e4 Abstract The interest in heat related illnesses has grown substan tially, largely because of the effects of climate change and an in creased frequency of heat waves Heat related illnesses are a spe[.]
e4 Abstract: The interest in heat-related illnesses has grown substantially, largely because of the effects of climate change and an increased frequency of heat waves Heat-related illnesses are a spectrum of disorders ranging in severity from heat cramps to heat exhaustion to heat stroke Heat stroke is a life-threatening emergency that is driven by induction of the systemic inflammatory response syndrome and multiorgan dysfunction Within the pediatric population, children younger than years and adolescents are at highest risk of heat-related illness For optimal patient outcomes, treatment of heat stroke requires rapid cooling and maintenance of organ perfusion Key words: heat-related illness, heat exhaustion, heat stroke, systemic inflammatory response syndrome 114 Accidental Hypothermia BJƯRN GUNNARSSON AND CHRISTOPHER M.B HEARD • There is no uniform classification of accidental or unintentional hypothermia, but it can be categorized as mild (35°C–32°C), moderate (32°C–28°C), severe (,28°C), or profound (,24°C or ,20°C).1–3 The Swiss staging system of hypothermia is based on clinical findings at the scene that roughly correlate with core temperature.4–7 The stages correspond to the four categories mentioned earlier, as well as stage V: death due to irreversible hypothermia (,13.7°C) This is a serious and, in many cases, preventable health problem that can cause marked depression of bodily functions to such degree that live victims may appear clinically dead.3,8–10 Each organ system may be affected.11 However, our understanding of accidental hypothermia is based largely on case reports, observational studies, and studies in animals, as controlled studies of serious hypothermia in humans are not available for obvious reasons Physiology Humans have a high capacity to dissipate heat but a relatively poor capacity to increase heat production Thus, humans rely heavily on environmental regulation in the form of clothing and warm shelter to maintain normal body temperature The body can compensate to a great degree for mild hypothermia The hypothalamus sends signals that produce cutaneous vasoconstriction, increased muscle tone, and increased metabolic rate When muscle tone reaches a 1332 • Accidental or unintentional hypothermia is a potentially lethal complication of exposure to cold It can occur as a result of exposure to cold air, water immersion/submersion, or snow burial Risk factors include accidents, neglect, toxins, mental disorders, and violence Information about the duration and severity of cold exposure, scene details, and any other associated injuries may help in the selection of the appropriate facility and rewarming methods Many organ systems are affected by hypothermia There is a marked depression of cerebral blood flow and oxygen use in patients with hypothermia Rescuers should initiate resuscitation on all patients with hypothermia unless a patient has a frozen chest or any other obvious nonsurvivable injury The hallmark of rescue in all individuals with hypothermia is prevention of further heat loss, careful • • • • • PEARLS transport, and rewarming Avoiding excess activity and rough movements of patients with hypothermia is important because this may precipitate cardiac dysrhythmias Various techniques have been used for in-hospital resuscitation from deep hypothermia, but no controlled studies in which rewarming methods are compared exist, and rigid treatment protocols cannot be recommended Active external rewarming has been shown to be effective Extracorporeal rewarming of blood is the preferred method, however, to resuscitate patients with hypothermia and cardiac arrest or cardiovascular instability Prediction of patient outcome is difficult, and some children with presumed dismal prognosis have had intact survival The decision to terminate resuscitative efforts must be based on the unique circumstances of each case certain level, shivering thermogenesis begins The clinical manifestations depend on the severity, acuity, and duration of temperature reduction; the patient’s age; premorbid conditions; and superimposed disease states Children are more susceptible to hypothermia than adults because of a large surface area relative to body mass and less subcutaneous tissue Neonates have a capacity for nonshivering thermogenesis primarily by metabolism of brown fat; however, this is at the cost of greatly increased oxygen consumption Therefore, neonates are extremely sensitive to relatively minor deviations from a neutral thermal environment Central Nervous System Cerebral oxygen requirement decreases with cooling, and mentation is progressively impaired Mild hypothermia may be associated with confusion, dysarthria, and impaired judgment.11 Deep tendon reflexes are depressed at core temperature below 32°C because of slowed peripheral nerve conduction As body temperature drops, many victims no longer complain of cold Shivering thermogenesis ceases at about 31°C Pupillary responses decline and dilated unreactive pupils may be noted at temperatures below 30°C Victims may experience hallucinations and sometimes paradoxically remove their clothes The electroencephalogram shows abnormal activity at temperatures less than 32°C; at 20°C, the electroencephalogram may appear consistent with brain death.12 CHAPTER 114 Accidental Hypothermia J 1333 may include peripheral vasoconstriction and blunted response to antidiuretic hormone Renal vasoconstriction and ischemia to the kidney may lead to oliguria and acute tubular necrosis in those with severe hypothermia Progressive hypokalemia develops during hypothermia, probably because of the shifting of potassium from the extracellular to intracellular compartment, and significant hyperkalemia may develop during rewarming.28,29 The electrocardiogram manifestation of hyperkalemia may be obscured or attenuated by hypothermia.13 Renal replacement therapy may be required for renal failure and has also been used for rewarming.30,31 Coagulation • Fig 114.1 Characteristic J or Osborne wave of hypothermia closely follows QRS It may be mistaken for a T wave with narrow QT interval if the true T wave is not appreciated The slightly rounded peak distinguishes it from R9 of bundle branch block Cardiovascular The initial cardiovascular responses are vasoconstriction, tachycardia, and increased cardiac output Further hypothermia results in decreased pacemaker and conduction velocity, causing bradycardia, heart block, and prolongation of PR, QRS, and QT intervals.13 The myocardium becomes irritable, and atrial fibrillation is common when core temperature is below 32°C The risk of ventricular arrhythmias is substantial below 30°C.8,13,14 A recent review of witnessed hypothermic cardiac arrest revealed that it occurred at a mean body temperature of 23.9°C and in no cases with a temperature 30°C.15 The electrocardiogram may show characteristic J or Osborne wave following the QRS complex (Fig 114.1).16–19 The presence of this wave is not pathognomonic for hypothermia and has no prognostic implications.13,20 Myocardial contractility, cardiac output, and systemic blood pressure are often decreased dramatically in hypothermic victims These changes may be persistent during and after rewarming.21 Hypovolemia due to cold-induced diuresis and capillary leakage may potentiate the problem.12,21,22 Respiratory Hypothermia affects tissue oxygenation through several complex physiologic mechanisms Initially, the respiratory rate may be increased As hypothermia worsens, the respiratory center becomes depressed and hypoventilation causes carbon dioxide retention, although carbon dioxide production decreases with increasing hypothermia Respiratory arrest is a late occurrence Suppression of cough and mucociliary reflexes leads to atelectasis and pneumonia Oxygen delivery to the tissues is further compromised by shifting of the oxyhemoglobin dissociation curve to the left.12 Blood gas analyzers warm blood to 37°C before analysis.23 In patients with hypothermia, arterial blood gases show higher oxygen and carbon dioxide levels and a lower pH than a patient’s actual values The best approach to interpretation is to compare the uncorrected blood gas values with the normal values at 37°C (Alpha-Stat strategy).14,24–26 Renal Renal injury may occur either because of hypothermia or during the rewarming process.27 The mechanisms involved in cold diuresis Hypothermia inhibits the intrinsic and extrinsic pathways in the clotting process The degree of coagulopathy, however, is often underestimated because dynamic coagulation tests are generally performed at 37°C in the laboratory Bone marrow suppression and splenic sequestration can lead to thrombocytopenia, and platelets become dysfunctional.12,14 This leads to increased bleeding tendency; the combination of hypothermia and trauma carries a grave prognosis.32,33 Treatment The hallmark of rescue in all individuals with hypothermia is prevention of further heat loss, careful transport, and rewarming.5,12 Wet clothes should be removed, and the individuals should be insulated and shielded from wind and cold Paying special attention to the head and neck is important because radiant heat loss from those areas can be profound Collapse of deeply hypothermic victims around the time of rescue may be explained by further cooling, circulatory collapse due to hypovolemia, or arrhythmias, sometimes triggered by procedures such as central venous catheterization.5,8,14 Sudden changes in hydrostatic conditions may contribute during extrication of victims from cold water This has been attributed to sudden fall in blood pressure and inadequate coronary blood flow, precipitating ventricular fibrillation.34,35 Victims should therefore be kept horizontal during rescue, if possible.35 Resuscitation Detecting signs of life in patients with deep hypothermia may be difficult Therefore, the rescuer should assess breathing and then pulse for 60 seconds.5,8 Chest compressions should be started immediately if the patient is pulseless with no detectable signs of circulation and may have to be continued for hours before extracorporeal rewarming can be started.8,36-41 Victims in cardiac arrest have survived when resuscitation after rescue was delayed by as much as 70 minutes.42 The advantages of endotracheal intubation outweigh the minimal risk of triggering ventricular fibrillation with the procedure If cervical spine injury is suspected, the neutral position must be maintained with manual cervical stabilization Care should be taken not to overventilate the patient’s lungs because this can increase ventricular irritability.43 Defibrillation can be tried up to three times for ventricular tachycardia or fibrillation If arrhythmia is resistant to three shocks in a patient with deep hypothermia, then further defibrillation attempts should be deferred until core temperature is 30°C or higher.5,44 However, a systematic review of animal models performing resuscitation from ventricular fibrillation in severe hypothermia revealed much higher return of spontaneous circulation rates in studies 1334 S E C T I O N X I I Pediatric Critical Care: Environmental Injury and Trauma administrating vasopressor medications.45 The hypothermic heart may also have a reduced response to pacing and cardioactive medications, but evidence for medication efficacy and risk of accumulation to toxic levels, if administered repeatedly, is limited and based mainly on animal studies.5,45–50 The European Resuscitation Council Guidelines for Resuscitation 2015 recommend withholding resuscitation medications until the patient has been warmed to core temperature of 30°C or higher and doubling the interval between doses compared with normothermic patients.5 The American Heart Association Guidelines, on the other hand, are less clear on this and conclude that it may be reasonable to consider administration of a vasopressor during cardiac arrest caused by hypothermia.9,51 Rewarming The rewarming of victims who are conscious with mild hypothermia and still able to shiver can be achieved with passive techniques (e.g., blankets, wool cap, warm shelter) Active external rewarming with chemical heat packs or forced warm air can be useful in the prehospital setting to prevent further heat loss in victims with moderate hypothermia Some researchers claim that standing and exercising immediately after rescue may be hazardous, as this may accentuate core temperature afterdrop.2 The term core temperature afterdrop refers to a continued decrease in core temperature with potential for clinical deterioration of a victim after rescue There are probably two mechanisms behind this One is the inevitable equilibration of temperature between the periphery and core.52 It follows that the magnitude of afterdrop is greater if cooling is rapid because the temperature gradient between the surface and core is greater.53 The other mechanism is increased blood flow to the cold periphery and return of cold blood to the core (convection), possibly causing cardiovascular instability.54 Any intervention that causes increased blood flow to the periphery, including exercise, can potentially accentuate the afterdrop, but the significance of this in the clinical management of hypothermia victims is still being debated.2,3,8,52,55 However, avoidance of excessive activity and abrupt movements of any hypothermia victim seems prudent Attempts to rewarm the victim of hypothermia should not delay transport to the hospital.5 It is difficult to rewarm hypothermic patients during transport, and no specific recommendations can be given The use of mechanical cardiopulmonary resuscitation (CPR) devices should be considered if body size fits the device, as this increases safety and may improve outcome.5,8,36,56-58 Fig 114.2 shows a simple triage and management algorithm, a slight modification of a previously published algorithm.59 Several techniques have been used in-hospital for active rewarming of hemodynamically stable patients, but no controlled clinical trials exist in which rewarming methods are compared This includes warming with forced air, warm humidified gases, warmed IV fluids (up to 42°C), and lavage of gastric, peritoneal, pleural, or bladder cavities with warmed fluids (40°C).3 There are many reports of successful use of different techniques in children with severe hypothermia, even with cardiac arrest.60–69 Thoracic lavage using two large chest tubes can accomplish a temperature Accidental hypothermia Yes Signs of circulation Start active external rewarming Circulation stable and adequate >28°C Start resuscitation Hypotension/ ventricular arrhythmia