Hypothermia is a commonly encountered issue when patients undergo surgery and frequently associated with patient discomfort and shivering. Decrease in temperature can result in significant detrimental effects. Strategies to reduce heat loss should be one of our patient concerns and care.
Heat loss occurs primarily from the skin of a patient to the environment through several processes, including radiation, conduction and convection, and evaporation. Heat from core body tissues is transported in blood to subcutaneous vessels, where heat is lost to the environment through radiation. This manner of heat loss is the basis for the familiar technology used to sense and identify the locations of persons in buildings who are out of normal view. Radiation is the major source of heat loss in most surgical patients.
Conduction refers to loss of kinetic energy from molecular motion in skin tissues to surrounding air. For this to be effective, warmed air or water must be moved away from the skin surface by currents, a process called convection. Conduction account for ∼15% of body heat loss.
Roughly 22% of heat loss occurs by evaporation, as energy in the form of heat is consumed during the vaporization of water. Water evaporates from the body even when not sweating, but mechanisms that enhance sweating increase evaporation.
Skin temperature rises and falls with the temperature of a patient’s surroundings. Core temperature normally remains between 97°F and 100°F, even while environmental temperatures fluctuate from as low as 55°F to as high as 130°F.2 this is due to a remarkable thermoregulatory system that is conventionally organized into three components: afferent sensing, central control, and efferent responses.
Causes for inadvertent hypothermia include not only patients’ exposure to a cold room environment and their inability to initiate behaviour responses, but the proclivity of anaesthetics to promote heat loss. Volatile anaesthetics, propofol, and older opioids such as morphine induce heat loss through vasodilation. This process is enhanced by the fact that these drugs, directly impair hypothalamic thermoregulation in a dose-dependent manner. Opioids also depress overall sympathetic outflow, which further inhibits any attempts at thermoregulation. The depressant effect on the hypothalamus results in an elevated threshold for heat response, results in vasoconstriction and shivering. It is notable that nitrous oxide depresses thermoregulation to a lesser extent than volatiles. Midazolam seems to have no influence.
Following induction of general anaesthesia, the decline in body temperature occurs by vasodilation combined with a lowered cold threshold in the hypothalamus allows a redistribution of body heat from core tissues to skin, where heat is lost primarily through radiation.
Curiously, regional anaesthesia also produces hypothermia. Hypothermia is very common following spinal and epidural anaesthesia. Blockade of afferent fibres from large regions obviously prevents cold input to the hypothalamus.
Hypothermia is defined as a core temperature <35°C .Perioperative hypothermia may produce a multitude side effects.
Other than shivering, the most common complications associated with hypothermia are (1) a threefold increase in morbid myocardial events,(2) a threefold increase in the risk of surgical wound infection, and (3) an increase in blood loss and transfusion requirements. Adverse cardiovascular events can follow intraoperative depression of cardiac output and heart rate. Hypothermia during the postoperative period markedly impairs thermal comfort, and physiologic stress leads to increases in heart rate, blood pressure, and oxygen consumption. Hypothermia most likely contributes to wound infection through impairment of immune function and through thermoregulatory vasoconstriction, which, in turn, diminishes oxygen delivery to surgical sites. Even mild hypothermia hampers blood clotting, inhibition of platelet function, coagulation cascade are also impaired.
Drug metabolism can be markedly decreased by hypothermia. The pharmacodynamics and pharmacokinetics of muscle relaxants and volatile anaesthetics are likewise altered.
Although hypothermia is generally regarded as deleterious, it can be beneficial in some situations. Hypothermia decreases the overall metabolic rate by 8% per °C . Oxygen demand drops and this allows aerobic metabolism to continue through greater periods of compromised oxygen supply, thereby reducing the production of anaerobic by-products such as superoxide radicals and lactate. Additional protection can be attributed to decreased release of excitatory neurotransmitters, reduced synthesis and release of kinases and pro-inflammatory cytokines, and decreased apoptosis. In addition, hypothermia lowers intracranial pressures and cerebral perfusion pressure.
Hypothermia may cause patient discomfort in awake patient under regional anaesthesia. Recovery is prolonged not only because a sense of coldness alters mentation and delays awakening, but because drug metabolism is reduced in general anaesthesia.
As with most complications in anaesthesia, prevention is the best management.
Temperature Monitoring
Temperature monitoring is a standard for patients undergoing general anaesthesia, although brief procedures for less than an hour especially for patients at risk for hypothermia such as small children, the elderly, and others who are noticeably frail.
Disposable thermocouple and thermistor probes are available for monitoring core temperature. Preferred sites include tympanic membrane, oesophagus, nasopharynx, and rectum. These sites constitute anatomical areas of highly perfused tissues.
Pre-warming
The patient can be pre-warmed before induction with forced-air systems to minimize the drop in core temperature that results from redistribution. Warm cotton blankets do not increase peripheral body temperature significantly, but they will comfort the patient and will at least minimize normal heat loss by minimizing the patient’s skin exposure.
Room Conditions
The operating room temperature is the most important factor in influencing heat loss due to radiation and convection from skin, and to evaporation from surgical wounds when they are large. The operating room should be warmed to greater than 24°C (ie, 76°F) during induction and while the patient is prepped and draped. Forced-air systems placed over patients are most effective and provide both insulation and active cutaneous warming. Warming of patients via skin surface apposition is most effective intraoperatively. Patient positioning is important in heat conservation. Placing of the arms and legs medially and tucking the patients with blankets to maintain the extremities against the body will also diminish the amount of heat loss.
Warming of IV Fluids
Warming of fluids by warmer tubings can only help to minimize heat loss. Unfortunately, it is not possible to warm patients by administering heated fluids as it causes denaturing proteins.
Postoperative shivering is a common complication following anaesthesia. Even a small decrease of 0.5°C may induce shivering. Patients often identify feeling cold as one of the most unpleasant aspects of their treatment, sometimes worse than any pain associated with the procedure. Shivering is not only subjectively unpleasant but is physiologically stressful because it elevates blood pressure, heart rate, oxygen consumption.
Postoperative shivering should be treated with warming of the patient, most effectively via forced-air systems. Warm blankets may not warm the patient significantly, as explained earlier, but they certainly make the patient subjectively feel better.
Post-anaesthetic shivering can also be treated with a variety of drugs, including clonidine, physostigmine and meperidine. However, they are known to reduce the threshold for shivering, which suggests actions on the central thermoregulatory system rather than in the periphery.
Hypothermia is extremely common for patients undergoing surgery more than 30 minutes due to the exposure and decreased temperature maintained (usually less than 21°c) in theatre complex area .Also in cardiac Cath lab , MRI are the other places where patient encounter the colder atmosphere. For those appropriate clothing and wrapping blankets help to avoid shivering. Major consequences of hypothermia include delayed recovery, unplanned elective ventilation, reduced resistance to surgical wound infection, impaired coagulation, and longer duration of postoperative shivering. Efforts to maintain intraoperative body core temperature higher than 36°C will prevent significant complications, improving the quality and safety of anaesthesia care for our patients.
Dr. K. Mahalakshmi Consultant Anaesthesiologist Kauvery Hospital Chennai
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