With the progress of medicine and the improvement of people’s living conditions, the per capita life expectancy in China has increased significantly. In recent years, the proportion of elderly patients in the consultation population has been rising, and the number of elderly patients undergoing surgery is also increasing, and advanced age is no longer a contraindication to surgery. Body temperature is an important vital sign of the body. Under physiological conditions, the body maintains a dynamic balance between heat production and heat dissipation through the thermoregulatory system, so that the body temperature is maintained at a constant 37.0±0.4. Under anesthesia, a number of factors impair the thermoregulatory function of the patient, resulting in a passive drop in body temperature.Sessler et al. defined a core body temperature of 34°C to 36°C as shallow hypothermia, and without intervention, 40% to 60% of the patients were in a state of shallow hypothermia after surgery. A higher incidence of postoperative shallow hypothermia has been observed in the elderly, and there is a strong correlation with postoperative complications, especially postoperative pulmonary infections. And the perioperative thermal insulation method is simple and safe. It can effectively reduce surgical complications, this paper gives a review on the influencing factors of perioperative hypothermia in elderly patients, its hazards and its prevention and treatment. 1, body temperature and thermoregulation 1.1 Body temperature The temperature of the human body includes body core temperature (core temperature) and body surface temperature (shell temperature). Body core temperature, also known as core temperature, refers to the temperature of internal organs, represented by the rectum and esophagus temperature. Due to different metabolic levels, the temperature of internal organs is slightly different, but this difference is small, the body temperature is generally around 37 ℃, the brain temperature is close to 38 ℃, the liver temperature is the highest, up to 38 ℃, the kidneys, the pancreas and the duodenum temperature is lower. Circulating blood flow is an important medium for transferring heat within the body. Body temperature can fluctuate by 1°C at different times of the day in adults and by about 0.5°C with the menstrual cycle in women. The accuracy of temperature control is similar in men and women, whereas in the elderly the accuracy of temperature control decreases. Body surface temperature mainly refers to the skin temperature, which is generally lower than the body core temperature, unstable, and varies greatly between parts, with an obvious temperature gradient from the surface to the inside. 1.2 Thermoregulation Thermoregulation includes behavioral thermoregulation (e.g. exercise for warmth, active heat preservation or cooling measures, etc.) and autonomic thermoregulation (e.g. chills, arteriovenous shunt, vasoconstriction, etc.), and perioperative thermoregulation is autonomic thermoregulation. The thermoregulation of the human body consists of three links: temperature receptors, hypothalamus and thermoregulatory responses, which keep the body temperature at a relatively constant level through three forms of heat production, heat dissipation and heat distribution. 1.2.1 Temperature receptors: (1) Peripheral temperature receptors: mainly distributed in the whole body skin, certain mucous membranes and visceral organs, belong to the temperature-sensitive free nerve endings, including cold receptors and heat receptors. The number of cold receptors is more than that of heat receptors, in which the number of skin cold receptors is about 4 to 10 times that of heat receptors, and the discharge frequency is much higher than that of heat receptors. Therefore, peripheral thermoreceptors mainly feel cold stimuli: (2) central thermoreceptors: distributed in the spinal cord, medulla oblongata, brainstem reticular formation and hypothalamus, they are neurons sensitive to temperature changes, including heat-sensitive neurons that increase the frequency of impulse release when the temperature rises, and cold-sensitive neurons that increase the frequency of impulse release when the temperature falls. These two kinds of neurons are mainly distributed in the preoptic region/anterior hypothalamus (PO/AH), in which there are significantly more heat-sensitive neurons than cold-sensitive neurons, indicating that the central thermoreceptors mainly feel warm and hot stimuli. 1.2.2 Hypothalamus According to the theory of hypothalamus, PO/AH of hypothalamus is an important part of the integration mechanism of thermoregulatory center. The central temperature-sensitive neurons in the spinal cord, medulla oblongata and brainstem reticular formation converge in the PO/AH area with the incoming temperature information, and the PO/AH receives, integrates and compares the temperature information with the temperature threshold, then triggers the autonomic thermoregulatory response, and regulates the skin blood vessel diastasis, secretion of the sweat glands, the activity of the skeletal muscles and the metabolic level of the organs involved in the endocrine system to maintain the body temperature by means of the efferent nerves. of organ metabolic levels to maintain a relatively constant body temperature. 1.2.3 Thermoregulatory Responses The hypothalamus maintains normal body temperature by regulating sweat glands and blood vessels and by inducing chills. The core temperature at which the thermoregulatory defense response is triggered is called the threshold (threshold), which is typically 37°C. When the body core temperature is higher than the thermal response threshold, the frequency of heat-sensitive neurons emitting impulses increases, causing increased heat dissipation, which is manifested by active precapillary vasodilatation, sweating, and decreased heat production, causing the body temperature to fall back to 37℃. When the body core temperature is lower than the cold response threshold, cold-sensitive neurons emit impulses to increase the frequency of heat-sensitive neuron activity is weakened, causing an increase in heat production, such as thyroid secretion to increase organ metabolism to raise the level of vascular constriction of arterial short-circuit vasoconstriction so that the blood flow is reduced, thereby reducing heat dissipation, shivering through the involuntary muscle movement so that the basal metabolic rate increased to 2 ~ 3 times the normal value, 25% ~ 50% of the sweat glands secretion stops! The chills increase the basal metabolic rate to two to three times the normal value through involuntary muscle movement, stopping 25% to 50% of the sweat gland secretion, thus bringing the body temperature back to 37℃. The core temperature between the thermal response threshold and the cold response threshold is called the interthreshold range (interthreshold range), about 0.2 ℃, within this range of temperature change will not trigger the autonomous thermoregulatory response. 2. Factors influencing perioperative hypothermia 2.1 Age As a result of the aging process, older adults have thinner skin and an elevated ratio of body surface area to body weight, which makes it easier for them to dissipate heat in a cold surgical environment. In the elderly, muscle atrophy and reduction in number and resting muscle tone are reduced, resulting in a decrease in myotonic heat production. Elderly skin vasoconstriction response ability to reduce, in the cold stimulation of vasoconstriction response threshold is lower than that of adults [1], and the elderly themselves thermoregulation ability to decline, compensatory capacity is weakened, more susceptible to environmental influences 2.2 room temperature Room temperature 32 ℃ or more than 32 ℃, and more than 3h general anesthesia surgery, 75% ~ 85% of the patient’s body temperature can be elevated to 38 ℃. When the room temperature is below 21℃ and the operation time is more than 2h, the hypothermia of all patients can be reduced to 34～36℃, and the degree of decline is especially obvious in elderly patients. 2.3 Anesthesia methods and anesthesia drugs Both general anesthesia and intrathecal anesthesia can impair the normal precise core temperature regulation. The interthreshold range of core temperature under general anesthesia can reach 4°C, 20 times the normal value. All general anesthetics trigger a dose-dependent decrease in core temperature, which in turn triggers a cold defense response, and most of them, such as isoproterenol, have direct vasodilatory effects, whereas muscarinic medications paralyze the skeletal muscles, losing the thermogenic effect of increasing muscle tone and inhibiting the patient’s shivering. Therefore, almost all patients with general anesthesia have shallow hypothermia in the postoperative period The process of body temperature decline under general anesthesia can be divided into three time phases, namely, the redistribution – linear – plateau time phase. The first time phase occurs within 1h after induction of anesthesia, anesthetics make the central thermoregulation damaged, the body heat is transferred from the center to the periphery, the core temperature drops 1℃～1.5℃ rapidly; the second time phase is 2～3h after induction of anesthesia, the heat is lost to the surroundings through the skin’s radiation and convection, the core temperature can drop slowly; the third time phase is when the body’s core temperature drops to a certain threshold of cold response , triggering thermoregulatory vasoconstriction, which reduces skin heat dissipation and maintains a constant body temperature. However, if no insulation measures are taken at this time, heat is still being lost and body temperature can fall further. The reasons for the decrease in core body temperature in the early stages of intrathecal anesthesia are the same as those for general anesthesia, only to a slightly lesser extent, but limited to the lower extremities. Unlike general anesthesia, because intrathecal anesthesia blocks centrally mediated thermoregulatory vasoconstriction of the lower extremities from the periphery, body temperature decreases linearly and continuously without a plateau period. In addition, intrathecal anesthesia blocks the afferent nerves of peripheral thermoreceptors, and local anesthetic dilates the peripheral vasculature, which increases the skin temperature so that the hypothalamus mistakenly believes that the blocked area is warm and does not trigger a cold response, at which time the threshold can drop by 0.5℃. General anesthesia compounded with intrathecal anesthesia results in redistributed hypothermia earlier than intrathecal anesthesia and general anesthesia alone, with a faster rate of temperature decline in the linear phase, making it more likely that severe hypothermia will occur. 2.3 Intraoperative operation The temperature and humidity of inhaled gas during mechanical ventilation are not properly adjusted, large areas of exposed skin, wiping the patient’s skin with volatile disinfectant such as iodine, alcohol, etc., prolonged exposure of the thoracic and abdominal cavities, flushing of the body cavities with cold liquids, and intravenous dripping of large amounts of unheated liquids and stored blood can lead to a large amount of heat dissipation. It has been reported that the input of 1L of room temperature crystalloid or 200mL of 4℃ library blood can make the body temperature drop 0.25℃. 3, perioperative hypothermia harm to the body hypothermia can reduce the body’s metabolic rate, ischemia and hypoxia of the tissues have a certain protective effect, but at the same time the harm to the body is also multi-faceted. 3.1 Effects on general anesthesia drugs During constant rate infusion of isoproterenol, the plasma drug concentration of hypothermic patients was higher than that of those with normal body temperature. Under shallow hypothermia, the blood concentration of fentanyl increases with decreasing body temperature; the clearance of midazolam decreases with decreasing body temperature; the delayed onset of action, prolonged duration of action, and possible prolonged recovery time of vinculobromine and atracurium. Tissue solubility of volatile anesthetics increases at hypothermia, and the patient’s recovery time from anesthesia is prolonged due to the need to exhale more volatile anesthetics. 3.2 Induced respiratory complications Hypothermia causes bronchospasm and increased bronchial secretion; it inhibits the medullary respiratory center and suppresses the cough reflex; it weakens the ciliary movement and weakens the protective airway reflex. Hypothermia makes the tissue because of oxygenated hemoglobin dissociation curve left shift, the utilization of oxygen decreased and hypoxia, tissue hypoxia coupled with coagulation dysfunction, can cause lung injury. Chest wall stiffness increases in the elderly, respiratory muscle strength becomes weaker, lung reserve capacity decreases, and perioperative hypothermia is more likely to induce respiratory complications such as pulmonary infection, which is one of the high-risk factors for perioperative death in the elderly. 3.3 Increased cardiovascular adverse reactions Zhang Zhixiong et al. found that the steady state blood concentration of cold was elevated after stress in sympathetic excitability while the change in vagal excitability was not significant. Increased sympathetic tone increases heart rate, myocardial contractility, and cardiac output. At the same time, increased sympathetic tone also causes peripheral vasoconstriction, increased circulatory resistance, increased myocardial ischemia and arrhythmia. Hypothermia increases blood viscosity, which not only increases vascular resistance, but also causes venous stagnation and reduced oxygen supply to local tissues, which further causes deep vein thrombosis. Frank et al. showed in a prospective randomized study that elevated norepinephrine in hypothermic patients increased the incidence of adverse cardiac events by 30%. In addition, hypothermia causes hypokalemia, which is positively correlated to some extent, and can cause arrhythmias such as ventricular tachycardia and ventricular fibrillation. In moderate hypothermia (28°C to 32°C), the patient’s heart rate slows down and is prone to atrioventricular block. Elderly people are often combined with cardiovascular diseases. In the elderly due to myocardial interstitial fiber hyperplasia so that myocardial compliance decreases, myocardial contractility is weakened, the cardiac reserve function decreases, the elderly vagal tone ageing increase and the reduction of cardiac pacemaker cells, sinus heart rate is slow [15]. Therefore the elderly are more prone to adverse cardiovascular events in perioperative hypothermia. 3.4 Disturbances in coagulation mechanism It has been reported that when prothrombin time is measured at different temperatures, a decrease in temperature of 3°C increases the prothrombin time by approximately 10%. Platelet adhesion and aggregation are abnormal at low temperatures, and the availability of platelet activating factor is reduced. Cang Jing reported that perioperative hypothermia reduced the number of platelets, platelet activity and coagulation factor activity, resulting in inhibition of coagulation and prolongation of bleeding time. Therefore, hypothermia increases intraoperative blood loss by 16% and the relative risk of blood loss by 22%. Some scholars believe that hypothermia patients have an increased need for blood transfusion, but there are also those who hold different opinions. 3.5 Suppression of immune function, hypothermia decreases the phagocytosis ability and oxidative killing effect of neutrophils and increases the rate of wound infection. Hypothermia reduces collagen deposition in the wound, decreasing the healing ability of surgical incisions and prolonging hospitalization.Flores reported that the rate of wound infection in perioperative patients was 6.3 times higher than that in normothermic patients.3.6 Delayed awakening, Hypothermia has been shown to increase the risk of wound infection. 3.6 Delayed awakening Hypothermia prolongs the duration of action and recovery time of anesthetic drugs.There is a correlation between age and awakening time.Intraoperative hypothermia prolongs the stay of the elderly in the postoperative awakening room. 3.7 Chills The incidence of postoperative chills is 40% due to hypothermia, which increases the body’s oxygen consumption, increases the dose of analgesics, and increases the chances of wound dehiscence. Patients remember the pain of postoperative chills more than the pain of wounds, and the incidence of respiratory and cardiovascular adverse events increases in the elderly. 3.8 Other postoperative mortality rate of hypothermic patients is higher than that of patients with normal body temperature, especially in patients with severe trauma, under the vicious circle of hypothermia, acidosis and coagulation disorders, hypothermic patients have a significantly higher rate of death than those with normal body temperature. 4, perioperative hypothermia prevention and control Frank and other studies show that the cost of perioperative hypothermia prevention is only about one percent of the cost of perioperative hypothermia treatment. Therefore, prevention is better than treatment. 4.1 Healthcare professionals pay attention to perioperative thermal insulation: (1) anesthesiologists must raise awareness of the dangers of perioperative hypothermia, and intraoperative body temperature should be maintained at 36 ℃ or above. the ASA guidelines for perioperative thermal insulation suggest that patients with general anesthesia for more than 30 min and regional blockade with significant changes in expected body temperature (e.g., body cavity surgery, prolonged major surgery, etc.) should be monitored for body temperature. Currently, blood temperature measured by a sensor on the pulmonary artery catheter is considered the gold standard for core temperature measurement. In addition, the nasopharynx, esophagus, rectum, and tympanic membrane are also sites for core temperature monitoring. Preoperative prewarming can largely prevent or mitigate body heat redistribution. Maximizing the heating area can reduce the occurrence of thermal injury. (2) The temperature of the operating room should be 23℃～25℃, and the relative humidity is 60%～70%. For prolonged surgery, the exposed skin should be covered with warm saline gauze. Body cavity irrigation fluid should be warmed. Preoperative psychological reassurance given to the patient can reduce the decreased threshold for cold stimuli caused by the patient’s nervousness. (3) In addition to covering the patient on the way to and from the hospital, special attention should be paid to keeping the head, neck, and feet warm. 4.3 Body surface warming: (1) passive isolation Non-surgical area of the cover can reduce heat loss by 30%, the effect is mainly proportional to the area covered; (2) active warming Most patients need to be actively warmed in order to keep warm, and at present, the most effective non-invasive warming method is the forced airflow heating system and the electric heat-resistant heating blanket. Elderly people have low skin sensitivity and are prone to burns. If an electric blanket is used, the blanket should be covered with a disposable center sheet to prevent leakage and burns; if a warm blanket machine is used, heating the extremities is more effective than heating the trunk. 4.4 Heating infusion Fluid isothermal with the environment or frozen blood products can reduce body temperature, preheating should be preheated to 37 ℃ before infusion, infusion using infusion warmer to warm blood products and fluids, but this is still insufficient to elevate the core temperature, an effective method is to increase the metabolic rate of amino acids by intravenous drip. 4.5 Airway warming Moisturizing and heating filters can reduce respiratory heat loss, and also reduce the inhibition of respiratory cilia movement, but they are basically unhelpful in elevating the core temperature. 4.6 Postoperative heat preservation After surgery, damp clothes should be replaced in time to keep the body dry, and covered with a good quilt. Appropriate methods of warming are used according to the patient. In summary, elderly patients are more prone to lower body temperature due to physiologic decline, and a series of complications associated with hypothermia have more serious consequences for the elderly. Therefore, perioperative thermal insulation is particularly important for elderly patients.