Perioperative hypothermia

Abstracts

JUSTIFICATIVA E OBJETIVOS: A hipotermia ocorre freqüentemente durante o período peri-operatório, sendo, porém, raramente diagnosticada e tratada. A hipotermia pode ser benéfica ou prejudicial ao paciente dependendo da situação e do procedimento específicos. Este artigo teve como objetivo realizar uma revisão da literatura sobre as indicações e complicações da hipotermia, assim como seu diagnóstico, prevenção e tratamento. CONTEÚDO: São apresentadas as principais causas e complicações da hipotermia peri-operatória, bem como os seus benefícios. CONCLUSÕES: A hipotermia pode ocorrer durante o ato anestésico-cirúrgico devido à inibição direta da termorregulação pelos anestésicos, à diminuição do metabolismo e à perda de calor para o ambiente frio das salas cirúrgicas, mesmo com a utilização de aquecimento ativo. Quando ocorre de forma inadvertida, pode estar associada a numerosas complicações, mas, quando bem indicada, pode proteger órgãos vitais, como o sistema nervoso central e o miocárdio. A manutenção da normotermia reduz os efeitos indesejáveis da hipotermia, sendo a prevenção através do aquecimento, o método mais efetivo. Estratégias de aquecimento ativo ou passivo devem ser empregadas e os tremores musculares devem ser adequadamente tratados, prevenindo desconforto e o aumento da demanda metabólica.

COMPLICAÇÕES; FISIOLOGIA, Temperatura; HIPOTERMIA; HIPOTERMIA

REVIEW ARTICLE

Hipotermia no período peri-operatório^*

Hipotermia en el período perioperatorio

Camila B. Biazzotto^I; Márcio Brudniewski^I,II; André P. Schmidt^I,II; José Otávio Costa Auler Júnior, TSA^III

^IMédico Especialista em Anestesiologia pelo CET da Disciplina de Anestesiologia do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo

^IIMédico Preceptor do CET da Disciplina de Anestesiologia do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo

^IIIProfessor Titular da Disciplina de Anestesiologia do Hospital das Clínicas da Faculdade de Medicina da Universidade de São Paulo

^{Correspondence to}

SUMMARY

BACKGROUND AND OBJECTIVES: Perioperative hypothermia is a common event, however seldom diagnosed and treated. Hypothermia may be beneficial or noxious for the patient depending on specific situations and procedures. This paper is a literature review of hypothermia indications and complications, as well as its diagnosis, prevention and treatment.

CONTENTS: Major perioperative causes and complications of hypothermia are presented, in addition to its benefits.

CONCLUSIONS: Perioperative hypothermia may be due to direct anesthetic inhibition of thermoregulation, decreased metabolism and loss of heat to the cold environment of operating rooms, even with active warming. When it is inadvertent, it may be associated to several complications, but when adequately indicated, it may protect vital organs such as central nervous system and heart. Normothermia decreases undesirable hypothermia effects, being warming the most effective preventive method. Active or passive warming approaches should be adopted and muscle shivering should be adequately treated to prevent discomfort and increased metabolic demand.

Key Words: COMPLICATIONS: hypothermia; HYPOTHERMIA: cardioprotecion, neuroprotection; PHYSIOLOGY, Temperature

RESUMEN

JUSTIFICATIVA Y OBJETIVOS: La hipotermia ocurre frecuentemente durante el período perioperatorio, siendo, sin embargo, raramente diagnosticada y tratada. La hipotermia puede ser benéfica o perjudicial al paciente dependiendo de la situación y del procedimiento específico. Este artículo tiene como objetivo hacer una revisión de la literatura sobre las indicaciones y complicaciones de la hipotermia, así como su diagnóstico, prevención y tratamiento.

CONTENIDO: Son presentadas las principales causas y complicaciones de la hipotermia perioperatoria, bien como sus beneficios.

CONCLUSIONES: La hipotermia puede ocurrir durante el acto anestésico-quirúrgico debido a la inhibición directa de la termorregulación por los anestésicos, a la disminución del metabolismo y a la pérdida de calor para el ambiente frío de las salas quirúrgicas, mismo con la utilización de calentamiento activo. Cuando ocurre de forma inadvertida, puede estar asociada a numerosas complicaciones, pero cuando bien indicada puede proteger órganos vitales, como el sistema nervioso central y el corazón. La manutención de la normotermia reduce los efectos indeseables de la hipotermia, siendo la prevención a través del calentamiento el método más efectivo. Estrategias de calentamiento activo o pasivo deben ser empleadas y los temblores musculares deben ser adecuadamente tratados, previniendo el malestar y el aumento de la demanda metabólica.

INTRODUCTION

Core body temperature is one of the most strictly controlled physiological parameters. Human thermoregulating system allows ranges from 0.2 to 0.4 ºC around 37 ºC to maintain metabolic functions¹.

Unintentional perioperative hypothermia, defined as core blood temperature below 36 ºC, is a common event, due to direct thermoregulation inhibition of anesthetics, decreased metabolism and exposure to the cold environment of operating rooms^2,3. When body cavities are exposed during surgical procedures, there might be further heat loss⁴. Didactically, hypothermia is classified in mild (34 to 36 ºC), moderate (30 to 34 ºC) and severe (below 30 ºC)⁵.

Perioperative temperature monitoring provides early detection of hypothermia and may help perioperative thermal control. Body temperature maintenance during surgery is important because hypothermia is associated to several complications. However, when adequately indicated, it may protect vital organs in which ischemia is expected, such as neuronal and myocardial cells. Deep global hypothermia is the primary technique to protect central nervous system (CNS) in procedures requiring circulatory stop, but its use is limited due to the need of cardiopulmonary bypass (CPB)⁶.

Decreasing concentration of anesthetic drugs in the CNS during anesthesia recovery allows the body to restart its thermoregulating responses. Body temperature tends to return to normal. Residual anesthesia and opioids employed to control postoperative pain decrease the efficacy of such responses. The time needed for such may vary from 2 to 5 hours, depending on the level of hypothermia and the age of the patient⁷.

HUMAN THERMOREGULATION

Hypothalamus is the primary temperature regulation site, integrating thermal impulses from skin and deep tissues. When these impulses exceed the temperature thresholds in any direction, autonomic thermoregulating responses arise, which maintain temperature within adequate levels^8,9.

Thermoregulating control precision is similar for males and females^8,9, though decreased in elderly and severely ill patients. Major responses to hypothermia are skin vasoconstriction, nonshivering thermogenesis, shivering and behavioral changes. Skin vasoconstriction is the first and most important autonomic response to hypothermia, decreasing heat loss to the environment in 25%¹⁰.

Behavioral changes seem to depend more on skin temperature than on environment, allowing humans to live in places with temperature extremes¹¹. Skin blood flow may be divided in two compartments: nutritional, represented by capillaries, and thermoregulator, represented by arterial-venous shunts. The thermoregulating flow is mediated by norepinephrine in a₂-adrenergic receptors and may be decreased up to 100 times during hypothermia, especially in distal parts of the body¹².

Nonshivering thermogenesis is caused by increased metabolic heat production and oxygen consumption, without muscle work increase. Its major sources are skeletal muscles and brown adipose tissue. It is the primary neonate and infant thermoregulating mechanism¹³, with minor contribution in adults^14,16. The stimulation of a₃-adrenergic receptors on brown adipose nervous terminations is responsible for the production of heat¹⁷.

Muscle shivering is an involuntary activity present only during a maximum vasoconstriction state and, as nonshivering thermogenesis, it is not enough to maintain body temperature^1,18. Muscle shivering determines increase in oxygen consumption of approximately 200% to 600%, in addition to triggering sympathetic discharge, increased intracranial and intraocular pressure and myocardial ischemia^19-21. Thermoregulation efficacy varies according to age. The elderly have lower vasoconstrictor response as compared to youngsters, and lower shivering threshold, being prone to hypothermia even when exposed to normal temperature²². Children and neonates, especially prematures, have limited vasoconstriction and low efficacy or even no shivering²³. The lower the age, the higher the skin heat loss due to large relative body surface²⁴.

HEAT PRODUCTION, DISTRIBUTION AND LOSS

The body produces energy and totally converts it into heat according to its metabolic need. At rest, the major tissues responsible for producing heat are the brain, the liver and the heart. Major human metabolism substrates responsible for heat production are glucose (4.1 kcal.kg^-1), protein (4.1 kcal.kg^-1) and lipids (9.3 kcal.kg^-1). During anesthesia there is decrease in metabolic rate, oxygen consumption and heat production²⁵.

To help understanding heat distribution in the body, it may be divided in two major thermal compartments: I. Core - made up of highly perfused tissues in which temperature remains relatively constant and higher (major body organs and CNS). It represents 50% to 60% of total body mass and is responsible for the production of all energy converted into heat²⁶; II. Peripheral - made up of tissues with heterogeneous temperature, which varies according to the environment (upper and lower limbs, skin and subcutaneous tissue). It is the largest compartment, with temperatures in general 3 to 4 ºC below core temperature. However, this difference may increase or decrease during extreme thermal or pathological conditions^27,28.

The skin is a barrier between core and peripheral compartments and the environment, and approximately 90% of all heat produced is lost through its surface. Internal redistribution of heat in the body after anesthetic induction is the main cause of perioperative hypothermia and is a function of the temperature gradient between core and peripheral compartments. Several factors can influence this gradient, including room temperature, the amount of adipose tissue and anesthetic drugs. This mechanism is responsible for 81% of the core temperature reduction during the first hour after anesthetic induction and 43% in the two subsequent hours²⁹. Individual response intensity to heat distribution is unpredictable.

Vasoconstriction maintains core heat and decreases the loss to the environment. Conversely, vasodilatation promotes heat transfer to the periphery, which does not necessarily mean heat loss to the environment. Obese patients present lower incidence of perioperative hypothermia because they have a lower redistribution gradient due to more adipose tissue and more heat production³⁰.

Exposure to surgical environment leads to heat loss to the environment through four mechanisms: irradiation, conduction, evaporation and convection³¹. Irradiation is heat loss through radiant energy to walls and solid objects. It depends on the absolute temperature difference between two surfaces to the fourth, representing 70% of total heat loss at 22 ºC³².

Conduction depends on the temperature difference between two objects in contact and on the conductance between them. One example would be heat loss to the operating table metal surface. Heat is also lost thru the evaporation of fluids applied to the skin, sweating and fluid loss by the respiratory system, surgical wound and skin. Conduction and evaporation together sum other 15% of total heat lost during anesthesia and surgery. Convection is heat loss or gain by the passage of a fluid in a certain temperature through a surface with different temperature. It is more intense when there is air shift in large spaces and is responsible for the remaining 15% heat loss from the body to the environment. Blood flow exhibits both convective and conductive components, being the former more important than the latter, especially when patients remain in operating rooms at low temperatures³³.

TEMPERATURE MONITORING

In general, core temperature decreases 1 ºC in the first 40 minutes after anesthetic induction due to peripheral vasodilation and internal heat redistribution²⁹. All patients submitted to procedures lasting more than 30 minutes should have their temperatures monitored and maintained in approximately 36 ºC, unless hypothermia is specifically indicated. Patients submitted to major surgeries under regional anesthesia should also have their temperature monitored³⁴.

The site for the transducer placement should be carefully chosen according to the surgical procedure. Core temperature may be measured in the pulmonary artery, tympanic membrane, nasopharynx and distal esophagus^35-38.

Oral, axillary, vesical or rectal thermometers may give reliable peripheral temperature measurements, unless patients are under extreme thermal conditions, such as during rewarming after cardiopulmonary bypass.

During liver transplantation, the exposure of receptor’s diaphragm muscle to the cooled graft of the donor leads to decreased esophageal temperature. In this case, a vesical thermometer provides a closer reading to pulmonary artery temperature³⁹. In open chest procedures, esophageal temperature also does not thoroughly reflect core temperature due to cavity exposure to room temperature, being recommended the use of tympanic membrane or nasopharynx. Tympanic membrane seems to be the ideal core temperature monitoring site because it is very close to CNS and precisely reflects brain temperature, provided the sensor is well positioned³⁷. During cardiac procedures, vesical temperature is a reasonable alternative when urinary flow is adequate. However, when it is low, measured temperature does not correspond to real temperature⁴⁰. Axillary temperature corresponds to core temperature when the arm is adequately abducted. It is however less reliable than vesical and esophageal measurements⁴¹.

ETIOLOGY OF PERIOPERATIVE HYPOTHERMIA

General Anesthesia

Anesthetic induction is responsible for about 20% reduction of metabolic heat production, in addition to abolishing physiological thermoregulation responses normally triggered by hypothermia. If temperature is not actively maintained, hypothermia may arise. Most anesthetics have vasodilating properties and all of them change core temperature control by inhibiting thermoregulating responses against cold, such as vasoconstriction and muscle shivering^42-44. Opioids⁴⁵ and propofol⁴⁶, for example, linearly decrease vasoconstriction and shivering threshold. Halogenate agents^47,48, on the other hand, decrease the response threshold to cold in a non-linear pattern.

Consequently, thermoregulating responses of anesthetized patients will be triggered at a lower temperature as compared to non-anesthetized patients¹. Care should be taken to warm patients and reduce their temperature loss to the environment before hypothermia triggers peripheral vasoconstriction. Once vasoconstriction is triggered, it becomes highly effective to prevent additional hypothermia¹⁰. Anesthetic-induced vasodilation mildly increases skin heat loss⁴⁹, suggesting that this is not the major cause of post-anesthetic hypothermia. Primary mechanism is heat redistribution from the core to the peripheral compartment by circulatory conduction and convection, leading to decreased core temperature and increased peripheral temperature, however without changing mean body temperature and body heat content^33,50. The same effect has been shown during regional anesthesia⁵¹.

Hypothermia during general anesthesia is developed in three stages. First there is a fast core temperature decrease by redistribution after anesthetic induction. This is followed by a linear temperature decrease (0.5 to 1 ºC/h) as long as the difference between the rates of heat metabolic production and loss to the environment persists. When the threshold temperature is reached, vasoconstriction is triggered creating a heat flow restriction between compartments, providing less internal heat distribution and heat loss to the environment.

The maintenance of metabolic heat production, in spite of continuous loss, generates a temperature plateau which is able to reestablish normal gradient between compartments. The last stage is then reached, characterized by a new thermal balance, now at a lower value^52-54. When patients recover from anesthesia under hypothermia, shivering is promptly triggered to compensate intraoperative heat deficit and increase core temperature. Primary shivering consequences are increased oxygen consumption, extreme discomfort and difficult monitoring.

Regional Anesthesia

Spinal anesthesia inhibits central thermoregulating control through peripheral sympathetic nervous system and motor nerves block, abolishing vasoconstriction and shivering^55-57. Redistribution is limited to lower limbs and remains as the major cause of perioperative hypothermia for those patients. Its magnitude varies according to patients’ initial thermal status and may be attenuated by warming lower limbs before anesthetic induction⁵⁸.

Since there is less initial redistribution in patients under regional anesthesia, linear hypothermia will be triggered at a higher temperature, resulting in 50% less temperature decrease as compared to general anesthesia. Linear hypothermia is developed at a lower rate, since metabolic heat production rate remains close to normal. The presence and extension of sympathetic and motor blocks prevent the development of thermoregulating vasoconstriction and this linear stage is not interrupted, as seen with general anesthesia.

As a consequence, patients submitted to major procedures under regional anesthesia are at increased risk of severe hypothermia³⁴. Body areas free from sympathetic and motor block may trigger thermoregulating responses if lower shivering threshold is reached, provided that the patient is not elderly⁵⁹ or is not excessively sedated^45,46. However, shivering restricted to upper extremities is relatively ineffective and insufficient to prevent additional hypothermia. Regional anesthesia-induced hypothermia is common⁶⁰ and depends more on surgical procedure magnitude and duration than on patient’s characteristics. Temperature measurement and diagnosis of hypothermia are uncommon, unless when it is already expected.

Among different monitoring sites during spinal anesthesia, it has been observed that more accurate measures are obtained from rectal temperature due to compensatory skin vasoconstriction above blockade level^61,62.

Combined Anesthesia

Combined anesthesia poses the highest risk of unintentional perioperative hypothermia. Initial redistribution in the four limbs promptly leads to hypothermia and the linear stage is developed at a higher rate. Regional anesthesia per se decreases vasoconstriction threshold and when superimposed to general anesthesia their effects are added. As result, vasoconstriction is lately triggered and at lower temperatures⁶³. On the other hand, general anesthesia inhibits shivering, which could increase internal heat production during spinal anesthesia. But the most important factor of the association is the abolishment of lower limbs vasoconstriction due to peripheral nerves block. Central vasoconstriction alone is ineffective, and temperature continues to drop, reaching no plateau^62,64,65.

CONSEQUENCES OF PERIOPERATIVE HYPOTHERMIA (CHART I)

Currently, perioperative normothermia is a recognized goal. However, inadvertent moderate hypothermia is often seen. Further studies are still needed to outline mechanisms of hypothermia-related complications and to develop practices to reestablish perioperative thermal balance.

Cardiovascular System

Adrenergic responses to moderate perioperative hypothermia lead to energetic myocardial demand and consumption imbalance. This in turn may result in acute myocardial infarction, which is one of the major unpredictable causes of perioperative morbidity/mortality. Studies indicate that perioperative myocardial ischemia is not associated to shivering alone, but rather to hemodynamic stress produced by cold-induced sympathetic-adrenal activation^19,66,67.

Even mild hypothermia increases catecholamine serum levels leading to tachycardia, hypertension, systemic vasoconstriction and myocardial oxygen supply and demand unbalance, in addition to increasing myocardial irritability¹⁹. Franck et al., in a randomized study (active warming versus routine) with 300 high risk patients to determine whether moderate perioperative hypothermia would increase the incidence of cardiovascular events, have observed that the incidence of myocardial ischemia and ventricular tachycardia was 3 times higher in hypothermal patients²¹. Hypothermia per se does not trigger coronary vasoconstriction, but is associated to increased cardiac work.

So, it may predispose to myocardial ischemia if the patient has some degree of coronary obstruction⁶⁷. Other complication probably related to hypothermia is deep vein thrombosis (DVT) due to vasoconstriction favoring venous stases and hypoxia. Further studies are still needed to confirm this hypothesis⁶⁸.

Coagulation System

Although platelet count is normal during hypothermia, morphological changes suggestive of platelet activation are obseved⁶⁹, especially when the hypothermal blood is exposed to platelet activators⁷⁰, such as CPB circuit and atherosclerotic plaques. Tests such as prothrombine time (PT) and activated partial thromboplastin time (APTT) remain normal because they are performed at 37 ºC regardless of patient’s temperature⁷¹. When performed at patient’s temperature, they show changes due to decreased coagulation cascade enzyme reaction speed⁷². Fibrinolysis is not affected at temperatures below 34 ºC and is enhanced during hyperthermia, what leads to the hypothesis that hypothermia-induced coagulopathy does not result from excessive fibrinolysis⁷³.

Thromboelastogram studies suggest that hypothermia affects more clot formation than degradation⁷⁴. Two randomized controlled studies have confirmed that a 0.5 ºC decrease in core temperature in patients submitted to hip arthroplasty under regional anesthesia is associated to higher blood loss and higher need for allogeneic transfusion^56,75.

Immune System

The incidence of operative wound infection is related to decreased subcutaneous oxygen tension⁶⁸ and it has been proven in humans that it is decreased by thermoregulating vasoconstriction. Hypothermia has a direct effect on cell and humoral immunity^50,52, and an indirect effect through decreased O₂ supply to peripheral tissues^76,77. Decreasing core temperature in 1.9 ºC triples the incidence of surgical wound infection after colon resection and increases in 20% hospital stay, which also increases hospital costs. It is still not well established whether hypothermia is related to other infections such as pneumonia^78-80.

Hormonal Changes

During hypothermia, steroid secretion is initially unchanged. However, as it persists, steroid suppression may occur⁸¹. Thyroxin production is increased due to increased thyroid-stimulating hormone (TSH) release⁸². The release inhibition and decreased activity of insulin, in addition to decreased renal glucose loss and increased catecholamine secretion result in hyperglycemia⁸³. However, hypoglycemia is seen in 40% of patients. During rewarming there might be severe hypoglycemia with brain injury. So, there should be rigid glycemic control and hyperglycemia should not be treated during hypothermia⁸⁴.

PHARMACOKINETICS AND PHARMACODYNAMICS (CHART II)

Atracurium duration is less dependent on core temperature because 3 ºC decrease increases its duration in 60%⁸⁵. Neostigmine efficacy in reverting neuromuscular block is not changed during moderate hypothermia⁸⁶. Propofol plasma concentration increases 30% during continuous infusion when patient’s core temperature is decreased 3 ºC. Inhalational anesthetics tissue solubility is increased in hypothermal patients and anesthesia recovery is prolonged since higher amount of anesthetics has to be eliminated. In addition, one can expect a 5% halothane and isoflurane MAC decrease for each degree of core temperature reduction⁸⁷. Lenhardt et al. have published a clinical randomized double-blind trial with 150 patients submitted to major surgical procedures and have related perioperative hypothermia to prolonged anesthetic recovery and longer PACU stay, even when discharge criteria did not include normothermia⁷⁴.

Shivering

General anesthesia deeply inhibits body thermoregulating defenses against hypothermia and this is the reason why shivering is seldom seen in anesthetized patients. Anesthetic recovery allows the reappearance of thermoregulating responses. Shivering is then triggered to compensate intraoperative heat output and to increase core temperature at the expenses of increased metabolic rate and oxygen consumption⁷. This sensation is extremely uncomfortable, and would justify prevention and treatment by itself, even in the absence of other hypothermia-related complications⁸⁸. Shivering is not effective during regional anesthesia, and when severe, it may interfere with the procedure or even prevent the mother of holding her child after a Cesarean section. Elderly and weak patients have less effective shivering as compared to young patients²².

Electrolytic Changes

Hypothermia is related to hypokalemia, hypophosphatemia and hypomagnesemia, but clinical significance of such changes is not adequately established^89-91.

Other Changes

Bupivacaine cardiotoxicity is increased during hypothermia⁹². Pulse oximetry is also changed by decreased peripheral perfusion triggered by vasoconstriction^93,94, which may impair monitoring.

BENEFITS OF HYPOTHERMIA

Hypothermia protects against neuronal and myocardial ischemia. Under adequately controlled conditions, it promotes well known brain protection in patients with neurological disorders. There are ongoing studies looking for adequate active cooling methods and drugs to decrease body thermoregulating responses to hypothermia, which would make cooling a fast and safe method.

Studies are still needed to confirm the benefits of hypothermia in patients with stroke, subarachnoid hemorrhage (SAH) and severe brain trauma (SBT). Minor brain temperature decrease (2 to 4 ºC) may reduce ischemia-induced neurological injury. Just as hypothermia may bring benefits, hyperthermia, even late, may worsen neurological recovery after stroke and its prevention is also important^95-97.

Brain Protection

Neurosurgery, carotid endarterectomy, cardiopulmonary bypass and hemodynamic instability may lead to brain ischemia. Some studies advocate moderate hypothermia for severe TCE. Marion et al. have shown that induced moderate hypothermia for 24 hours in patients with Glasgow comma scale between 5 and 7 has accelerated neurological recovery⁹⁸. However, Hartung and Cottrell have questioned the statistical analysis of such study⁹⁹. To date, there are no human studies ratifying moderate hypothermia during surgical procedures. Hindman et al. have investigated patients submitted to cerebral aorta aneurysm clipping and have observed that induced hypothermia during HSA has improved clinical recovery¹⁰⁰.

Myocardial Preservation

During myocardial ischemia, changed cell metabolism associated to reperfusion injury contributes to cell death extending from necrosis epicenter to periphery. Hypothermia protects myocardium against ischemia, but the exact protection mechanism is still undefined. A possible explanation would be decrease in metabolic demand associated to myocardial ATP reserves preservation during ischemia, maintaining cell integrity^101,102.

Most myocardial oxygen consumption is related to its electromechanic activity. Since cardioplegia abolishes heart contractions, hypothermia brings minor additional contribution¹⁰³.

Myocardial hypothermia may be induced during cardiac procedures regardless of systemic temperature, and during reperfusion, myocardial normothermia is promptly reestablished. This way, it is possible to promote myocardial protection without exposing patients to the risk of systemic hypothermia⁷⁶.

Recent studies have shown that hypothermia does not promote coronary vasoconstriction in healthy adults and, as opposed to what would be expected, an increased myocardial perfusion is observed. However, it has not been proven that acute myocardial infarction (AMI) patients benefit from hypothermia and myocardial protection has not yet been established⁶⁷. There are only experimental evidences that mild hypothermia promotes post-AMI protection^104-106.

Other Benefits

Hypothermia seems to be beneficial for Adult Respiratory Distress Syndrome (ARDS) patients¹⁰⁷, as well as for patients susceptible to malignant hyperthermia (MH). An experimental study has shown lower MH incidence in susceptible patients submitted to hypothermia, in addition to less severe crises once triggered⁷⁷.

PREVENTION AND TREATMENT

The risk-benefit analysis of high-risk surgical patients favors the maintenance of core normothermia in the perioperative period. Randomized studies have confirmed that normothermia decreases complications³⁰. The most effective method to maintain normothermia is prevention by previous warming^108-113. After anesthetic induction with no previous warming, hypothermia occurrence is common, even if active intraoperative warming is employed.

REDISTRIBUTION PREVENTION

Redistribution hypothermia is a relatively slow and difficult to treat process. Previous warming increases peripheral compartment heat content, decreasing redistribution gradient. Skin warming with circulating air at 43 ºC for one hour transfers enough heat to decrease redistribution effects. This warming does not increase core temperature, but sweating and thermal discomfort may occur after prolonged warming¹¹⁴.

Warm air circulation (thermal blanket) is the most effective noninvasive warming method currently available and increases core temperature in 0.75 ºC/hour in mean. Another method to increase peripheral heat content and prevent redistribution is patient’s previous vasodilatation. This consists on the administration of vasodilators from 12 h up to 1 h before anesthesia (20 mg oral or 10 mg sublingual nifedipine). The administration should allow time enough for the thermoregulating system to increase peripheral temperature without the risk of decreasing core temperature¹¹⁵.

INTRAOPERATIVE WARMING

Room temperatures above 23 ºC will maintain or reestablish normothermia during anesthesia, however they generate thermal discomfort for the surgical-anesthetic team and worsen their cognitive performance. As a consequence, active or passive warming strategies should be adopted. Passive warming is a low cost and effective method, consisting in intraoperatively covering and warming all possible skin surfaces with sheets, blankets or plaids. It may decrease heat loss in 30%¹¹⁶. Blankets warming do not generate additional heat transfer, just making them more comfortable¹¹⁷.

Active warming, the most effective method, may revert already installed hypothermia. Total area to be covered is critical. Anterior region warming is more effective than warming the region in contact with the operating table since little heat is loss to it. Blankets or mattresses with water circulation are only beneficial when placed over the patient. Electric blankets may also be used¹¹⁶. Skin warming is effective when thermoregulating vasoconstriction has already been installed. Peripheral vasodilatation induced by anesthetic agents promotes intercompartment heat transfer, helping the transfer of heat applied to skin surface to the core compartment¹¹⁸. Warm solutions are useful when infused in a rate over 2 liters in 1 hour¹¹⁹. One liter of crystalloid at room temperature decreases core temperature in 0.25 ºC¹²⁰. Warming and humidification of gases administered to the patient have minor impact on body temperature²⁴.

SHIVERING PREVENTION AND TREATMENT

Normothermia is the primary method to prevent shivering, which is common in the post-anesthetic recovery unit. When present, it should always be treated due to all its previously mentioned consequences, in addition to the discomfort it brings.

Most anesthetic agents promote dose-dependent decrease of shivering threshold, even in the absence of hypothermia, which justifies shivering prevention in the post-anesthetic recovery unit, even when there has been no significant core temperature change during surgery.

Postoperative shivering is treated with active skin warming and/or opioids. Any drug decreasing shivering threshold will effectively treat it. Opioids are first choice drugs because they promote minor sedation in addition to controlling pain, which in general coexists with shivering.

Skin contains 20% of all body thermal afferents and heat transfer increases temperature above shivering threshold, leading to their abolishment. Patients recovering from general anesthesia should be submitted to warming of all possible skin surfaces. Those recovering from spinal anesthesia should have only the skin surface with preserved sensitivity warmed to prevent burnings. A limitation of this technique is its low ability to increase core temperature¹²¹. Alfentanil and sufentanil are knowingly effective, but none is to be compared to meperidine, which promotes a disproportional decrease in shivering threshold, with unknown action mechanism though. Clonidine may also be used^{85,86,122-124}. Shivering during regional anesthesia may be treated with systemic or spinal opioids¹²⁵.

CONCLUSION

Although perioperative normothermia is an accepted primary goal, unintentional hypothermia is frequent during anesthetic-surgical procedures due to several mechanisms, specially the internal heat redistribution between core and peripheral compartments, which is a direct function of their temperature gradient. Hypothermia during anesthesia develops in three stages. General anesthesia usually ends up in a thermal balance state, which may reestablish the gradient between compartments.

This balance is not reached during regional anesthesia since thermoregulating vasoconstriction and shivering are abolished due to sympathetic nervous system and motor nerves block. Combined general and regional anesthesia poses an additional risk of unintentional perioperative hypothermia.

Hypothermia triples the incidence of adverse myocardial events, significantly increases intraoperative bleeding and the need for transfusion, as well as the incidence of surgical wound infection and hospital stay. Pharmacokinetics and pharmacodynamics of most anesthetics are also changed, prolonging anesthetic recovery.

Children, elderly, obese and severely ill patients exhibit abnormal responses to hypothermia. On the other hand, deep global hypothermia is the primary brain protection technique for procedures requiring circulatory stop. The exact myocardial protection mechanism, widely used during cardiac surgeries, is still not defined, as well as it is not confirmed that AMI patients benefit from hypothermia. It is critical, then, to prevent and treat perioperative hypothermia aiming at decreasing noxious consequences and patients’ discomfort.

REFERENCES

Submitted for publication June 10, 2005

Accepted for publication October 21, 2005

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*

Recebido da Received from Disciplina de Anestesiologia da Faculdade de Medicina da Universidade de São Paulo, São Paulo, SP

Publication Dates

• Publication in this collection
  22 Mar 2006
• Date of issue
  Feb 2006

History

• Received
  10 June 2005
• Accepted
  21 Oct 2005