Hypothermia

Hypothermia pathophysiology, diagnosis, prehospital care, hospital care, emergency care, hypothermia management

1. HYPOTHERMIA Munkhtulga G. 2015 “ZEPH” Young Pathophysiologist student club

2. Definition, classification Core body temperature of less than 35°C (95°F) and can be clinically stratified by the core temperature into mild ( 35°-32°C /95°-89.6°F ), moderate ( 32°-30°C /89. S0-8 6°F), and severe ( <30°C /86°F) subtypes. Primary healthy person is unable to compensate for an excessive exposure to cold tem­peratures Secondary comorbid medical condition ( eg , hypothyroidism, sepsis, intoxication) disrupts a patient's normal thermoregulatory processes.

3. Hypothermia occurs as the body loses heat from 1 of 4 major mechanisms : conduction, convection, evaporation, and radi­ation . Convective (windy environments) and conductive (cold and wet exposures) mechanisms are responsible for most cases of accidental hypothermia.

4. More common in elderly 65< y/o Patients with an initial core body temperature <23°C (73 .4°) typi­cally do not survive, and the overall mortality rate of patients with hypothermia is approximately 40%.

5. Conditions causing hypothermia in children by physiologic mechanism Substrate deficiency Hypoglycemia Central nervous system conditions that disrupt hypothalamic and autonomic temperature regulation Traumatic brain or spinal cord injury (including neurosurgical procedures) Intracranial hemorrhage Stroke Brain tumors ( eg , craniopharyngioma or astrocytoma) Congenital brain malformations (eg, anencephaly or absence of the corpus callosum) Familial dysautonomia Drug overdose with agents that cause central nervous system depression and vasodilation Ethanol Opioids Clonidine Benzodiazepines Barbiturates Antipsychotic agents ( eg , chlorpromazine) Antidepressants ( eg , amitriptyline) Generalized infection* Sepsis Meningitis Encephalitis

6. Conditions causing hypothermia in children by physiologic mechanism Endocrine or metabolic diseases that impair energy utilization or basal metabolic rate Adrenal insufficiency Hypothyroidism Hypoparathyroidism Hypopituitarism Diabetes mellitus Menkes disease ( Menkes kinky hair syndrome) Organic acidemias Aminoacidemias Increased insensible losses due to disruption of skin Burns Epidermolysis bullosa and other weeping dermatoses Decreased calorie intake with decreased energy production and loss of insulating subcutaneous fat Malnutrition Anorexia nervosa Other Hyponatremia ( eg , water intoxication) Episodic spontaneous hypothermia with hyperhidrosis • Child abuse and maltreatment

7. International Commission for Mountain Emergency Medicine hypothermia scale Hypothermia (HT) stage Clinical assessment Expected core temperature, C° (F°)* HT I Clear consciousness with shivering 35-32 (95-89.6) HT II Impaired consciousness without shivering 32-28 (89.6-82.4) HT III Unconsciousness (circulation present) 28-24 (82.4-75.2) HT IV Apparent death (circulation absent) resuscitation possible <24-13.7 (<75.2-56.7) ** HT V Death due to irreversible hypothermia Resuscitation not possible <13.7-9 (56.7-48.2) ** * Clinical assessment may be used alone to stage hypothermia when core temperature measurement in the field is not available. ** The core temperature at which irreversible hypothermia with death occurs is not well defined.

8. Pathophysiology Body temperature reflects the balance between heat production and heat loss . Heat is generated by cellular metabolism (most prominently in the heart and liver) and lost by the skin and lungs via the following processes: Evaporation – Vaporization of water through both insensible losses and sweat Radiation – Emission of infrared electromagnetic energy Conduction – Direct transfer of heat to an adjacent, cooler object Convection – Direct transfer of heat to convective air currents

9. In response to a cold stress, the hypothalamus attempts to stimulate heat production through shivering and increased thyroid, catecholamine, and adrenal activity. Sympathetically mediated vasoconstriction minimizes heat loss by reducing blood flow to peripheral tissues, where cooling is greatest

10. Other physiologic effects Respiration progressively becomes slow, shallow, irregular, and then absent. Blood volume markedly decreases because of extravasation due to vascular leak and a profound "cold diuresis" caused by erroneous signaling to the kidney about blood volume. Cold diuresis is worsened by failure of renal concentrating function. The hydrostatic effects of immersion also promote shock after water rescue

11. Other physiologic effects Hypothermia increases myocardial irritability. Thus, ventricular fibrillation (VF) is a frequent problem in severe hypothermia. Potential triggers of VF to be avoided during treatment include rough patient handling, patient exertion, core temperature afterdrop (further cooling of the body after being removed from cold exposure), administration of room-temperature fluids, direct stimulation of the myocardium ( eg , subclavian central lines, and rewarming shock).

12. Clinical presentation, History usually obvious in patients with significant exposures. Patients may present in wet clothing, be found outdoors in the cold weather , or be inappropriately dressed for the environment in which they live . In the US, most hypothermic patients are either intoxicated or suffer from an underlying psychiatric ill ness or dementia . (drunk people)

13. Clinical presentation The history or presentation may be less obvious for patients with mild hypothermia or unknown exposures . Said patients typically present with nonspecific neurologic findings , including dizziness, confusion, slurred speech, or ataxia .

14. PE start by assessing and addressing the patient's airway, breathing, and circulation (ABCs) and vital signs. may present with unstable airways or absent pulses . Carefully measure the patient's core body tempera­ture by inserting a specialized "low-reading" probe into the bladder , rectum, or esophagus. majority of standard ED thermometers will not record temperatures below 34.4°C (94°F).

15. The hypothermic heart is very sensitive to movement, and rough handling of the patient may precipitate arrhythmias, including ventricular fibrillation. Take care to avoid jostling the patient during the physical examination or the performance of essential procedures.

16. Assessment A rectal probe thermometer is practical in most cases. In patients with severe hypothermia, particularly those requiring endotracheal intubation, an esophageal probe provides a near approximation of cardiac temperature. Either device may be inaccurate: rectal probe readings may rise following peritoneal lavage or fall if adjacent to cold feces; esophageal probe readings may rise due to warm air transmitted through the trachea. The reliability of tympanic thermometers in the setting of significant hypothermia has NOT been established

17. Ancillary studies Fingerstick glucose Electrocardiogram (ECG) Basic serum electrolytes, including potassium and calcium BUN and creatinine Serum hemoglobin, white blood cell, and platelet counts Serum lactate Fibrinogen level Creatine phosphokinase Arterial blood gas, uncorrected for temperature Chest radiograph (care must be taken to avoid jostling the patient)

18. Arterial blood gas Metabolic acidosis, respiratory alkalosis, or both Electrolytes No consistent abnormality Glucose Increased, decreased, or no change White blood cell and platelets counts Decreased due to splenic sequestration Hemoglobin, hematocrit Increased due to hemoconcentration Lipase May be increased due to hypothermia-induced pancreatitis Prothrombin and partial thromboplastin times Increased in vivo due to inhibition of coagulation cascade, despite normal reported values Electrocardiogram Prolongation of PR, QRS, QT intervals ST segment elevation T wave inversions; Osborn J wave Atrial fibrillation or sinus bradycardia Chest radiograph Aspiration pneumonia, vascular congestion, pulmonary edema

19. Inhibition of the enzymes of the coagulation cascade from hypothermia leads to a bleeding diathesis. Because tests of coagulation ( eg , prothrombin time, partial thromboplastin time) are always performed at 37ºC, the laboratory will report deceptively "normal" results despite an obvious in vivo coagulopathy. Treatment consists of rewarming; administration of clotting factors is ineffective.

20. Oxygenation should be monitored continuously; however, the response time of pulse oximeters placed on the finger is slowed by hypothermia. Probes placed on the ears or forehead appear to be less influenced by decreased body temperature and the associated peripheral vasoconstriction. Assessment of oxygenation is also complicated by the fact that arterial blood gas analyzers operate at 37ºC, which is (by definition) greater than the body temperature of a hypothermic patient.

21. How do you diagnose hypothermia?

22. Hypothermia is diagnosed by identification of a core body temperature that is <35ºC (95ºF) Children with suspected hypothermia should have their temperature taken with a low-reading rectal probe thermometer or, ideally, the core temperature should be measured using nasopharyngeal, esophageal, bladder, or central venous catheter temperature probes The clinician should not use standard clinical thermometers in hypothermic patients because they do not read below 34ºC (93ºF). In addition, the oral, axillary, infrared, and indirect tympanic membrane sites are unreliable.

23. Although true tympanic temperatures may closely reflect core temperatures, indirect infrared tympanic thermometers, typically available in the emergency department, are subject to artifact after cold water exposure and have not been shown to be reliable in patients with moderate or severe hypothermia . When core temperature assessment is not readily available, assessment of shivering, level of consciousness, and vital signs provide a means of estimating the initial stage of hypothermia

24. Initial management

25. Successful resuscitation of the hypothermic child requires rapid attention to supportive care (airway, breathing, circulation), assessment and treatment of injury or other medical conditions, and rewarming interventions that vary based upon core body temperature and the presence or absence of circulation

26. Stage Rewarming methods* Notes All   Remove wet clothing Check ECG, continuously monitor cardiac rhythm Provide dry insulation Monitor core temperature Warm airway humidity Monitor circulation Warm IV (40-44°C)• Warmed volume support Mild (32-35°C , 90-95°F )   Active external Watch for afterdrop Moderate (28-32°C , 82-90°F )   Active external (forced air preferred) Watch for afterdrop Direct heating methods (eg, hot packs, heating pads) may cause burns Severe (<28°C , <82°F ) - or if methods above prove ineffective Circulation intact Left pleural lavage or forced air rewarming Δ As above, plus ensure warming is effective Circulation absent Extracorporeal rewarming (ECR) Left pleural lavage (if ECR is unavailable)

27. Prehospital rescue and triage Suspicion is vital! Should consider all children who have altered mental status or who require critical care . Measure core temperature (low reading thermometer/clinical signs)

28. Prehospital declaration of death clinical appearance of death, including rigidity, is typical of severe hypothermia Possible or confirmed severe hypothermia is a strong contraindication to the declaration of death in the prehospital setting.   Successful resuscitation has occurred in patients with asystole and prolonged avalanche burial (35 minutes or longer) when the airway was patent (not filled with snow or ice) upon extrication

29. Circumstances that do permit prehospital declaration of death include: Rescue is impossible or poses undue risk of serious injury or death to the prehospital providers Injuries are clearly lethal Valid “do not resuscitate” order is in place Some experts also suggest that the prehospital declaration of death may be appropriate in a patient after prolonged avalanche burial (35 minutes or longer) in the combined presence of an obstructed (snow-packed) airway and asystole upon extrication

30. Prehospital care and transport Patients should be extracted from the cold environment in a horizontal position, if possible. Prehospital providers should avoid patient exertion and rough handling.

31. Many hypothermic patients arrive at the hospital colder than they left the scene. Rescue , transport, and treatment involve several risks for iatrogenic cooling. During transport, rescuers should do all that is possible to prevent further patient heat loss. Key interventions include: Removal of wet clothing Gentle insulation of the patient with blankets or other items ( eg , sleeping bag) Warming of the transport vehicle Provision of warm IV fluids

32. We prefer to avoid active external rewarming in transport for children. According to 2 trials If active rewarming is to be performed, rescuers should avoid warming of the extremities and should provide rapid infusion of warmed normal saline with appropriate monitoring of cardiac rhythm.

33. Initial hospital care Hypothermic children should be maintained in a horizontal supine position and handled gently with minimization of movement . remove wet clothing and insulate the victim from further exposure ( eg , dry blankets). The application of external heat, although almost automatic in many centers, should be avoided when circulation is impaired.

34. Initial hospital care - OXYGEN Administration of heated, humidified oxygen is an acceptable therapy for all hypothermic patients. The amount of rewarming provided by this treatment is limited. The ideal gas temperature, based on the highest delivered temperature that will not cause harm, is 45°C (113°F). However, even at 41°C , inspiration of humidified oxygen prevents further heat loss.

35. Initial hospital care – IV FLUID Any intravenous (IV) fluid ( eg , normal saline), administered in volume should be heated to 40° to 44°C (104° to 111°F ). Because?? Avoid delivering fluid that is less than normal body temperature (37°C (98.6°F)) which can worsen core hypothermia

36. Delivery of warmed IV fluids requires the use of specific technologies, such as trauma-style fluid warmers with short, large-bore insulated, or countercurrent tubing.

37. Rewarming techniques Passive rewarming Active external rewarming Active internal rewarming Non-invasive (Noninvasive techniques include the provision of heated, humidified oxygen and warmed intravenous normal saline) Invasive Extracorporeal

38. Passive rewarming removing cold or wet clothing applying dry insulation, such as blankets, in a warm environment . should be accompanied by appropriate active rewarming techniques when treating children with hypothermia

39. Active external rewarming apply heat externally to the patient. These include forced air rewarming, radiant heat, and the application of chemical heat packs or plumbed water heating pads.  

40. However, they have the potential to promote afterdrop (further cooling), hypotension (rewarming shock), ventricular fibrillation, or asystole in patients with moderate or severe hypothermia. These adverse effects arise from mobilization of cold and acidic blood into the central circulation, especially if the extremities are warmed early. Thus, when these methods are used, the trunk should be warmed first. should be avoided in patients with severe hypothermia or absent circulation

41. Invasive active internal rewarming heated saline lavage of the left pleural space, bladder, stomach, and/or peritoneum. Invasive techniques are suggested for patients with severe hypothermia (core temperature <28ºC (82ºF)), especially with some preserved circulation, or those with inadequate response to initial rewarming

42. Extracorporeal active internal rewarming Extracorporeal techniques include: cardiac bypass and extracorporeal membrane oxygenation (ECMO). These are suggested for children with severe hypothermia and absent circulation or in patients for whom other techniques prove ineffective.

43. Supportive care moderate hypothermia or severe hypothermia require intensive supportive care of the airway, breathing, and circulation in addition to rewarming.

44. Airway and breathing Warmed, humidified 100 percent oxygen via nonrebreather mask for all patients Bag-valve-mask ventilation in children with hypoventilation and when preparing for endotracheal intubation Endotracheal intubation in patients with respiratory failure, uncompensated shock, or cardiac arrest

45. Chest compression clinician start cardiopulmonary resuscitation without delay if there are no signs of life

46. Stage Rewarming methods* Notes All   Remove wet clothing Check ECG, continuously monitor cardiac rhythm Provide dry insulation Monitor core temperature Warm airway humidity Monitor circulation Warm IV (40-44°C)• Warmed volume support Mild (>34°C, 93°F)   Active external Watch for afterdrop Moderate (30-34°C, 86-93°F)   Active external (forced air preferred) Watch for afterdrop Direct heating methods (eg, hot packs, heating pads) may cause burns Severe (<30°C, 86°F) - or if methods above prove ineffective Circulation intact Left pleural lavage or forced air rewarming Δ As above, plus ensure warming is effective Circulation absent Extracorporeal rewarming (ECR) Left pleural lavage (if ECR is unavailable)

47. Post-resuscitation care Pulmonary edema, coagulopathy, and renal failure are common complications after rewarming that require continued care. These conditions usually resolve within a few days. Neurologic abnormalities may resolve quickly with rewarming or persist for weeks to months even in cases where they eventually resolve.

48. PREDICTION OF NEUROLOGIC OUTCOME ??? Glasgow Coma Scale, have not proved useful for hypothermic patients