hypothermia
The first phase of heat loss
The first phase of heat loss is rapid and is due largely to a core-to-peripheral redistribution of body heat during the first hour of anaesthesia (Matsukawa et al, 1995). This occurs via two mechanisms. the first lowers the temperature threshold needed for reflex vasoconstriction to take place, thus a lower temperature is needed for stimulation of the thermoregulatory centre in the hypothala-mus (Sessler, 1997). The second mechanism is direct vasodilation caused by the anaesthetic agent (Sessler, 1997). The extent of body heat redistribution depends on the initial core-to-peripheral temperature gradient which is in-fluenced by the temperature of the patient's pre-anaesthetic environment; therefore patients kept in a warm environment prior to anaesthesia redistribute heat little compared with those kept in a cold environment (Sessler, 1997).
The second phase of heat loss
The core temperature continues to fall in the second phase of intraoperative hypothermia but at a slower rate (Sessler, 2000). The second phase occurs when heat loss exceeds metabolic heat production, which decreases by 20-30% during anaesthesia. Heat is lost from the body via four routes: radiation, convection, conduction and evaporation (Table 1).
Table 1. Routes of heat loss during the second phase of perioperative hypothermia
| Radiation | The transfer of heat from one surface (e.g. the body) to another via photons independent of the temperature of the intervening air. Most heat loss occurs via radiation |
| Convection | The movement of air reduces the build up of heat near the skin surface by displacing warmed air with cooler air. Convection is the basis for the popular ‘wind-chill’ factor and is the second most important source of heat loss (Sessler, 2000) |
| Conduction | The direct transfer of heat from one surface to an adjacent surface (e.g. the body to the operating table) |
| Evaporation | Evaporation results in heat loss when moisture dissipates into the air, pulling heat with it (e.g. surgical preparation solutions, surgical incisions) |
The third phase of heat loss
The final phase of intraoperative hypothermia is a core temperature plateau that usually develops after 2–4 hours of anaesthesia where the core temperature remains the same (Sessler, 2000).
Physiological impact of hypothermia
Perioperative hypothermia places stress on hepatic and renal function, the cardiovascu-lar, respiratory and central nervous systems. As Box 1 shows there are many physiological consequences to perioperative hypothermia. A trial performed by Frank et al (1995) investigated the relationship between periopera-tive changes in body temperature and plasma levels of stress hormones. By comparing two groups of human patients it was found that mild hypothermia resulted in an increased frequency of tachycardia, hypertension, systemic vasoconstriction and an imbalance between myocardial oxygen supply and demand due to increased levels of circulating catecholamines (Frank et al, 1995). These results are echoed by another trial performed by Frank et al (1997) involving 300 human patients undergoing non-cardiac surgery who already had cardiac risk factors. These patients were three times more likely to develop adverse myocardial outcomes when hy-pothermic, and maintaining normothermia perioperatively was in fact associated with a reduced incidence of morbid cardiac events and ventricular tachycardia. These findings can be extrapolated to veterinary patients and show the importance of monitoring temperature and maintaining normothermia under anaesthesia.
Box 1.Physiological impact of hypothermia
- Reduced hepatic metabolism
- Cardiovascular compromise
- Decreased ventilation
- Impaired oxygen delivery to tissues
- Renal compromise
- Reduction in cerebral blood flow
Hypothermia is detrimental to hepatic metabolism. Lowered enzymatic activity within the liver can result in depressed conjugation and detoxification that may prolong anaesthetic action (Cabell et al, 1997; Armstrong et al, 2005). Due to this reduced hepatic metabolism, less anaesthetic is required with a low body temperature and if this is not taken into account, a prolonged recovery from anaesthesia may result with excessive depression of the central nervous, cardiovascular and respiratory systems (Murison, 2001).
Mild to moderate hypothermia may also have detrimental renal effects causing a cold diuresis due to an increased glomerular fil-tration rate, vasoconstriction and diminished sensitivity to antidiuretic hormone (Armstrong et al, 2005). If hypothermia becomes severe this could lead to a decrease in renal flow, a decreased glomerular filtration rate, blood sludging (the clumping of eryth-rocytes which interferes with blood circulation), ischaemia or cold renal tubular damage; the ultimate damage being acute renal tubular necrosis (Armstrong et al, 2005).
Impaired platelet function and clotting enzyme function due to hypothermia may increase blood losses during surgical procedures. One possible reason for this could be that the enzymes involved in the coagulation cascade are temperature dependent (Frank et al, 2000, Putzu et al, 2007). Results of studies differ in opinion over whether blood loss is increased during surgical procedures when hypothermia is present (Johansson et al, 1999; Winkler et al, 2000).
Kurz et al (1996) tested the hypothesis that mild hypothermia may increase patients’ susceptibility to perioperative wound infections by causing vasoconstriction and impaired immunity. Two hundred human patients undergoing colorectal surgery were randomly assigned to two groups (hypother-mic and normothermic). Colorectal surgery is high risk for infection, however only 6% of the normothermic group of patients developed infection opposed to 19% of the hypo-thermic group. Interestingly, when looking at low-risk clean surgical procedures, a retrospective study of 777 dogs and cats found that mild perioperative hypothermia was not a significant risk factor for post-operative wound infections (Beal et al, 2000). Instead, duration of anaesthesia was revealed as the significant risk factor independent of the duration of surgery.
Patient warming methods
Considering the range of physiological effects of hypothermia, it is important to maintain normothermia. There are three types of patient warming: passive and active surface warming and active core warming (see Step-by-step guide). Passive warming aims to prevent further heat loss from the patient, active surface warming increases the air temperature around the patient therefore increasing the patient's temperature, and active core warming aims to increase the patient's core temperature (Table 2) (Figure 1).
Table 2. Patient warming methods.
| Passive surface warming | Active surface warming | Active core warming |
|---|---|---|
|
|
|
Forced-air warming (FAW) works by blowing warm air through a pipe (Figure 2) and into a disposable single use blanket placed either over the patient or around the patient as in Figure 3. The warm air permeates through the blanket at a set temperature and onto the patient.
In human and veterinary research FAW has compared favourably with other methods of patient warming and can prevent hypothermia and maintain normothermia (Lindwall et al, 1998; Machon et al, 1999). To reach its full potential, FAW must be initiated as soon as possible to prevent the initial rapid decline in body temperature. FAW is most effective however once the patient is fully draped with the ‘trapped’ warm air circulating underneath the drapes.
Although this method is seen as a safe option, there are potential risks to the patient particularly if the system is misused. The blankets must always be used attached to the piping, as there is a risk of thermal burns if the piping alone is used to blow warm air directly over the patient. It is also possible that the warm air may blow hair and debris onto the surgical site increasing the risk of infection (Pfiedler, 2009). This means delaying the use of FAW until the drapes are placed therefore protecting the surgical area, but potentially leaving the patient with no warming system for a long period of time. A study of 16 human patients undergoing aortic surgery suggested that use of FAW does not result in bacterial contamination of the operating theatre atmosphere as a result of the air blowing over the patients’ skin, mobilizing and dispersing skin organisms (Huang et al, 2003).
The cost of FAW has long been an issue with general veterinary practitioners due to the cost of the equipment, electrical running costs, and the need for single-use disposable blankets. The most recent warming technology to eliminate these disadvantages is the Hot Dog™ patient warming system with a fabric conductive mat (Figure 4).
It enables the temperature of the mat to be controlled accurately via low voltage to avoid the problems of thermal burns. The mat is cleanable and, importantly for cost, reusable. It can be placed either on top, below or around the patient and quickly warms up to the required temperature. In research when compared with a combination of a warm water mattress and FAW, the Hot Dog™ system maintained significantly higher core, skin and rectal temperatures during anaesthesia (Ayers and Riedesel, 2010).
Traditional electric heat pads, ‘hot hands’ (latex gloves filled with warm water) and radiant heat can all assist in maintaining body temperature (Tan et al, 2004). However, there are potential problems associated with these and similar methods which put patients at great risk from thermal burns (Dunlop et al, 1989). Traditional heat pads may exceed safe temperatures for patients, it is also difficult to maintain stable temperatures with radi-ant heat lamps and if placed too close to the patient these may cause burns. The temperature of water placed into hot water bottles and latex gloves may be too hot and the use of these is not recommended in any animal that is unable to move away from the heat source.
Hypothermia can occur due to the administration of cool intravenous fluids; heating up the cool fluid requires a transfer of heat from the body tissues to the fluid and in doing so causes the core temperature to drop. Electric fluid warmers are a simple method of active core warming where an infusion giving set is passed through a heated device (Figure 5). These devices are relatively inexpensive and their small size means they are easy to store. The effectiveness of warming intravenous fluids with an electric fluid warmer prior to administration has been debated (Leben and Tryba, 1997). To significantly warm a hypo-thermic patient, intravenous fluids would require administration at temperatures above 40°C; however these higher temperatures can be detrimental to blood cells (Werlhof, 1996). Using a fluid warmer alone is not as effec-tive as convective warming methods such as FAW; however fluid warmers still have their place in the prevention of hypothermia, and when used in conjunction with FAW prove successful in maintaining core temperature (Leben and Tryba, 1997).
When using a fluid warmer the positioning is very important to ensure the warmed flu-ids enter the patient's circulation as soon as possible. The fluid warmer has to be placed as close to the proximal end of the giving set as possible or the fluid will have cooled considerably by the time it reaches the patient. Infusion rates should also be considered. A high infusion rate may mean the fluid passes through the warmer at such a rate that the fluid warmer cannot heat the fluid fast enough, therefore reducing its effectiveness.
When deciding on a warming method it is important to take into account the type of surgery to be performed. A larger surface area exposed to heat loss (such as during laparotomy) will require multiple methods of patient warming to prevent hypothermia, such as a convective system, warm peritoneal lavage and a fluid warmer; whereas surgery involving a smaller area (i.e. stifle surgery) may need only a convective system (Ng et al, 2006; Leung et al, 2007).
Step-by-step guide to preventing perioperative hypothermia using active surface warming
- Ideally a standard of practice protocol (SOP) approach should be used on every patient prior to the animal being anaesthetized.
- The patient's temperature should be taken prior to the administration of premedication and recorded.
- The pre-medication should be carefully selected according to the patients American Society of Anesthesiologists status (2012) incorporating any patients that are high risk at developing hypothermia for example neonates and underweight patients.
- Once the premedication has been administered the patient should be housed in a warm ward to help maintain normothermia.
- Pre-operative warming may be required if temperature declines.
- Once the patient is anaesthetized the patient should ideally be placed on a heating devise immediately following induction and remain on the heating devise throughout the anaesthetic (dependent on regular temperature readings).
- Ideally continuous temperature monitoring should be performed throughout the anaesthetic with an oral probe if available.
- Rectal temperature should be taken ideally every 5 minutes and recorded if equipment is not available for continuous monitoring.
- The aim is to achieve normothermia throughout the anaesthetic (37.8-39.2oC).
- If the patient's temperature declines below 37oC during active surface warming then consideration into additional warming methods should be utilzed such as a intravenous fluid warmer used in conjunction with active surface warming.
- If the patient's temperature rises above the low end of normal then consideration should be given to lowering the temperature of the active surface warming device if possible or switching the heating device off.
- Care must be taken not to over heat the patient as when heating devices are switched off the patient's temperature may still increase.
- The veterinary surgeon should be advised if and when the patient becomes hypothermic.
Pre-operative warming
Due to the initial core-to-peripheral redistribution of body heat following induction of anaesthesia and the subsequent rapid drop in temperature during the first hour, research has been undertaken to ascertain if pre-op-erative warming sufficiently minimizes hypothermia. Just et al (1993) performed a small study involving 16 human patients undergoing total hip arthroplasty. These randomly assigned patients were either subjected to pre-operative warming for 90 minutes with FAW, or unwarmed pre operatively. Core and mean skin temperatures were measured and substantial differences were seen between the two groups. During the first hour of anaesthesia, core temperature in the unwarmed group decreased more than twice as much as the pre-warmed group. By the end of the procedure, the pre-warmed group had much higher temperatures than the unwarmed group. Post-operative shivering, which can cause considerable discomfort, was also monitored and seen in seven of the unwarmed patients and none of the pre-warmed patients. The results of this research emphasize the importance of the type of environment patients are kept in prior to anaesthesia. If it is not possible to actively warm patients prior to anaesthesia they should at the very least be housed in a warm environment to reduce the core-to-peripheral redistribution of body heat seen in the first phase of hypothermia.
Peripheral skin warming
There is an abundant supply of arteriovenous shunt vessels located within the distal extremities of humans and many animals compared with a smaller number found in the head or trunk. These shunts are vital in thermoregu-lation, involved in vasodilation and constriction, thus functioning as effector organs for dissipation or conservation of heat (Cabell et al, 1997). A study conducted by Cabell et al (1997) used circulating warm water mattresses on differing areas of dogs’ bodies to determine which was the most effective. Thirty-two dogs were involved in the study and randomly assigned to one of three groups (Figure 6).
Interestingly, Cabell et al (1997) found active peripheral warming of the extremities was significantly more effective in maintain-ing normothermia than either single or double truncal warming. It seems by using the vasoactive and thermoregulatory role of the distal extremities along with protection of the periphery from contact with the cooler environmental temperatures may prove ben-efcial. It appeared that this method was effective against all stages of perioperative hypothermia. Redistribution hypothermia was minimized during phase one, total body heat loss during phase two was reduced and the vasoconstrictive response in phase three was avoided due to the prevention of significant intraoperative hypothermia.
Recommendations for the veterinary nurse
It is important for the veterinary nurse to individually assess every patient prior to anaesthesia. Some patients need extra care to avoid perioperative hypothermia due to their size, age, state of health prior to anaesthesia and the type of surgery they will be undergoing.
The effectiveness of all heat prevention methods is dependent on the individual assessment of the patient regarding the above criteria. For example, FAW alone may not prevent perioperative hypothermia in a 15-year-old cat undergoing a laparotomy. This case may benefit from additional strategies such as being kennelled in a warm environment for 1 hour prior to induction, the covering of its extremities to prevent heat redistribution or the usage of an infusion heater during surgery. Conversely a 5-year-old Labrador undergoing patella surgery may benefit very well from FAW alone.
Ideally a multimodal approach to treating perioperative hypothermia should be taken and it is recommended that all methods of patient warming are used with the knowledge of evidence-based research.
Conclusion
The pre-operative temperature, age and weight of the patient along with the environmental temperature play important roles in determining the degree of hypothermia suffered. Understanding the three phases of perioperative hypothermia alongside the knowledge of effective patient warming methods and excellent patient monitoring enhances the management of these cases.
The extent of redistribution hypothermia (phase one) depends heavily on the temperature of tissue in the peripheral thermal compartment. Pre-operative skin-surface warming of patients appears to limit this redistribution, and sufficiently minimize the occurrence of hypothermia (Just et al, 1993).
Once the final phase of intraoperative hypothermia occurs and the core temperature plateaus, the patient becomes severely hy-pothermic. Core temperature will remain the same and limited heat transfer occurs between the peripheral and central thermal compartments. Hypothermia becomes extremely difficult to treat at this point with the risks to the patient increasing dramatically; for this reason, preventing the onset of peri-operative hypothermia is vital rather than attempting treatment after it has developed.
Key Points
- Perioperative hypothermia is a common problem during anaesthesia in dogs and cats.
- Anaesthetics depress the central nervous system, leading to decreased sensitivity of the hypothalamus to changes in body temperature.
- Most heat loss during anaesthesia occurs via radiation.
- Perioperative hypothermia is associated with metabolic, cardiovascular, respiratory and neurological and immunological consequences.
- Preventing hypothermia by using a combination of warming methods is the key to maintaining normothermia during anaesthesia.
- To answer the CPD questions on this article visit www.theveterinarynurse.com Approved by Harper Adams University College