Temperature control is a fine balance between heat loss and heat gain. When an animal undergoes surgery, the normal mechanisms responsible for generating and conserving heat are suppressed so the balance is heavily tipped in favour of heat loss. In addition the theatre environment and surgical procedure will impose large thermal stresses on the patient.
While it is well documented that surgical patients lose heat, the negative impact of such heat loss is greatly underestimated. A possible reason why this problem has not received the attention it deserves may be because the consequences of a decreased body temperature are invariably masked by, and mistaken for, the effects of surgery, rather than sometimes simply the fact that the patient is cold (McNeil, 1998). Although efforts aimed at averting hypothermia peri and intra-operatively exist, patients continue to enter the post-operative recovery period hypothermic with temperatures frequently below 34°C in cats and dogs (Dunlop, 2010). If nothing is done to correct this, the patient will continue to lose heat in the post-operative period with potential serious complications.
How do patients lose heat in the post-operative period?
Losses occur via convection, conduction, evaporation and radiation (Figure 1).
- In the post-operative period convection losses occur via transfer of heat from the animal to the air, highlighting the need for adequate patient coverings.
- Conduction losses occur as a result of transfer of heat from the animal to a surface that is cooler, for example, a stainless steel kennel, thus adequate bedding material should be provided.
- Evaporative losses occur when moisture in contact with the skin dissipates into the air, thus careful drying of the patient is essential post operatively.
- Heat naturally radiates from the patient's body into the environment; air conditioning ventilation systems add to the chill factor by creating air currents and draughts, so careful positioning of the postoperative patient away from draughts is essential (McNeil, 1998).
Negative impact of hypothermia
Effects on pharmacokinetics and dynamics
The enzymes that moderate organ function and metabolize most drugs are highly temperature sensitive. It is therefore not surprising that drug metabolism is temperature dependent. As body temperature falls, metabolism is slowed and liver function is impaired; this delays breakdown of anaesthetic drugs which in turn delays recovery (Robertson, 2007). Decreased metabolism of anaesthetic drugs may also cause depressed mentation in post-operative patients. Moderate hypothermia will result in a reduced blood supply to the kidneys, urine output will be decreased, and waste products such as urea and electrolytes will accumulate in the blood stream, further compounding mental confusion (Fox, 1993).
Effects on the cardiovascular system
The effects of hypothermia on the cardiovascular system may be particularly dangerous and include a depression in heart rate and cardiac output (and hence arterial blood pressure) (Murison, 2001). As core temperature falls, the myocardium becomes more irritable and the sino-atrial node beats more slowly. This is in part associated with increases in circulating catecholamines. Changes in cardiac rhythm may be noted and at temperatures approaching 32.2°C asystole or fibrillation may occur (Robertson, 2007).
Pressure sore development is a potential complication following surgery if the patient has been allowed to become cold, because constriction of blood vessels supplying the skin means that there will be a reduced blood flow and therefore a lack of oxygen in the tissues. Cells deprived of oxygen continue to work but produce lactic acid. Immobility on the operating table and in the recovery period means that tissues are subjected to prolonged pressure, a lack of oxygen and the presence of lactic acid meaning that there is an increased risk of pressure sore development (McNeil, 1998).
The end result of this will not be seen in the immediate post-operative period, because the death of the tissue occurs deep within and may take days to manifest as a breakdown of the skin. In humans it has been suggested that the more delicate skin of elderly patients is most at risk, along with that of the debili tated patient, whose cells may already lack the nutrients necessary to recover from the effects of pressure (Bridel, 1993), and it is likely this can be extrapolated to veterinary patients.
Effects on the respiratory system
Shivering, or involuntary contractions of the body, occur in order to increase body heat. Because no physical work is performed by the muscles, all the energy generated is used to raise the temperature of the body. However, working muscles require a huge amount of oxygen; this may be detrimental in the post-operative period, as shivering increases oxygen demand by up to 400% (Robertson, 2007). Increases in oxygen demands occurring after surgery may be dangerous, as this is just as the patient is least able to deal with it, and just as breathing complications can be expected. This increase in oxygen demand, combined with a decreased ventilatory drive can lead to hypoxaemia (Robertson, 2007). Consideration should be given to providing supplemental oxygen during recovery from anaesthesia in hypothermic patients until normothermia is achieved. This may be of particular importance in patients with poor pulmonary function (Greene, 2002).
Evidence from human nursing literature suggests that patients often recall feeling cold in the immediate post-operative period, reporting this as the worst period of their hospitalization, sometimes rating feeling cold to be worse than the surgical pain (Frank et al, 1995). Shivering is reported as extremely painful for patients with large wounds and assaulted muscles (McNeil, 1998). Frank et al (1995) also reported that thermal discomfort is physiologically stressful and contributes to observed increases in post-operative blood pressure, heart rate and plasma catecholamine concentrations.
The respiratory system can be further compromised by post-operative hypothermia; when the body becomes cold a greater affinity develops between haemoglobin and oxygen, they become more closely bound together, and haemoglobin becomes reluctant to release oxygen where it is needed. This means that despite pulse oximetry readings indicating that the oxygen saturation levels look reasonable, delivery of oxygen to the tissues may be considerably lower than estimated/expected (McNeil, 1998).
Effects on the neurological system
Mild-to-moderate hypothermia can cause a reduction in cerebral blood flow and impair autoregulation, which may lead to derangements in mentation. A study undertaken by Michenfelder and Theye (1968) demonstrated that cerebral blood flow decreased by approximately 5% for each 1°C drop in body temperature. Severe hypothermia is associated with abnormal neurological signs ranging from depression to coma (Michenfelder and Theye, 1968).
Monitoring temperature
The temperature of the patient should be read and recorded as soon as possible after it is settled into the recovery area. This will provide a baseline reading to which all subsequent readings can be compared. Hamilton (2003) stated that in normothermic patients, the temperature should be monitored every 30 minutes initially. For patients showing signs of hypothermia however, the temperature should be checked every 15 minutes. A simple mercury, digital or aural thermometer is essential in order to record the temperature of patients. It is also advisable to have a room thermometer available to record the temperature of the recovery environment. This can prove especially important for small and exotic patients, from which core temperature readings can prove difficult to obtain (Hamilton, 2003).
Warming devices
Within the protocol for managing post-operative hypothermia, the main objective of the veterinary nurses’ aid consists of detecting and recording hypothermia, as well as preventing further heat loss by means of passive surface rewarming and correcting the deficit by means of active surface rewarming and active core rewarming.
Although re-warming is possible in the post-operative period, great care must be taken as rapid re-warming is not tolerated by some surgical patients. In cold conditions, peripheral vasoconstriction will reduce perfusion and hence heat loss in the extremities; this is protective. If excessive warmth is applied to these areas, it may lead to vasodilation, diverting warm blood away from the core and actually increasing heat loss. If the patient is hypotensive or in shock, the reduction in arterial blood pressure produced by the decrease in systemic vascular resistance may prove detrimental as it will decrease blood flow to vital organs such as the brain and the heart.
Great care must be taken in the choice of heating device in order to prevent thermal burns.
Passive surface rewarming
The goal of passive surface rewarming is to prevent further heat loss from the patient. This involves placing blankets over the patient to decrease convection losses as well as placing blankets between the patient and the kennel floor to decrease heat loss via conduction. Blankets can reduce heat loss by approximately 30% (Matsukawa et al, 1995) thus it is essential they are repositioned after patient monitoring has taken place. Bubble wrap and foil blankets are also effective insulators, and can be disposed of if they become soiled; it is important to remember however that these items are not an actual heat source, so will not warm up already cold patients.
Active surface rewarming
The goal of active surface rewarming methods is to increase the air temperature around the patient, thereby reducing the temperature gradient between the body surface and the environment, and lowering the convection and conduction heat loss. Examples of this type of rewarming are detailed below.
- Electrical heat pads: these have been used extensively within veterinary practice for many years and there are now several types available. While they may be useful on occasions, heat pads do have limitations. Not all are thermostatically controlled, while some are more effective when a weight is applied to them. All heat pads carry the risks associated with having an electrical cable close to the animal. In addition, some monitors, for example electrocardiographs, may show electrical interference when used with an electric heat pad (Murison, 2001). It should also be remembered that electric heat pads can cause severe skin burns (Dunlop, 2010); these devices must always be insulated from the patient's skin surface.
- Warm air circulating devices: these are produced by several companies with the Bair Hugger® (Arizant Healthcare Inc, USA) perhaps being the most well known. The system consists of a heating unit that forces warmed air into a disposable paper and plastic quilt-like blanket. The warm air inflates the blanket then flows towards the patient, surrounding them with a ‘shell’ of warm air. A number of warm air heating devices have now been designed specifically for the veterinary market, as many of the devices used within a veterinary setting are intended for use on humans. The main disadvantage with this is that most blankets for people are designed to lie ‘on top’ of the patient. Animals have a hair-coat, which differs from people, so heating is more effective if applied in such a way that it rises into the hair where it is trapped, forming a warm layer. Typical human warm air blankets are manufactured with 1 to 2 mm holes punched in the surface layer. Dunlop (2010) reported that this can result in high air flow ‘jetting’ through these holes, causing a wind tunnel effect which can cool small animals. Warm air systems are now available that are specifically designed to warm animals in cages, with adaptors to duct warm air through metal door grills and with blankets designed to fit dog or cat cages (Advanced Anaesthesia Specialists, Oxfordshire) (Dunlop, 2010).
- Active core rewarming: the goal of this method is to provide heat centrally to rapidly warm the body core; the use of warmed intravenous fluids is an example of this type of warming. The warming of intravenous fluids for surgical patients is commonplace within veterinary practice, however fluids rapidly lose heat and return to their ambient temperature; this is further compounded by the low infusion rates used in small animals. Burch-man et al (1989) observed that the temperature of a bag of preheated intravenous fluid decreased by 1.4°C over 4 minutes during a rapid infusion. For this reason intravenous fluids need to be constantly warmed. Electric fluid warmers, whereby the giving set fits into a thermal track, are available but are often underused in veterinary practice. These devices provide constant heat which is thermostatically controlled. In the absence of a dedicated machine, passing a loop of the fluid giving set through warm water at 38°C (Dix et al, 2006) in a bowl or kidney dish (close to the point of venous access to the patient) will have a similar effect, but needs frequent water changes to remain effective. Dix et al (2006) compared four methods of maintaining the temperature of intravenous fluids in veterinary practice. The study showed that the most efficient method of maintaining the temperature was the use of a heat retention cover over a pre-warmed fluid bag, combined with 30 cm of the giving set wrapped around a latex glove filled with warm water (hot hand). This was improved if the giving set was also pre-warmed. It is important to note however, that such methods are better at helping prevent further heat loss rather than warming hypothermic patients.
The type of rewarming intervention selected will depend on the cause and severity of the hypothermia. Patients with mild hypothermia may be effectively treated with passive surface rewarming; however active surface and active core rewarming techniques need to be considered for more severely compromised patients. Whatever type of warming method is selected, it is important to note that temperature monitoring during rewarming is essential in order to prevent hyperthermia.
Conclusion
On admission to the recovery ward, initial assessment of body temperature, along with interventions aimed at restoring normothermia should prevail. Veterinary nurses play a pivotal role in preventing hypothermia and intervening for patients with hypothermia in order to minimize the physiological effects as well as to provide comfort. Nursing interventions aimed at decreasing cutaneous heat loss include minimal exposure of the patient, the use of blanket layering, and passive and active surface rewarming. Decreasing shivering of post-operative patients may promote patient comfort while minimizing oxygen consumption. Warming hypothermic patients in recovery is slow, consumes nurse's time and may potentially be a cause of thermal burns (Dunlop, 2010). For these reasons, a team approach to minimizing hypothermia in the peri, intra and post–operative period is considered best practice. All members of the surgical team should be aware of how heat may be lost while the patient is under their particular care, and actions that may be taken to minimize further heat loss. Although heat loss during surgery is inevitable, simple precautionary measures can considerably reduce the amount of heat lost, which can improve the patient's short and long-term recovery.
Key points
- Hypothermia is frequently encountered in the post-operative patient.
- Patients can lose heat via a variety of means.
- Hypothermia is associated with impairment and abnormalities in various organs and systems.
- Monitoring of core body temperature is essential post operatively.
- A combination of passive and active rewarming techniques may be utilized for warming post-operative patients.