Control of body temperature by the brain.
The hypothalamus controls core body temperature using thermoreceptors located in the skin, spinal cord, brain, and deep central nervous tissue  .
Body maintenance mechanisms of normothermia.
The body has various mechanisms for maintaining body temperature. In healthy individuals, core temperature is regulated with a tolerance of about ±0.2 °C. These narrow limits above and below the 37 °C set point define the interthreshold range, or the acceptable limits of body temperature. When external factors drive core temperature above this range, the body takes corrective action.
- Vasoconstriction: small blood vessels near the surface of the skin constrict, reducing blood flow and thus heat loss.
- NST: "Non shivering thermogenesis," is an increase in metabolic rate that occurs primarily in infants.
- Shivering: This involves involuntary muscle contractions that generate heat. Because it uses large amounts of energy, it is the body's last resort when other strategies fail. 
When heat loss is less than required, i.e., the body becomes too warm, the body responds with:
- Vasodilation: small blood vessels near the surface of the skin dilate, increasing blood flow and removing excess heat.
- Sweating: The body secretes fluid, which evaporates and cools the skin surface. 
Hypothermia under anesthesia
Normally, a 40-year-old person produces about 70 kilocalories of heat per hour while awake. In the anesthetized state, the metabolic heat production of the same person drops to about 42 kilocalories per hour. 
Thus, under anesthesia, the ability of the hypothalamus to regulate temperature is impaired and the range between thresholds expands to 4 °C. As a result, anesthetized patients are unable to regulate their body temperature. The patient becomes warmer or colder before thermoregulatory responses are triggered. 
During anesthesia, the patient's hypothermia triggers vasoconstriction, which does not result in shivering due to the muscle relaxants that are often administered. Under the influence of anesthesia, the threshold for vasoconstriction is lowered to the point that the arteriovenous shunts, located primarily in the fingers, toes, and nose, are opened. Blood flows unimpeded to the periphery and the body can no longer maintain the temperature gradient between the core and the periphery. 
Impairment of thermoregulation due to anesthesia.
Patients under general anesthesia are unable to regulate their core body temperature to the optimal set point through their behavior and must rely on their autonomic thermoregulatory system. However, the body's autonomic thermoregulatory system is impaired in its ability to function under anesthesia.
Anesthetic drugs inhibit the autonomic system by reducing metabolism and suppressing the thermoregulatory response. 
Main causes of perioperative hypothermia
- the administration of anesthetics during general anesthesia
- the cool temperatures prevailing in most operating rooms
- exposed body parts and incisions during surgery
- the infusion of cold fluids and blood
Heat loss is exacerbated by the cold environment in most operating rooms. The room temperature is usually 16.0 - 21.0 °C. 
According to the laws of thermodynamics, heat always moves from a warmer location or object to a colder area or object. Thus, a significant amount of the patient's body heat is lost to the much cooler environment of the operating room.
The surgical procedure itself results in increased heat loss from the patient. During surgery and surgical preparation, large areas of skin are exposed to the cool environment. Surgical incisions that expose internal organs to the cold surgical air often result in profound internal hypothermia, which exacerbates the natural heat transfer.
Effects of unintended hypothermia.
There are many documented adverse effects of unintended perioperative hypothermia, including:
- Wound infection
- Myocardial ischemia and cardiac dysfunction
- Prolonged and altered drug effects
- Increased mortality
- Chills and thermal discomfort and
- Delayed awakening from anesthesia .
Positive effects from maintaining normothermia.
Studies have shown that maintaining normothermia can lead to positive outcomes such as:
- Reduction in wound infection rates
- Decreased likelihood of myocardial infarction
- Shortened length of stay in the intensive care unit
- Lower mortality rate
- Reduced use of blood products
- Lower likelihood of needing a transfusion
- Savings can range from $2,500 to $7,000 per patient .
Therefore, it is highly desirable to maintain a normothermic temperature perioperatively. Pre-warmed blankets and heating systems are suitable for this purpose, as is the warming of fluids such as infusions, EC, FFP, etc. prior to administration.
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