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Logo of tjarTurkish Journal of Anaesthesiology and Reanimation
Turk J Anaesthesiol Reanim. 2013 October; 41(5): 188–190.
Published online 2013 October 1. doi:  10.5152/TJAR.2013.64
PMCID: PMC4894098

The Turkish Anaesthesiology and Reanimation Society Guidelines for the prevention of inadvertent perioperative hypothermia

1. Definitions

a. What is inadvertent perioperative hypothermia?

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).

b. What is the role of anaesthesia in the development of inadvertent hypothermia?

Heat loss from the body is directly associated with age, gender, body surface area, type and duration of the operation, ambient temperature and the duration of mechanical ventilation (1, 2).

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).

c. Phases of Perioperative Hypothermia

  • Phase 1: In the first 60 minutes, core body temperature is decreased by 0.5–1.5°C due to thermal redistribution.
  • Phase 2: After internal redistribution, heat from core to peripheral tissues heat loss continues. This period coincides with the 2nd to 4th hours of anaesthesia. Body temperature falls below 35°C.
  • Phase 3: This is the 3th to 4th hours of anaesthesia. In this period, peripheral vasoconstriction occurs and the core temperature is stabilized at 33–35°C (5).

d. Complications that can occur due to inadvertent Perioperative Hypothermia (6)

  1. The duration of action of hypnotic drugs and neuromuscular blocking agents is prolonged,
  2. Intraoperative intraoperative blood loss and the need for blood transfusion increases,
  3. Cardiac problems increase and may lead to mortality,
  4. Duration of post anaesthesia recovery is prolonged,
  5. Shivering increases oxygen consumption, and also decreases the thermal comfort of the patient,
  6. The development of surgical wound site infections is eased,
  7. The incidence of postoperative nausea and vomiting increases,
  8. Duration of hospital stay and costs increase.

e. Monitoring of Inadvertent Perioperative Hypothermia

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:

  1. Tympanic membrane,
  2. Nasopharynx,
  3. Distal oesophagus,
  4. Skin/axillary region,
  5. Urinary bladder,
  6. Pulmonary artery catheter.

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):

  1. ASA grade II–IV, being female,
  2. Patients with a preoperative body temperature below 36°C,
  3. Patients who were sedated and premedicated,
  4. Patients undergoing major or intermediate surgical intervention,
  5. Patients with concomitant cardiovascular diseases,
  6. Patients undergoing combined regional and general anaesthesia,
  7. Patients over 70 years of age,
  8. Patients with a systolic blood pressure above 140 mmHg

f. Techniques used in the prevention of Inadvertent Perioperative Hypothermia

a) Passive insulation

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).

b) Active Warming Techniques

  1. Forced-air warming devices: These systems decrease heat loss via radiation’ and increase the temperature by convection. They may increase the body temperature by approximately 0.75°C/hour in the intraoperative period. They are recommended to be used with blankets specific to the blowers. They are effective in active warming of the patients having preoperative hypothermia. These blankets should cover as much part of the body as possible 10).
  2. Electric heating blankets (Resistive Systems): These are as effective as the forced-air warming devices. They can warm multiple independent body parts. These systems are reusable and may be cost effective. They contain water or special gels. The electric blankets which use covers with electrical wires are not suitable (11). Local skin burns can occur in overweight patients lying in the same position for a long time, and in paediatric patients.
  3. Radiant heaters: They don’t affect convective heat losses. They are generally used in paediatric cases. Their warming effects change according to the distance between the operating table and the device (12).
  4. Intravenous fluid, blood, blood-product warmers: They are not effective when used solely. They should be used if patients will receive more than one litres of fluid at room temperature (12).
  5. Heat and moisture exchanging filters: They may be used along with the other warming techniques. They improve the comfort of the patient. (9).
  6. Negative pressure warming devices: Extremities are covered by special coverings, the blanket is heated to 44–46°C and 30–40 mmHg negative pressure is applied. It is a newly-developed technique (13).

g. When should warming of the patient begin in the prevention and management of Inadvertent Perioperative Hypothermia?

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).

2. Guidelines for the prevention and treatment of Inadvertent Perioperative Hypothermia

Aim: To decrease the difference between core and peripheral body temperature and to maintain the core temperature between 36 and 37°C.

Preoperative Period

  1. During the visit before anaesthesia, the patient and the relatives should be informed that the hospital and the operating room are much more colder than their homes. They should be reminded that they can bring socks and blankets from their houses, and ask for additional sheets and blankets from the staff if they feel cold.
  2. The ward nurses should be educated about the temperature measurement devices. They should be informed that they should transfer the patients to the operating room when the patients’ body temperature is at least 36°C, and apply active warming in patients with a body temperature below 36°C. The patients should be transferred to the operating room with their socks on. On arrival at the operating room, the patient should be taken to the recovery unit. The ambient temperature of the recovery unit should be 22–24°C.
  3. The patients should be transferred to the recovery unit at least 20 minutes before the surgery. Body temperature should be measured. In patients with a body temperature ≥ 36°C, passive insulation should be applied in order to prevent heat loss. Patients with a body temperature below 36°C should be actively warmed. Forced-air warming systems and suitable blankets should be used for this purpose. The patients who are at risk or who had been premedicated should receive special care.
  4. The patient should not be transferred from the recovery unit to the operating room until body temperature is ≥36°C.
  5. The body temperature of the patients taken into the operating room should be recorded and anaesthesia induction should not be started in those with a body temperature below 36°C.

In the steps mentioned thus far, with regard to patient comfort, the body temperature should be measured from the tympanic or oral route.

Intraoperative Period

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

  1. The core temperature of the patient, whose body temperature is measured before anaesthesia induction, should be monitored from distal oesophagus after induction.
  2. If oesophageal temperature monitoring is not available, temperature should be measured from the tympanic membrane at 15 minute-intervals.
  3. It should be considered that patients are naked in the operating room; therefore the ambient temperature should be maintained at temperatures above 21°C.
  4. When the patients are covered by surgical drapes, provided that active heating is started, the ambient temperature can be reduced to allow better working conditions for the surgical team.
  5. If intravenous fluids, blood and blood-products over 1000 mL will be administered to the patients, fluids should be warmed to 37°C, using special warming units.
  6. Patients at risk, even if they will undergo an intervention <30 minutes, should be actively warmed.
  7. Forced air warming systems should be set at maximum temperature, and then adjusted to maintain a patient temperature ≥36°C. Active warming should be discontinued when the body temperature increases to ≥37°C.
  8. The irrigation solutions used should be warmed up to 38–40°C.

Postoperative Period

Postoperative period involves the time between the arrival of the patient to the recovery unit and postoperative 24 hours.

  1. Body temperature of the patients should be measured immediately on arrival at the recovery unit. Measurements should be taken continuously or at 15-minute intervals until the patient is transferred to the ward.
  2. The patients should not be transferred to the ward until their body temperature exceeds 36°C.
  3. Passive insulation should be applied in patients with a body temperature ≥36°C at the arrival to the recovery unit.
  4. Active warming should be used in patients with a body temperature<36°C, using forced air warming systems and blankets. This process should continue until the patient’s body temperature reaches to 36°C.
  5. Patients with a body temperature ≥ 36°C should be covered by at least one blanket during transfer to the ward.
  6. On arrival at the ward, body temperature of the patient should be measured at 4-hour intervals. At least two blankets should be provided for a patient.
  7. Active warming should be used in patients who have a body temperature <36°C in the ward; in this case, body temperature should be measured at 30-minute intervals.

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.


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