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Inadvertent perioperative hypothermia is the decrease of core body temperature below 36ºC between the preoperative period (1 hour before anaesthesia) and the postoperative period (the first 24 hours after anaesthesia) (1).
In normal conditions, core temperature is regulated by the hypothalamus. When the body temperature is 37°C, a little deviation of 0.2 degrees centigrade causes the thermoregulatory system to function; regulating the production and distribution of heat, and body temperature is kept stable. Intravenous and inhalational anaesthetics inhibit the hypothalamus and increase the regulation range from 0.2°C to 4°C; making the thermoregulatory system react in extremes of body temperature. In addition to all these, the patient under anaesthesia lying motionless and naked in the cold operating room, inhalation of cold gases and heat loss from open body cavities lower the body temperature (2, 3).
Vasodilatation that occurs due to sympathetic blockade induced by neuroaxial blocks leads to a more rapid heat loss. Neuroaxial anaesthesia also inhibits heat production by shivering. Deep hypothermia (34.5–35.5°C) lasts until the block is completely resolved. Combined central neuroaxial block and general anaesthesia leads to a deeper hypothermia (34.5°C). The synergistic effects of both anaesthesia techniques lower the vasoconstriction threshold below 1°C than that occurs during general anaesthesia (4).
According to the recommendations of the American Society of Anaesthesiologists (ASA), temperature monitoring should be carried out in patients who are at a high risk of body temperature changes under anaesthesia. However, temperature monitoring should be performed in all patients undergoing an intervention longer than 30 minutes. Temperature monitoring can be performed from the following sites:
The most accurate measurements of core temperature can be obtained from the distal oesophagus and pulmonary catheter. Temperature monitoring via the urinary bladder is preferred in the intensive care units. Heat loss from open body cavities (e.g. abdomen) and changes in the urinary flow cause body temperature measurements to be less accurate during abdominal surgery (7).
The patients who are at high risk of hypothermia under anaesthesia are as follows: (8):
Passive insulation is used to prevent hypothermia in patients with a body temperature ≥ 36°C. Cotton-wool blankets, socks and head coverings can be used in wards and recovery units. Surgical covers, metalized plastic sheets can be used in the operating rooms. Heat loss can be decreased by 30% by passive insulation. Its effect is directly proportional to the covered surface area (9).
Perioperative period starts 1 hour before surgery, and continues until the subsequent 24 hours. Therefore, warming of patients has to be carried out in the preoperative, intraoperative and postoperative period. Currently, it is recommended to begin warming the patient in the preoperative period. The reason for this is to decrease the difference between core and peripheral temperature, and to prevent internal redistribution. Patients undergoing general anaesthesia, are recommended to be warmed 20 minutes, if possible or at least 10 minutes before surgery (2, 14, 15).
Aim: To decrease the difference between core and peripheral body temperature and to maintain the core temperature between 36 and 37°C.
In the steps mentioned thus far, with regard to patient comfort, the body temperature should be measured from the tympanic or oral route.
Body temperature should be monitored in all surgeries lasting for more than 30 minutes. Intraoperative period starts with the induction of anaesthesia, and continues untill the patient is transferred to the recovery unit. A “Critical incident form” should be completed for patients determined to have hypothermia before anaesthesia induction
Postoperative period involves the time between the arrival of the patient to the recovery unit and postoperative 24 hours.
Body temperature measurement is suggested to be performed from the tympanic membrane in the postoperative period.
NOTE: This guideline is prepared upon the request of the Administrative Board of the Turkish Anaesthesiology and Reanimation Society in order to help the anaesthetists in the prevention of inadvertent perioperative hypothermia.