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 is increasing, 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 loss through the thermoregulatory system, so that the body temperature is maintained at a constant level of 37.0±0.4℃. Sessler et al. defined a core body temperature of 34°C to 36°C as shallow hypothermia, and if no intervention is taken, 40% to 60% of patients will be in a state of shallow hypothermia after surgery. Chen Peilian et al. observed a higher incidence of postoperative shallow hypothermia in the elderly and a strong correlation with postoperative complications, especially postoperative pulmonary infections. While perioperative insulation methods are simple, safe and can effectively reduce surgical complications, this paper presents a review of the factors influencing 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 the core temperature (core temperature) and the surface temperature (shell temperatureo core temperature, also known as the core temperature, refers to the temperature of the internal organs, represented by the rectal and esophageal temperature. Due to different levels of metabolism, the temperature of internal organs varies slightly, but this difference is small. The human body temperature is generally around 37℃, the brain temperature is close to 38℃, the liver temperature is the highest, up to 38℃, the kidney, pancreas and duodenum temperature is lower, and the circulating blood flow is an important medium for heat transfer in the body. The body temperature of adults can fluctuate from day to day at different times by 1℃, and that of women fluctuates by about 0.5℃ with the menstrual cycle. The accuracy of temperature control is similar in males and females, while the accuracy of temperature control decreases in the elderly”. The body surface temperature mainly refers to the skin temperature, which is generally lower than the core temperature, unstable, and varies greatly from part to part, and there is a significant temperature gradient from the surface to the inside. 1.2 Thermoregulation There are behavioral thermoregulation (such as exercise for warmth, active insulation or cooling measures, etc.) and autonomic thermoregulation (such as chills, arteriovenous shunts, vasoconstriction, etc.), and perioperative thermoregulation is autonomic thermoregulation. The thermoregulation of human body is composed of three links: thermoreceptors, hypothalamus and thermoregulatory response, which maintain 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 body skin, some mucous membranes and visceral organs, belong to the temperature-sensitive free nerve endings, including cold receptors and thermoreceptors. The number of cold receptors is more than that of heat receptors, of which the number of skin cold receptors is about 4-10 times that of heat receptors, and the discharge frequency is much higher than that of heat receptors. Therefore, peripheral temperature receptors mainly feel cold stimuli; (2) central temperature receptors: distributed in the spinal cord, medulla oblongata, brainstem reticular formation and hypothalamus, are neurons sensitive to temperature changes, including heat-sensitive neurons that increase the frequency of impulse delivery when the temperature rises and cold-sensitive neurons that increase the frequency of impulse delivery when the temperature falls. These two types of neurons are mainly distributed in the preoptic region and the anterior hypothalamus (PO/AH), where there are significantly more heat-sensitive neurons than cold-sensitive neurons, indicating that the central temperature receptors mainly sense warm and hot stimuli. 1.2.2 The hypothalamic tuning point theory suggests that the PO/AH of the hypothalamus is an important part of the central integration mechanism of thermoregulation. The central temperature-sensitive neurons in the spinal cord, medulla oblongata, and brainstem reticular formation converge the perceived local tissue temperature change information together with the afferent temperature information in the PO/AH area. The PO/AH receives, integrates, and compares the temperature information with the temperature threshold, then triggers the autonomic thermoregulatory response, which regulates the skin vasodilation and contraction, sweat gland secretion, skeletal muscle activity, and endocrine system involvement through the efferent nerves. The body temperature is maintained at a relatively constant level by regulating the level of organ metabolism through the efferent nerves. 1.2-3 Thermoregulatory responses The hypothalamus maintains normal body temperature primarily through the regulation of sweat glands and blood vessels and by inducing chilling effects. The core temperature at which the thermoregulatory defense response is triggered is called the threshold (threshold), which is generally 37°C. When the core temperature is higher than the threshold of thermal response, the frequency of impulses issued by thermosensitive neurons increases, causing increased heat dissipation, which is manifested by active precapillary vasodilation, sweating, and reduced heat production, bringing the body temperature back down to 37℃. When the body nucleus temperature is lower than the cold response threshold, the frequency of impulses issued by cold-sensitive neurons increases and the activity of heat-sensitive neurons decreases, causing an increase in heat production: for example, thyroid secretion increases to raise the level of organ metabolism, arteriovenous short-circuit vasoconstriction reduces blood flow and thus heat dissipation, and involuntary muscle movements during chills increase the basal metabolic rate to 2-3 times its normal value”. The core temperature between the thermal response threshold and the cold response threshold is called the interthreshold range (interthreshold range), which is about 0.2°C. The temperature change within this range does not trigger the autonomic thermoregulatory response. 2. Factors influencing perioperative hypothermia 2.1 Age As a result of the aging process, older people have thinner skin and a higher ratio of body surface area to body weight, making it easier to dissipate heat in a cold surgical environment. Muscle atrophy and reduced number of resting muscles in the elderly leads to reduced myotonic heat production. The skin vasoconstriction response ability of the elderly is reduced, and the threshold of vasoconstriction response when stimulated by cold is lower than that of adults, and the elderly have a reduced ability to regulate their own body temperature and a weakened compensatory capacity, which makes them more susceptible to environmental influences. 2.2 Room temperature When the room temperature is above 32℃ and the general anesthesia surgery is more than 3h, the body temperature can rise to 38℃ in 75%-85% of patients. When the room temperature is below 2l℃ and the operation time is more than 2h, the body temperature of all patients can be reduced to 34-36℃, and the degree of decrease in body temperature is especially obvious in the low temperature environment of elderly patients. 2.3 Anesthetic modalities and anesthetic drugs Both general anesthesia and intralesional anesthesia can impair normal and precise core temperature regulation. The inter-threshold range of core temperature under general anesthesia can reach 4°C, which is 2O times the normal value All general anesthetic drugs trigger a dose-dependent decrease in core temperature, which subsequently triggers a cold defense response, and isoproterenol and other drugs mostly have a direct vasodilatory effect, while inotropic drugs paralyze skeletal muscle, losing the thermogenic effect of increasing muscle tone and suppressing the patient’s chills. Therefore, without intervention, shallow hypothermia is almost always present in patients under general anesthesia postoperatively. The process of decreasing body temperature under general anesthesia can be divided into 3 time phases, namely the redistribution I linear-plateau phase. The first phase occurs within 1 h after induction of anesthesia, when the anesthetic agent impairs central thermoregulation and the body heat is transferred from the center to the periphery, resulting in a rapid decrease in core temperature of 1°C to 1.5°C. The second phase is 2-3 h after induction of anesthesia, when heat is lost to the peripheral environment through radiation and convection of the skin, and core temperature can fall slowly; the third phase is when the body’s core temperature falls to a certain threshold of cold response The third phase is when the body’s core temperature drops to a certain cold response threshold, triggering thermoregulatory vasoconstriction, thus reducing skin heat loss and maintaining a constant body temperature. However, if no insulation measures are taken at this time, heat is still being lost and body temperature can drop further. The reason for the decrease in core body temperature in the early phase of endotracheal anesthesia is the same as that of general anesthesia, caused by redistribution of body heat, except that redistribution is confined to the lower extremities, so the degree of core temperature decrease is slightly less…. Unlike general anesthesia, there is a linear and sustained decrease in body temperature without a plateau period because endotracheal anesthesia blocks centrally mediated thermoregulatory vasoconstriction in the lower extremities from the periphery. In addition, intralesional anesthesia blocks the afferent nerves of the terminal temperature receptors, and the local anesthetic dilates the peripheral vasculature, causing the hypothalamus to mistakenly believe that the blocked area is warm and does not trigger a cold response, at which point the threshold can drop by 0.5°C. General anesthesia compound intravertebral anesthesia than simple intravertebral anesthesia or general anesthesia earlier redistribution of hypothermia, linear period of body temperature drops faster, more likely to occur severe hypothermia. 2.4 Intraoperative operation of mechanical ventilation when the temperature and humidity of the inhaled gas are not properly adjusted, large areas of exposed skin, wiping the patient’s skin with volatile disinfectants such as iodine or alcohol, prolonged exposure of the thoracic and abdominal cavities, flushing of the body cavity with cold liquids, intravenous infusion of large amounts of unheated fluids and reservoir blood can result in significant heat dissipation. It has been reported that the input of 1L of room temperature crystalloid or 200 mL of 4℃ blood can cause the body temperature to drop by 0.25℃. 3, perioperative hypothermia on the body’s harm Hypothermia can reduce the body’s metabolic rate, the ischemic and hypoxic tissue has a certain protective effect, but at the same time the harm to the body is also multifaceted. 3.1 Effects on general anesthetic drugs During constant rate infusion of isoproterenol, the plasma drug concentration of hypothermic patients is higher than that of those with normal body temperature. Under shallow hypothermia, the blood concentration of fentanyl increases with decreasing body temperature; the clearance rate of midazolam decreases with decreasing body temperature; the onset of action of vecuronium and atracurium is delayed, the duration of action is prolonged, and the recovery time may also be prolonged. The 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; suppresses the medullary respiratory center and inhibits the cough reflex; weakens ciliary movements and weakens the protective airway reflex. Hypothermia causes tissue hypoxia due to a leftward shift in the dissociation curve of oxygenated hemoglobin and a decrease in the utilization of oxygen. Tissue hypoxia combined with coagulation dysfunction can cause lung injury. Increased chest wall stiffness, weakened respiratory muscle strength and reduced lung reserve capacity in the elderly make perioperative hypothermia more likely to induce respiratory complications such as pulmonary infection, which is one of the high-risk factors for perioperative mortality in the elderly. 3.3 Increased cardiovascular adverse reactions Zhang Zhixiong et al. “” found that cold steady-state blood concentrations increased sympathetic excitability after stress while vagal excitability did not change significantly. Increased sympathetic tone increased heart rate, increased myocardial contractility, and increased cardiac blood output. Increased sympathetic tone also causes peripheral vasoconstriction, increased circulatory resistance, and increased myocardial ischemia and arrhythmias. Hypothermia increases blood viscosity, which increases vascular resistance and also increases venous stasis and decreases local tissue oxygen supply, further causing deep vein thrombosis. a prospective randomized study by Frank et al. showed that elevated norepinephrine in patients with shallow body temperature increased the incidence of adverse cardiac events by 30%. In addition, hypothermia causes hypokalemia, and the two are positively correlated to some extent, with hypokalemia causing arrhythmias such as ventricular tachycardia and ventricular fibrillation. When moderate hypothermia (28~C to 32%) is present, the patient’s heart rate slows down and atrioventricular block is likely to occur. The elderly are often combined with cardiovascular diseases. The elderly due to myocardial interrogative fiber hyperplasia myocardial compliance decreases, myocardial contractility decreases, cardiac reserve function decreases, the elderly vagus nerve tone ageing increase and the reduction of cardiac pacing cells, so that the sinus heart rate slow chills when the body oxygen consumption increases several times, for low cardiopulmonary reserve function, especially with myocardial ischemia in elderly patients, there is a risk of angina pectoris, so the elderly perioperative hypothermia more Therefore, perioperative hypothermia in the elderly is more likely to occur cardiovascular adverse events. 3.4 Disorders of coagulation mechanism Some studies have reported that when prothrombin time was measured under different temperature conditions, a 3°C decrease in temperature increased prothrombin time by about 10%. Platelet adhesion and aggregation are abnormal at low temperatures, and the availability of platelet-activating factors is reduced. Cang Jing reported that “perioperative hypothermia reduces the number of platelets, platelet activity and coagulation factor activity, resulting in inhibition of coagulation and prolonged bleeding time. Therefore, hypothermia increases intraoperative blood loss by 16% and the relative risk of blood loss by 22%It7]. It has been suggested that hypothermic patients have an increased need for blood transfusions”, but there are those who hold different views. 3.5 Suppression of immune function, decreased phagocytosis and oxidative killing of neutrophils at hypothermia, and increased wound infection rate. The rate of wound infection is 6.3 times higher in perioperative patients than in those with normal body temperature, according to Flores. 3.6 Delayed awakening Hypothermia prolongs the duration of action and recovery of anesthetic drugs, and there is a correlation between age and awakening time”, and intraoperative hypothermia prolongs the duration of stay in the postoperative awakening room in the elderly. 3_7 chills hypothermia makes the incidence of postoperative chills 40%, chills increase the oxygen consumption of the body, increase the dose of analgesics, and increase the chance of wound dehiscence, patients’ painful memories of postoperative chills are more than wound pain, chills also increase the incidence of respiratory and cardiovascular adverse events in the elderly. 3.8 Others: The 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 constituted by hypothermia, acidosis and coagulation disorders, the morbidity and mortality rate of hypothermic patients is significantly increased than that of normal body temperature patients. 4, perioperative hypothermia prevention Frank and other studies show that the cost of using perioperative hypothermia prevention is only about one percent of the cost of treating patients with perioperative hypothermia. Therefore, prevention is better than treatment. 4.1 Health care workers pay attention to perioperative hypothermia (1) Anesthesiologists must raise awareness of the dangers of perioperative hypothermia, and should maintain body temperature above 36 during surgery.ASA guidelines for the treatment of perioperative hypothermia recommend that patients with general anesthesia for more than 30 min and regional blocks with significant expected temperature changes (e.g., body cavity surgery, prolonged major surgery, etc.) should have their body temperature monitored intraoperatively. Currently, blood temperature measured by a sensor on the pulmonary artery catheter is considered the gold standard for core temperature measurement [22]. o In addition, the nasopharynx, esophagus, rectum, and tympanic membrane are also sites for core temperature monitoring; (2) the operating room temperature should be 23% to 25%°C, with a relative humidity of 60% to 70%. For prolonged surgery, the exposed skin should be covered with warm saline gauze; the body cavity flushing solution should be warmed; (3) in addition to covering the patient on the way, special attention should be paid to the warmth of the head, neck and feet; (4) psychological reassurance is given to the patient before surgery, which can reduce the patient’s threshold for cold stimulation caused by tension. 4.2 Body surface warming Pre-operative pre-warming can largely prevent or reduce body heat redistribution, and maximizing the warming area can reduce the occurrence of thermal injury. (1) passive isolation of the non-surgical area of the cover can reduce the heat loss by 30%, the effect is mainly proportional to the area covered; (2) active heating most patients need active heating to keep warm, the most effective non-invasive heating methods are forced airflow heating system and thermal resistance heating blanket elderly people with low skin sensitivity, easy to scald. If using electric blanket, the electric blanket should be covered with a disposable medium sheet to prevent leakage and burns; if using a warm blanket machine, heating the extremities is more effective than torso heating. 4.3 Heated infusion Fluids or frozen blood products that are isothermal to the environment can lower the body temperature, so it is advisable to preheat to 37℃ before infusion. 4.4 Airway warming For mechanically ventilated patients, the use of an anesthesia machine humidification module with a wet heat exchanger is effective in warming the inhaled gas, reducing respiratory heat loss and also reducing the inhibition of respiratory cilia movement, but it is basically unhelpful in raising core temperature. 4.5 Postoperative insulation Postoperative damp clothes should be changed promptly to keep the body dry and covered with quilts. According to the patient’s needs, appropriate methods of warming are used. In summary, elderly patients are more prone to intraoperative hypothermia due to their reduced physiological function, and a series of complications associated with hypothermia cause more serious harm to the elderly. Therefore, perioperative warming is particularly important for elderly patients.