Dr Denica Djiodjeva
Central Vet Clinic
Sofia, Bulgaria
Hypothermia is one of the most frequent and major anesthetic complications, occurring in at least 40% of patients. Unfortunately, too little attention is paid to this condition, which is associated with many pathophysiological changes that affect the patient before, during and after surgery. In a dog, hypothermia is considered a temperature below 37° C. As with prolonged procedures and operations, the risk increases. These are operations in which the abdominal cavity is open for a long time, in small animals under 2 kg, weak, cachectic, pediatric and geriatric patients.
Thermoregulation is a process in which the body strives to maintain a constant body temperature, regardless of external conditions, which ensures normal functioning of enzymes, coagulation and immune response. The normal physiological limits for a dog and a cat are 37.5˚ C to 39.2˚ C for a dog and 37.8˚ C to 39.5˚ C for a cat. For mild hypothermia, 37.0˚ C to 37.7˚ C is accepted; moderate, 35.8˚ C to 37.0˚ C ; severe, 33.6˚ C to 35.8˚ C ; and critical, less than 33.6˚ C or less. The normal body temperature (head and body) is about 38° C, and that of the peripheral parts is 2-4° C lower. Animals and humans, in addition to maintaining their body temperature within certain limits, can also produce it. Their body is conditionally divided into two parts, central (core), which generates heat, and peripheral, which regulates. The body’s regulatory mechanisms work to keep heat within normal limits. Under normal conditions, the production of heat is the result of the metabolic processes of the internal organs. When the blood passes through them, it warms up and reaches the periphery of the body through the cardiovascular system. The main organ that plays the role of a thermostat is the hypothalamus. When the blood passes through it, its temperature depends on what the body’s response will be in order to maintain the balance between heat gain and loss. From the hypothalamus, through afferent and efferent nerve pathways, vasoconstriction is induced, which occurs before the activation of other energy-consuming reactions, such as shivering. It is important to mention that the efferent response includes both types of regulation – behavioral and autonomic. Behavioral is the strongest response to rewarming, but requires awareness, which is absent during anesthesia. For this reason, the patient must rely on autonomic defense mechanisms, such as maintaining normal blood pressure, vasoconstriction, etc. When local anesthesia is used, vasoconstriction is reduced in the area, where it is administered and this increases heat loss. In addition to central thermoreceptors for heat and cold (in the hypothalamus, spinal cord, abdominal organs, brain stem, muscles), there are also peripheral ones in the skin.
According to the second law of thermodynamics heat can only flow by temperature gradient from the body that is warmer towards the periphery or the environment that is colder, therefore, the body can never be heated from the periphery to the core which is usually warmer than the outside.
As already mentioned, when the animal is under anesthesia, the thermoregulatory mechanisms are blocked. Anesthesia slows down behavioral defense mechanisms, reduces metabolic needs, hypothalamic function and muscle tone. Heat loss begins within the first minutes of premedication because all sedatives and tranquilizers block the hypothalamus. The highest heat loss is during the first 20 minutes of induction, due to its distribution from the center to the periphery of the body. For this reason, it is very important to prevent heat loss at the beginning of the anesthesia, through various methods that will be mentiont later.
At first, the main mechanisms of heat loss are four.
Convection- This is one of the most common ways of losing heat, which occurs when body heat is dissipated into the surrounding space through the air. The larger the surface of the body, the greater the heat loss. In animals, hair greatly interferes with this mechanism and it is important, with a larger shaved area and an open abdominal cavity during prolonged surgery. Conduction – occurs in direct contact of surfaces with different temperatures. For example, when lying on a cold operating table. This mechanism is especially important, when the patient is lying on a wet and cold surface. 
Radiation- The transfer of heat from one surface (e.g. the body) to another without direct physical contact. Radiation is received from the sun by any object exposed to sunlight. The heat load from solar radiation, can be significant in hot environments, where animals are exposed to sunlight for prolonged periods. When an animal is standing in bright sunlight, the amount of solar radiation absorbed may substantially exceed its own metabolic heat production.
Evaporation – evaporation of water at the surface of the body or respiratory tract results in heat loss and it’s approximately 22% of total body loss. 0.58 kilocalories of heat is lost for each gram of evaporated water. In human the evaporation is manifested like sweating but in animals due to the lack of sweat glands, it is expressed by panting. To prevent evaporation from the respiratory tract and a drop in body temperature during anesthesia, the oxygen flow can be reduced if this is compatible with the circuit used and the needs of the patient.
The main physiological disorders that occur with hypothermia are related to reduced liver metabolism, compromised cardiovascular system, reduced ventilation and oxygenation, compromised renal function, reduced cerebral flow. All these factors also influence the slower post-anesthesia recovery. In human medicine, there are many more studies on the subject and more specifically on the direct impact of hypothermia on the body. The most frequently observed are delayed pharmacokinetic and dynamics of anesthetics, impaired coagulation, a threefold increase in the risk of cardiac problems in high-risk patients, an increased likelihood of difficult wound healing and infection, leukocyte migration and suppression along with impaired phagocytosis and neutropenia.
When liver metabolism and enzyme systems are reduced, the metabolism of most anesthetics such as acepromazine, propofol is also impaired. As well as anesthetics can directly block the hypothalamus, such as acepromazine and morphine. Inhalational anesthetics are affected by hypothermia by increasing their solubility but not slowing their potency. They also reduce the intensity of shivering, as a mechanism to conserve heat. It has not been proven, whether that hypothermic patients may take longer to recover from anesthesia because of larger amounts of anesthetic that need to be exhaled. But it’s for sure known that propofol, as one of the most commonly used anesthetics, is also affected by body temperature, as for hypothermia with 3° C down, its plasma concentration increases by 30%. The only drug tested so far, which does not effect thermoregulatory responses, is midazolam. The vasodilator effect of most of the anesthetics surpasses physiological vasoconstriction, which supports thermoregulation. As with vasodilation, there is a large loss of heat that comes from the center of the body and is lost to the periphery.
The negative effect of hypothermia on coagulation and blood has three main factors. It affects – platelet function, coagulation enzyme function and fibrinolytic function. As a rule, hypothermia increases blood viscosity, which leads to deterioration of perfusion. For every 1° C decrease, the hematocrit rises by 2%. This accordingly leads to false results that can be interpreted as blood loss. Since the function of the enzyme systems is disturbed, this also affects blood clotting. PTT, PT increase significantly, there is temporary thrombocytopenia and reduced platelet function occur due to impaired synthesis of thromboxane B2. The morphology of the platelets also changes. There is a hypothesis according to which hypothermia results in coagulopathy by reducing the availability of platelet activators. This hypothesis is supported by the following observations: (a) The generation of thrombin, a potent platelet agonist, decreases under hypothermic conditions, and (b) hypothermia results in the release of a circulating anticoagulant with heparin-like effects. (1)
Due to the vasoconstriction that occurs, the oxygenation of the tissues is reduced and hence their slower healing. Direct suppression of neutrophil function is also a factor influencing healing in addition to the immunosuppressive effect, reducing leukocyte migration, neutrophil phagocytosis and production of ILF 1, 2, 6 and TNF.
In order to avoid all complications of hypothermia, different methods are used for pre-during and post-operative warming of patients. Typically, in the preparation of the animal for surgery, towels are used to cover the table or the animal is wrapped. A heating pad is often placed on the surgical table. The use of fluid-warming devices, which largely support normothermia, is also appropriate. Various methods can be used such as putting socks on the paws, wrapping in bubble rap, placing hot water bottles, red infrared lamps. After surgery, the animal can be wrapped with a blanket and any of the methods of warming can be used. But some of the most effective methods of maintaining a normal body temperature are warm air devices and warm water beds. According to a study comparing several methods of warming and prevention of heat loss, warm air is the most effective. (2) In addition to all the listed methods, it is important to reduce the time of the operation, especially in longer abdominal operations. Avoiding placing animals on cold metal tables, warm operating room.
It is advisable to warm up by 1-2° C per hour and under constant monitoring, because complications can occur from trivial burns to more serious systemic complications. Some of the underestimated ones are the so-called “afterdrop”, in which, despite the warming, the temperature of the animal continues to fall. This is caused by the return of cold blood from the peripheral limbs to the body, which makes it difficult to reach a normal temperature. It is important in such moments to warm up the body (chest, abdomen), and not the extremities. Afterdrop can cause deterioration of physiological parameters, cardiac arrhythmias and arrest.
Rewarming shock is very unknown and underestimated complicaton, which manifests itself in a sudden vasodilation with following drop in blood pressure and cardiac output. This results in increased metabolic demands and increased perfusion requirements. In this regard, there may also be areas of impaired perfusion that are hypoxic and lactate begins to form. During rewarming, these areas are reperfused and lactate re-enters normal oxidative pathways, consuming oxygen in the process. Because of the rewarming acidosis that has occurred, appropriate fluid therapy may be considered. Shivering is a normal response of the body, with which it tries to normalize its temperature, but on the other hand, it can also lead to additional complications, because additional oxygen consumption is needed and this can cause additional hemodynamic instability. The suppression of shivering by neuromuscular blockade is an effective method for decreasing O2 consumption. This method has been described in some human studies. (3) Monitoring during the warm-up should include as many indicators as possible, such as saturation, blood pressure, ECG, lactate, glucose.
