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• W2000595150 abstract "•Anesthetic medications and techniques can disrupt the thermoregulatory process in surgical patients. Temperature monitoring is essential in patients to preserve normothermia.•Even mild hypothermia may cause bleeding, myocardial events, infection, and postoperative pain.•Untreated, malignant hyperthermia leads to morbidity or death. The perioperative team must be familiar with its diagnosis and prepared with a treatment plan.•Immediate care for malignant hyperthermia demands the ready availability of a cart stocked with dantrolene, diluent, and adjunctive treatment drugs.•A patient with malignant hyperthermia requires continued postoperative care and, if necessary, transfer to an acute care facility. •Anesthetic medications and techniques can disrupt the thermoregulatory process in surgical patients. Temperature monitoring is essential in patients to preserve normothermia.•Even mild hypothermia may cause bleeding, myocardial events, infection, and postoperative pain.•Untreated, malignant hyperthermia leads to morbidity or death. The perioperative team must be familiar with its diagnosis and prepared with a treatment plan.•Immediate care for malignant hyperthermia demands the ready availability of a cart stocked with dantrolene, diluent, and adjunctive treatment drugs.•A patient with malignant hyperthermia requires continued postoperative care and, if necessary, transfer to an acute care facility. Homeotherms, including humans, are able to maintain a relatively constant temperature despite variations in their thermal environment. We normally maintain a narrow thermoregulatory threshold range of approximately 0.2°C, and little change in core temperature is required to trigger compensatory mechanisms to either cool or warm our core temperature back to normothermia. This article focuses on the mechanisms and consequences of hypothermia and hyperthermia in the surgical patient and reviews techniques to prevent and treat these conditions. Maintaining core temperature is a complex process. Initially, the hypothalamus was believed to be primarily responsible for temperature regulation, but it has since become clear that spinal cord pathways, abdominal organs, and skin surface temperature contribute to the activation of autonomic thermoregulatory mechanisms, including shivering, vasoconstriction, sweating, and vasodilation.1Kurz A. Physiology of thermoregulation.Best Pract Res Clin Anaesthesiol. 2008; 22: 627-644Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar The complexity and multiple locations of thermoregulatory control help explain why a wide range of anesthetic medications and techniques can disrupt normal temperature. Vasoconstriction and shivering take place when the core temperature decreases lower than the lower limit of the thermoregulatory threshold, whereas sweating and vasodilation represent a central response to a core temperature that exceeds the upper limit of the thermoregulatory threshold. Peripheral temperature is usually cooler than the core by 2° to 4°C, with the gradient maintained by peripheral vasoconstriction of arteriovenous shunts.1Kurz A. Physiology of thermoregulation.Best Pract Res Clin Anaesthesiol. 2008; 22: 627-644Abstract Full Text Full Text PDF PubMed Scopus (123) Google Scholar Core heat is conserved and heat dissipation to the environment is prevented to maintain normothermia in an unanesthetized person, but thermoregulation and other processes are often disrupted in an anesthetized patient. Hypothermia is defined as a core body temperature less than 36°C.2Sessler D.I. Mild perioperative hypothermia.N Engl J Med. 1997; 336: 1730-1737Crossref PubMed Scopus (508) Google Scholar Despite an effective homeothermic thermoregulatory system, patients can become hypothermic perioperatively, when the thermoregulatory threshold is widened under the effects of sedation, general anesthesia, or regional anesthesia (Fig. 1).3Sessler D.I. Central thermoregulatory inhibition by general anesthesia.Anesthesiology. 1991; 75: 557-559Crossref PubMed Scopus (41) Google Scholar Anesthetic agents disrupt a patient’s normal thermoregulatory mechanisms in different ways,4Ikeda T. Kim J.S. Sessler D.I. et al.The volatile anesthetic, isoflurane, alters shivering patterns and reduces maximum shivering intensity.Anesthesiology. 1998; 88: 866-873Crossref PubMed Scopus (42) Google Scholar, 5Kurz A. Go J.C. Sessler D.I. et al.The narcotic alfentanil slightly increases the sweating threshold but markedly reduces the vasoconstriction and shivering thresholds.Anesthesiology. 1995; 83: 293-299Crossref PubMed Scopus (158) Google Scholar, 6Kurz A. Xiong J. Sessler D.I. et al.Another volatile anesthetic, desflurane, reduces the gain of thermoregulatory arteriovenous shunt vasoconstriction in humans.Anesthesiology. 1995; 83: 1212-1219Crossref PubMed Scopus (62) Google Scholar, 7Stoen R. Sessler D.I. The thermoregulatory threshold is inversely proportional to isoflurane concentration.Anesthesiology. 1990; 72: 822-827Crossref PubMed Scopus (110) Google Scholar including the loss of behavioral mechanisms (ie, putting on a jacket when cold) because of loss of consciousness to inhibition of autonomic responses, such as shivering and vasoconstriction (Fig. 2). The development of hypothermia during general anesthesia can be described in 3 phases (Fig. 3).Fig. 3Phases of temperature loss.(From Sessler DI. Temperature regulation and monitoring. In: Miller RD, editor. Miller’s anesthesia. 7th edition. Philadelphia: Churchill Livingstone; 2010. p. 1538; with permission.)View Large Image Figure ViewerDownload Hi-res image Download (PPT) The first or redistribution phase of core temperature heat loss occurs shortly after induction of general anesthesia (Fig. 4). Peripheral vasodilation transfers core body heat to the periphery and subsequently to the environment. With general anesthesia, a core temperature decline fails to trigger normal thermoregulatory vasoconstriction (ie, the body’s thermostat is reset to defend a temperature <37°C). As a result, the normal core/peripheral temperature gradient becomes smaller, with greater heat loss to the environment. Most of the heat loss during a general anesthetic from core/peripheral redistribution occurs within the first hour of the anesthetic and, in the absence of active warming, the patient’s core temperature can decline 1°C to 1.5°C.8Matsukawa T. Sessler D.I. Sessler A.M. et al.Heat flow and distribution during induction of general anesthesia.Anesthesiology. 1995; 83: 961-967Crossref PubMed Scopus (221) Google Scholar Redistribution contributes to approximately 80% of heat loss in the first hour, but is not the only contributor.8Matsukawa T. Sessler D.I. Sessler A.M. et al.Heat flow and distribution during induction of general anesthesia.Anesthesiology. 1995; 83: 961-967Crossref PubMed Scopus (221) Google Scholar General anesthesia can decrease metabolic heat production by approximately one-third, and exposure to a cool ambient environment, administration of cool intravenous fluids, and the application of cool skin-cleaning solutions can result in up to 5% of additional first-phase heat loss. Without active warming, core temperature declines linearly during the second hour, resulting in the loss of another 1°C to 2°C of core temperature. Core to peripheral heat redistribution contributes only 40% of heat loss during the second phase.8Matsukawa T. Sessler D.I. Sessler A.M. et al.Heat flow and distribution during induction of general anesthesia.Anesthesiology. 1995; 83: 961-967Crossref PubMed Scopus (221) Google Scholar Most heat loss during the second phase occurs as a result of decreased metabolic heat production in the setting of cool intravenous fluid administration and the exposure of a large body surface area or large wounds. Heat is lost via radiation, convection, evaporation, and conduction, and outpaces the diminished metabolic heat production during a general anesthetic. The third phase of intraoperative hypothermia is the plateau phase, typically during the third hour of general anesthesia.9Kurz A. Sessler D.I. Christensen R. et al.Heat balance and distribution during the core temperature plateau in anesthetized humans.Anesthesiology. 1995; 83: 491-499Crossref PubMed Scopus (182) Google Scholar The plateau phase is characterized by a relatively constant core temperature even with increased surgical time. Heat loss continues, but core temperature is preserved by the restoration of the core/periphery temperature gradient, at a lower temperature, and the activation of autonomic thermoregulatory mechanisms.10Belani K. Sessler D.I. Sessler A.M. et al.Leg heat content continues to decrease during the core temperature plateau in humans.Anesthesiology. 1993; 78: 856-863Crossref PubMed Scopus (72) Google Scholar Plateau phase core temperatures typically reach between 34°C and 35°C. A major regional anesthetic causes vasodilation and the loss of afferent signaling to the hypothalamus within the affected area. Anesthetized tissue not only loses heat rapidly but also loses heat without signals to the central regulatory system that would activate vasoconstriction and shivering. As a result, patients may subjectively feel warm despite hypothermia (Fig. 5). With vasodilation, redistribution of heat loss is similar to that in the first phase of a general anesthetic and in the case of epidural blockade, core temperature decreases 0.8°C ± 0.3°C during the first hour.11Matsukawa T. Sessler D.I. Christensen R. et al.Heat flow and distribution during epidural anesthesia.Anesthesiology. 1995; 83: 961-967Crossref PubMed Google Scholar Heat loss from redistribution is less than during general anesthesia because part of the body is spared vasodilation, but redistribution remains the primary mechanism of heat loss during regional anesthesia. In the linear phase, heat loss through radiation, convection, evaporation, and conduction may still exceed metabolic heat production, and core temperature may continue to decrease over hours 1 to 3 of the anesthetic. A patient under the influence of a prolonged major regional anesthetic (eg, an epidural block) may not achieve a thermal steady state or plateau phase. Vasoconstriction and shivering do not return during regional anesthesia (as they do during general anesthesia), because of peripheral blockade of the anesthetized area. Infiltration of a local anesthetic that does not result in major conduction blockade does not affect thermoregulation perioperatively, but many patients who undergo a procedure with infiltration of a local anesthetic also may receive intravenous sedation or analgesia. Opioids and propofol, which disrupt normal thermoregulation,5Kurz A. Go J.C. Sessler D.I. et al.The narcotic alfentanil slightly increases the sweating threshold but markedly reduces the vasoconstriction and shivering thresholds.Anesthesiology. 1995; 83: 293-299Crossref PubMed Scopus (158) Google Scholar, 12Matsukawa T. Kurz A. Sessler D.I. et al.Propofol linearly reduces the vasoconstriction and shivering thresholds.Anesthesiology. 1995; 82: 1169-1180Crossref PubMed Scopus (267) Google Scholar put patients at risk for perioperative hypothermia. Midazolam is unique among anesthetic agents, because it does not seem to appreciably alter thermoregulation.13Kurz A. Sessler D.I. Annadata R. et al.Midazolam minimally impairs thermoregulatory control.Anesth Analg. 1995; 81: 393-398PubMed Google Scholar The risk of hypothermia in sedated patients should be considered even if most of the analgesia is provided by infiltration of local anesthetics. Mild hypothermia can have deleterious effects perioperatively on coagulation,14Rajagopalan S. Mascha E. Na J. et al.The effects of mild perioperative hypothermia on blood loss and transfusion requirement: a meta-analysis.Anesthesiology. 2008; 108: 71-77Crossref PubMed Scopus (517) Google Scholar wound healing and infection,15Kurz A. Sessler D.I. Lenhardt R. Perioperative normothermia to reduce the incidence of surgical wound infection and shorten hospitalization.N Engl J Med. 1996; 334: 1209-1215Crossref PubMed Scopus (2107) Google Scholar, 16Melling A.C. Ali B. Scott E.M. et al.Effects of preoperative warming on the incidence of wound infection after clean surgery: a randomized controlled trial.Lancet. 2001; 358: 876-880Abstract Full Text Full Text PDF PubMed Scopus (501) Google Scholar patient satisfaction, recovery time,17Lenhardt R. Marker E. Goll V. et al.Mild intraoperative hypothermia prolongs postanesthetic recovery.Anesthesiology. 1997; 87: 1318-1323Crossref PubMed Scopus (415) Google Scholar drug metabolism, and the rate of perioperative myocardial events. Although controversy persists regarding the clinical significance of coagulopathy that results from mild hypothermia, the investigators of 1 meta-analysis concluded that mild hypothermia, even a decrease of less than 1°C from core normothermia, is sufficient to increase procedural blood loss by 16%.14Rajagopalan S. Mascha E. Na J. et al.The effects of mild perioperative hypothermia on blood loss and transfusion requirement: a meta-analysis.Anesthesiology. 2008; 108: 71-77Crossref PubMed Scopus (517) Google Scholar Mild hypothermia is believed to increase the risk of wound infection by several mechanisms. Vasoconstriction reduces blood flow to wounds, lessening subcutaneous oxygen tension locally and predisposing to wound infection and delayed healing.18Sheffield C.W. Sessler D.I. Hopf H.W. et al.Centrally and locally mediated thermoregulatory responses alter subcutaneous oxygen tension.Wound Repair Regen. 1996; 4: 339-345Crossref PubMed Scopus (104) Google Scholar Hypothermia also may impair immune defenses so that bacterial contamination results in infection.19van Oss C.J. Absolom D.R. Moore L.L. et al.Effect of temperature on the chemotaxis, phagocytic engulfment, digestion and O2 consumption of human polymorphonuclear leukocytes.J Reticuloendothel Soc. 1980; 27: 561-565PubMed Google Scholar A 2°C core temperature decline from normothermia tripled the rate of wound infection after colon resection,15Kurz A. Sessler D.I. Lenhardt R. Perioperative normothermia to reduce the incidence of surgical wound infection and shorten hospitalization.N Engl J Med. 1996; 334: 1209-1215Crossref PubMed Scopus (2107) Google Scholar so maintaining normothermia perioperatively has the potential to decrease the chances of a surgical wound infection.16Melling A.C. Ali B. Scott E.M. et al.Effects of preoperative warming on the incidence of wound infection after clean surgery: a randomized controlled trial.Lancet. 2001; 358: 876-880Abstract Full Text Full Text PDF PubMed Scopus (501) Google Scholar Hypothermia also may affect myocardial events. Although metabolic demand from shivering may have a small effect on myocardial events, increased release of catecholamines20Kurz A. Thermal care in the perioperative period.Best Pract Res Clin Anaesthesiol. 2008; 22: 39-62Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar in the hypothermic patient may predispose to myocardial events. Patients with hypothermia of less than 2°C had a greater than double risk of a myocardial event than did a normothermic group.21Frank S.M. Fleisher L.A. Breslow M.J. et al.Perioperative maintenance of normothermia reduces the incidence of morbid cardiac events: a randomized clinical trial.JAMA. 1997; 277: 1127-1134Crossref PubMed Google Scholar Mild hypothermia also slows the metabolism of many drugs typically used as anesthetic agents. Hypothermia alters the pharmacokinetics of medications such as propofol, neuromuscular blocking agents, and even the volatile anesthetic agents.20Kurz A. Thermal care in the perioperative period.Best Pract Res Clin Anaesthesiol. 2008; 22: 39-62Abstract Full Text Full Text PDF PubMed Scopus (126) Google Scholar A decrease in core temperature of 3°C increased propofol plasma concentrations and prolonged duration of neuromuscular blockade with atracurium.22Leslie K. Sessler D.I. Bjorksten A.R. et al.Mild hypothermia alters propofol pharmacokinetics and increased the duration of action of atracurium.Anesth Analg. 1995; 80: 1007-1014PubMed Google Scholar A hypothermic patient may suffer from delayed awakening and a longer duration of neuromuscular blockade. Mildly hypothermic patients may increase metabolic heat production by shivering postoperatively, an unpleasant sensation for most people and even more so when postoperative wounds are fresh. As a result, postoperative hypothermia may contribute to patient dissatisfaction and prolong the recovery period.17Lenhardt R. Marker E. Goll V. et al.Mild intraoperative hypothermia prolongs postanesthetic recovery.Anesthesiology. 1997; 87: 1318-1323Crossref PubMed Scopus (415) Google Scholar, 23Sessler D.I. Rubinstein E.H. Moayeri A. Physiological responses to mild perianesthetic hypothermia in humans.Anesthesiology. 1999; 90: 1609-1616Crossref PubMed Scopus (43) Google Scholar The consequences of mild perioperative hypothermia have led to policies to encourage the maintenance of perioperative normothermia. The basic monitoring standards of the American Society of Anesthesiologists state that “every patient receiving anesthesia shall have temperature monitored when clinically significant changes in body temperature are intended, anticipated or suspected.”24ASA Basic Anesthetic Monitoring, Standards For. Available at: http://www.asahq.org/For-Members/Clinical-Information/Standards-Guidelines-and-Statements.aspx. Accessed October 9, 2012.Google Scholar An American Society of Plastic Surgeons task force on patient safety advised that office-based facilities have temperature monitoring equipment and the ability to adjust temperature and actively warm patients.25Iverson R.E. ASPS task force on patient safety in office-based surgery facilities.Plast Reconstr Surg. 2002; 110: 1337-1342Crossref PubMed Scopus (82) Google Scholar Facilities that do not have such capabilities should accept patients only for procedures expected to last 2 hours or less that involve no more than 20% of the body surface area. The standards of the Surgical Care Improvement Project require that patients have at least 1 documented temperature of 36°C within the 30 minutes before or 15 minutes after the documented anesthesia end time or the documented use of active warming intraoperatively.26Available at: http://www.jointcommission.org/specifications_manual_for_national_hospital_inpatient_quality_measures.aspx. Accessed October 9, 2012.Google Scholar The Center for Medicare and Medicaid Services has also adopted this criterion for reimbursement. Thus, the prevention of perioperative hypothermia not only optimizes patient outcomes, but is used as a metric for institutional performance. Maintaining normothermia and treating hypothermia require reliable and accurate core temperature monitoring, which is not always easy to obtain. An invasive pulmonary artery catheter provides accurate monitoring of core temperature but is impractical for most patients, especially in the ambulatory setting. The direct application of a temperature probe to the tympanic membrane provides a good monitor of core temperature,27Benzinger M. Tympanic thermometry in surgery and anesthesia.JAMA. 1969; 209: 1207-1211Crossref PubMed Scopus (123) Google Scholar but more readily available infrared tympanic temperature monitors fail to reliably approximate core temperature unless properly placed directly in front of the tympanic membrane.28Imamura M. Matsukawa T. Ozaki M. et al.The accuracy and precision of four infrared aural canal thermometers during cardiac surgery.Acta Anaesthesiol Scand. 1998; 42: 1222-1226Crossref PubMed Scopus (47) Google Scholar Nasopharyngeal measurement also reasonably approximates core temperature.29Cork R.C. Vaughan R.W. Humphrey L.S. Precision and accuracy of intraoperative temperature monitoring.Anesth Analg. 1983; 62: 211-214Crossref PubMed Scopus (194) Google Scholar Distal measurement of esophageal temperature is a good method of temperature monitoring used by anesthesiologists during a general anesthetic, but for a nonintubated patient, esophageal temperature probes are impractical. Additional measurement sites include the rectum and bladder, but temperature values can be affected by local factors, including urine output. In an ambulatory setting, rectal measurement of temperature can be a good option for patients who undergo a lower body regional anesthetic with attendant perineal blockade. Cutaneous temperature monitoring may be used as a proxy for core temperature measurement but is better suited for trending than for accuracy. Axillary temperature monitoring works best when the temperature probe is placed over the patient’s axillary artery and the corresponding arm is tucked beside the torso.30Sessler D.I. Temperature monitoring and perioperative thermoregulation.Anesthesiology. 2008; 109: 318-338Crossref PubMed Scopus (451) Google Scholar, 31Lodha R. Mukerji N. Sinha N. et al.Is axillary temperature an appropriate surrogate for core temperature?.Indian J Pediatr. 2000; 67: 571-574Crossref PubMed Scopus (49) Google Scholar Adhesive liquid crystal thermometer strips can be useful for core temperature monitoring if placed properly. The skin of the forehead is typically 2°C cooler than the core, and this relationship is well maintained as core temperature changes.32Ikeda T. Sessler D.I. Marder D. et al.The influence of thermoregulatory vasomotion and ambient temperature variation on the accuracy of core temperature estimates by cutaneous liquid crystal thermometers.Anesthesiology. 1997; 86: 603-612Crossref PubMed Scopus (51) Google Scholar Even if an accurate core temperature reading is elusive, monitoring can at least identify a trend of cooling or heating that threatens normothermia. The 2 methods of maintaining a patient’s temperature perioperatively are passive and active warming. Passive techniques prevent the loss of heat without specifically adding any heat to the system. Blankets, surgical drapes, and passively humidified respiratory gases are examples of passive warming. Although passive warming can limit heat loss, it has not been shown to prevent hypothermia.33Ng S.F. Oo C.S. Loh K.H. et al.A comparative study of three warming interventions to determine the most effective in maintaining perioperative normothermia.Anesth Analg. 2003; 96: 171-176PubMed Google Scholar Unlike passive techniques, active warming adds external heat. Radiant heat lamps, higher ambient (room) temperatures, forced air blankets, and fluid warmers are examples of active warming methods. Forced air warmers are the most commonly used active warming devices. Comparisons of forced air warming devices with passive devices such as cotton blankets and reflective blankets have shown that forced air blankets are superior for maintaining normothermia.33Ng S.F. Oo C.S. Loh K.H. et al.A comparative study of three warming interventions to determine the most effective in maintaining perioperative normothermia.Anesth Analg. 2003; 96: 171-176PubMed Google Scholar, 34Horn E.P. Bein B. Bohm R. et al.The effect of short time periods of preoperative warming in the prevention of perioperative hypothermia.Anaesthesia. 2012; 67: 612-617Crossref PubMed Scopus (83) Google Scholar, 35Sessler D.I. Schroeder M. Merrifield B. et al.Optimal duration and temperature of pre-warming.Anesthesiology. 1995; 82: 674-681Crossref PubMed Scopus (139) Google Scholar To be effective, a forced air blanket must let warmed air circulate and must be in contact with the patient’s body. Peripheral heat content gain during the preoperative phase has been shown to diminish heat loss during the redistribution of heat from the core to the periphery in phase 1 of the intraoperative phase.35Sessler D.I. Schroeder M. Merrifield B. et al.Optimal duration and temperature of pre-warming.Anesthesiology. 1995; 82: 674-681Crossref PubMed Scopus (139) Google Scholar Administration of intravenous fluids at room temperature can lower body temperature by 0.25°C per liter,36Leslie K. Sessler D.I. Perioperative hypothermia in the high-risk surgical patient.Best Pract Res Clin Anaesthesiol. 2003; 17: 485-498Abstract Full Text Full Text PDF PubMed Scopus (130) Google Scholar but warmed intravenous fluids can help prevent perioperative hypothermia. The combination of forced air warming and warmed intravenous fluid administration (average 6 L) has been suggested to be superior to forced air warming alone, preventing perioperative hypothermia in major surgery.37Leben J. Tryba M. Prevention of hypothermia during surgery: contribution of convective heating system and warm infusion.Ann N Y Acad Sci. 1997; 813: 807-811Crossref PubMed Scopus (14) Google Scholar Fluids used for irrigation and tumescent infiltration can also contribute to the development of perioperative hypothermia. A comparison of lipoplasty patients treated with cool (27°C) versus heated (37°C) tumescent solution without additional active warming showed both groups to be hypothermic at the end of the 3.5-hour procedure.38Robles-Cervantes J.A. Martinez-Molina R. Cardenas-Camarena L. Heating infiltration solutions used in tumescent liposuction: minimizing surgical risk.Plast Reconstr Surg. 2005; 116: 1077-1081Crossref PubMed Scopus (11) Google Scholar The group who received heated tumescent solution did maintain a higher mean core temperature (35.7°C) than the group who received cool tumescence (34.9°C), showing that warming infiltration fluids does not prevent hypothermia without other active measures. Also, procedures that require large amounts of irrigation fluid or tumescent solution often result in exposure of greater patient body surface area, which may accelerate patient heat loss perioperatively. Thus, warming solutions for irrigation or infiltration are best used in combination with other active warming techniques, such as forced air warming devices and warm intravenous fluids. Most patients undergoing procedures with anesthesia, except those who receive infiltration with local anesthetics only, are at risk of hypothermia. Even mild hypothermia may negatively affect patient outcome by increasing bleeding, myocardial events, wound infections, postoperative pain, and recovery time. The risks of perioperative hypothermia must be recognized, and we must be vigilant in detecting, treating, and preventing it (Box 1). Temperature monitoring is essential to the preservation of patient normothermia. Monitoring is critical in procedures involving major regional anesthetics, because physicians may be unable to detect and patients unable to react to hypothermia. Maintenance of normothermia is best accomplished with the use of forced air warming devices over a sufficient period to minimize loss of core heat. A coordinated effort during the preoperative, intraoperative, and postoperative care phases is essential for preventing hypothermia.Box 1Risk factors for hypothermiaExtremes of age (geriatric and neonatal patients)Combined general and regional anesthesiaLow ambient OR temperatureLow preoperative patient peripheral temperatureThin body habitusHigh blood lossHigh-exposure surgery (ie, burns) Extremes of age (geriatric and neonatal patients)Combined general and regional anesthesiaLow ambient OR temperatureLow preoperative patient peripheral temperatureThin body habitusHigh blood lossHigh-exposure surgery (ie, burns) Perioperative hyperthermia results from excessive heat added to the patient, such as may ensue from overzealous attempts to prevent hypothermia, or from endogenous heat production in a wide range of medical conditions, including sepsis, hypothalamic dysfunction, thyroid dysfunction, substance abuse, neuromuscular disorders, and malignant hyperthermia (MH). This section focuses on MH. MH is a unique condition associated with commonly used anesthetic agents, which may cause serious morbidity or mortality if left unrecognized or if treatment is delayed. A pharmacogenetic disorder that follows an autosomal-dominant pattern with incomplete penetrance,39Hirshey Dirksen S.J. Larach M.G. Rosenberg H. et al.Future directions in malignant hyperthermia research and patient care.Anesth Analg. 2011; 113: 1108-1119Crossref PubMed Scopus (36) Google Scholar MH involves the disordered regulation of intracellular calcium, which prolongs and sustains skeletal muscle contraction and causes a hypermetabolic state. Triggering agents of MH are all volatile inhalational anesthetics (eg, sevoflurane) and succinylcholine. Nonvolatile inhalational anesthetics (eg, nitrous oxide) do not trigger MH. Succinylcholine, a depolarizing muscle relaxant commonly used to facilitate tracheal intubation and to treat airway emergencies such as laryngospasm, works differently from nondepolarizing muscle relaxants (eg, cisatracurium and rocuronium). A general anesthetic may include 1 or more triggering agents, making vigilance for MH imperative. A history of the signs and symptoms of MH in a patient or a relative during a previous anesthetic can indicate MH susceptibility, but a definitive medical history is often lacking. Patients may simply recall a relative who almost died in the operating room or had some other nonspecific perioperative course. When historical information is vague, the anesthesiologist is left to weigh the risks and benefits of avoiding triggering agents in a patient. It is not uncommon for physicians to dismiss the possibility of MH susceptibility in patients who have had uneventful general anesthetics in the past, but MH susceptibility is a result of many possible genetic mutations, with variable expression, and MH susceptibility cannot be excluded by a history of uneventful general anesthetics.40Halsall P.J. Cain P.A. Ellis F.R. Retrospective analysis of anaesthetics received by patients before susceptibility to malignant hyperpyrexia was recognized.Br J Anaesth. 1979; 51: 949-954Crossref PubMed Scopus (57) Google Scholar MH is rare, with estimates of 1 in 100,000.41Brady J.E. Sun L.S. Rosenberg H. et al.Prevalence of malignant hyperthermia due to anesthesia in New York State, 2001-2005.Anesth Analg. 2009; 109: 1162-1166Crossref PubMed Scopus (97) Google Scholar It is not uncommon for anesthesiologists to finish their careers without having to manage a case of MH. Although planning and preparing for rare but potentially catastrophic events is difficult, the Malignant Hyperthermia Association of the United States (MHAUS) is a resource for the prevention and treatment of MH. Any location that uses MH-triggering medications must be prepared to diagnose and treat and, if necessary, transfer a patient with MH for intensive care. Although preparation for MH requires costly economic and staff resources, the alternative of a bad outcome for a patient is costlier still.42Available at: http://www.aboutlawsuits.com/anesthesia-malpractice-lawsuit-malignant-hyperthermia-death-6174/. Accessed October 9, 2012.Google Scholar The American Association for Accreditation of Ambulatory Surgery Facilities (AAAASF) and the Accreditation Association for Ambulatory Health Care (AAAHC) address MH in their requirements for accreditation. The AAAHC refers to a statement from MHAUS that advises that centers in which triggering agents are used should have 36 vials of dantrolene available. The AAAASF directives give a detailed list of supplies, which includes dantrolene (12 vials, with immediate access to 24 more within 15 minutes), MHAUS treatment literature on the MH cart, a plan for transfer of patients to an acute care setting, and regularly documented staff drills and assigned roles during an MH crisis (Fig. 6).43The guidelines are available at: http://www.aaaasf.org/pub/htm. Accessed October 9, 2012.Google Scholar State regulations regarding MH preparation vary. Local regulations should be verified, because the required minimum number of stocked vials of dantrolene is different from state to state. Increased temperature in MH does not necessarily represent the best or earliest indicator of the condition. MH is fundamentally a hypermetabolic state, fueled by the abnormal and prolonged contraction of skeletal muscle, with increased carbon dioxide production secondary to increased metabolism. Excretion levels of carbon dioxide, which is cleared during ventilation, increase on end-tidal monitoring in patients with MH. In 1 review, hypercarbia was the first or only sign of MH at presentation in 38% of 255 patients.44Larach M.G. Gronert G.A. Allen G.C. et al.Clinical presentation, treatment, and complications of malignant hyperthermia in North America from 1987 to 2006.Anesth Analg. 2010; 110: 498-507Crossref PubMed Scopus (211) Google Scholar Other early indicators included sinus tachycardia (31%) and masseter spasm (20.8%). Rapid temperature increase, or temperature greater than 38.8°C, was the first sign in only 8.2% of the same patient population. Hypercarbia and sinus tachycardia during general anesthesia are not pathognomonic for MH, so other signs help support the diagnosis. In addition to ventilatory acidosis, a metabolic acidosis, as measured by arterial blood gas, is common. Muscle rigidity also may be present, along with increased plasma creatinine kinase levels from muscle breakdown. Rhabdomyolysis may produce cola-colored urine from myoglobinuria and hyperkalemia, the latter potentially leading to cardiac arrest. Death may also ensue from the extreme acidosis and temperature increase. Effective treatment of an MH crisis requires coordination and swift action. MHAUS has posters and a Web page with detailed treatment algorithms and a 24-hour hotline staffed by experts (Box 2). Triggering agents should be discontinued and intravenous dantrolene sodium, which lessens the calcium release that fuels the hypermetabolic state, should be administered as quickly as possible.45Kobayashi S. Bannister M.L. Gagopadhyay J. et al.Dantrolene stabilizes domain interactions within the ryanodine receptor.J Biol Chem. 2005; 280: 6580-6587Crossref PubMed Scopus (100) Google Scholar The reconstitution and administration of dantrolene can be labor intensive, and additional personnel should be mobilized to ensure a rapid response. Supportive care includes cooling of the patient, treating a metabolic acidosis with bicarbonate, monitoring and treating hyperkalemia, and administering diuretics and fluids to maintain urine output. Laboratory tests should include arterial blood gas analysis, creatinine kinase, myoglobin, electrolytes, and coagulation studies.Box 2MHAUS MH treatment recommendationsCALL the MH 24-hour hotline (for emergencies only)•United States: 1+800-644-9737•Outside the US: 00+1+303-389-1647Start emergency therapy for MH acute phase treatment1.Get help. Get dantrolene. Notify surgeon2.Dantrolene sodium for injection 2.5 mg/kg rapidly IV through large-bore IV, if possible3.Bicarbonate for metabolic acidosis4.Cool the patient5.Dysrhythmias: usually respond to treatment of acidosis and hyperkalemia6.Hyperkalemia7.Follow: ETCO2, electrolytes, blood gases, CK, serum myoglobin, core temperature, urine output and color, and coagulation studies CALL the MH 24-hour hotline (for emergencies only)•United States: 1+800-644-9737•Outside the US: 00+1+303-389-1647Start emergency therapy for MH acute phase treatment1.Get help. Get dantrolene. Notify surgeon2.Dantrolene sodium for injection 2.5 mg/kg rapidly IV through large-bore IV, if possible3.Bicarbonate for metabolic acidosis4.Cool the patient5.Dysrhythmias: usually respond to treatment of acidosis and hyperkalemia6.Hyperkalemia7.Follow: ETCO2, electrolytes, blood gases, CK, serum myoglobin, core temperature, urine output and color, and coagulation studies Transfer to an acute care facility should be considered soon after a diagnosis of MH, particularly if resources for laboratory studies and critical care management are lacking. Arrangements for transfer of patients to an acute care facility are best made in advance, and an agreement should already be in place between an ambulatory surgery center or office surgical suite and an acute care facility equipped to take care of patients in MH crisis. A guide for the transfer of patients with MH from an ambulatory surgery center to an acute care hospital makes several recommendations.46Larach M.G. Hirshey Dirksen S.J. Belani K.G. et al.Creation of a guide for the transfer of the malignant hyperthermia patient from ambulatory surgery centers to receiving hospital facilities.Anesth Analg. 2012; 114: 94-100Crossref PubMed Scopus (30) Google Scholar A mechanism should exist for rapid mobilization of transport (emergency medical services or otherwise), reliable communication between treating physicians and accepting physicians, and appropriate critical care and dantrolene resources at the receiving hospital. After successful treatment of the acute phase of an MH crisis, a patient still requires intensive monitoring. MH may recur despite initial dantrolene treatment, and repeat dosing of dantrolene is recommended. Once patients have recovered from an MH crisis, they must be counseled about the diagnosis and its hereditary ramifications. The newly diagnosed MH-susceptible patient should be educated on the condition and the implications for family members. There are many reasons why a patient may become hyperthermic in the perioperative period. Most of the time, the patient's comorbidities, such as infection with sepsis, endocrine disorders, and or iatrogenic overheating, are the cause. However, the rapid morbidity and mortality of untreated MH demand that all members of the perioperative team be familiar with the diagnosis and treatment of the condition. The possibility of MH must be considered any time a patient is cared for in a facility housing triggering agents. Even if there is no intent to use succinylcholine or volatile anesthetic agents, the presence of these agents in a facility requires an MH treatment plan. It is also critical to realize that patients with previous uncomplicated exposure to triggering agents may suffer an MH crisis with subsequent exposure to triggering agents. Institutional preparation for MH crises is as important as the education and training of the perioperative team. An MH cart fully stocked with nonexpired dantrolene, diluent, and adjunctive treatment drugs and equipment must always be at the ready in facilities housing triggering agents. Similarly, a plan for the continued care of a patient with MH must be operational at all times. Surgical facilities should have agreements in place for transfer of the patient with MH to an acute care setting with critical care capabilities, when such resources are lacking in the original location. MH is a rare disorder, with potentially catastrophic outcomes. Patient survival without permanent sequelae is possible with prompt diagnosis and treatment. All members of the perioperative team must be prepared to recognize and treat MH." @default.
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