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Accidental hypothermia

Accidental hypothermia

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Accidental hypothermia

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  1. Accidental hypothermia Author(s): Philip Miller MD, CCFP(EM) Date Created: July 2012

  2. Learning Objectives • Understand the physiology of hypothermia • Recognize predisposing factors • Have an approach to the management of accidental hypothermia • Understand the treatment of cardiac arrest scenarios

  3. Hypothermia • Epidemiology and Definitions • Pathophysiology • Clinical features • General approach • Advanced cardiac life support • Rewarming

  4. Key Concepts • Hypothermia simplified: • Take precautions to prevent ventricular fibrillation (VF) • Optimize conditions for return of spontaneous circulation (ROSC) in cardiac arrest scenarios • Identification and treatment of precipitants • Use of appropriate rewarming techniques

  5. Case • 2 year old, with Grandma who lost track of him by a lake • Found 2 hrs later, face down in the lake • No pulse, not breathing • Brought to Emergency Room • Rectal temperature 19°C • Asystole • How do you manage this patient? • Should you intubate him? • How long should you continue resuscitative efforts?

  6. What is hypothermia? • Definition: core temperature less than 35ºC

  7. Epidemiology • Actual incidence is not known and under-reported • Survival has been reported as low as 32% from avalanche victims and 70% in patients with alcohol or other drug intoxication

  8. Pathophysiological progression • 32-37ºC: vasoconstriction, shivering, and nonshivering thermogenesis (increase in basal metabolic rate) • 24-32ºC: decreased basal metabolic rate, less shivering • <24º: autonomic and endocrinological mechanisms for heat generation fail

  9. Approach to accidental hypothermia Goal: optimize heart for ROSC

  10. Approach to accidental hypothermia Cardiac arrest (usually VF or asystole) Signs of life CPR Defibrillate if indicated Rewarming techniques Post-warming care Identify and treat underlying precipitants

  11. Hypothermia myths • Hypothermia ‘only occurs in winter’ • Hypothermia ‘only occurs in northern latitudes, and high altitude’ • Although less common, hypothermia does occur in summer months, and in warm climates

  12. Accidental hypothermia - etiology Wilderness setting Urban setting Homicide or suicide Trauma Cold exposure Alcohol and drugs of abuse Submersion Sepsis/ infection DKA Immersion Hypoglycemia Psychiatric illness Mountain Accidents Metabolic – thyroid, Addison’s

  13. Epidemiology of predisposing factors Danzl, Ann Emerg Med 1987

  14. Mortality Danzl, Ann Emerg Med 1987

  15. Factors that affect thermoregulation: • Decreased heat production • Increased heat loss • Impaired thermoregulation

  16. Decreased heat production • Endocrinologic failure – hypopituitarism, hypoadrenalism, hypothyroidism • Malnutrition and decreased subcutaneous fat • Neonates: less subcutaneous tissue, ineffective shivering mechanisms, lack behavioural defense mechanisms • Elderly: decreased ability to sense cold, decreased adaptive behavioural mechanisms

  17. Increased heat loss • Exposure during resuscitation • Cold saline infusions • Loss of skin barrier (eg burns, skin conditions) • Ethanol

  18. Impaired thermoregulation • Centrally mediated lesions (subdural hematoma, malignancies, CVAs) • Medications (antidepressants, antipsychotics, anxiolytics)

  19. Normal physiological response • Shivering can double heat production and increase basal metabolic rate by 2-5x • However, increased heat production can only last a few hours because of fatigue and glycogen depletion

  20. How is heat lost? • Radiation – transfer by electromagnetic waves • Conduction – transfer by direct physical contact • Convection – heat loss to air and water vapour molecules circulating around the body • Evaporation – conversion of liquid to gas (eg sweat on body) • Respiration

  21. Hemodynamic changes • Progressive bradycardia (usually refractory to atropine) • Progressive decreased mean arterial pressure • Decreased cardiac index

  22. ECG changes • Osborn (J) wave – may appear at any temperature under 32ºC • Increasing PR interval • Increasing QRS interval • Long QT interval • Artifact from shivering may obscure ECG http://en.wikipedia.org/wiki/Osborn_wave Graham 2001

  23. Dysrhythmias • Cardiac rhythm disturbances can be from many factors • Acid base disturbances • Hypoxia • Sensitization of conduction system • Decreased refractory period

  24. Dysrhythmias • Atrial fibrillation • Ventricular fibrillation • Can be related to physical jostling, autonomic dysfunction, hypoxia, acid-base disturbances • Decreased threshold for ventricular dysrhythmias • Asystole can happen spontaneously below 25ºC

  25. Mechanical trauma causing VF? • Anecdotal reports in humans but limited evidence • A 2007 study on induced hypothermia in pigs: • Once below 25°C, 5/10 pigs developed VF • Below 25°C, 2/10 pigs developed VF after mechanical trauma (dropping backboard from 6 and 12 inches) Grueskin 2007

  26. Dysrhythmias Graham 2001

  27. Renal effects • Cold diuresis: • Kidneys excrete dilute urine • Cold water immersion may increase urine output by 3.5 times Danzl 2010

  28. Coagulation effects • Impaired enzymatic activity of clotting cascade (from cold) • Increased fibrinolytic activity • Platelet sequestration • Also hypercoagulability and intravascular clotting Danzl 2010

  29. Laboratory features • Arterial blood gas can give a falsely elevated pO2 and pCO2 and falsely lower pH • Falsely elevated hematocrit as a result of decreased plasma volume Danzl 2010

  30. Lab features • Lab tests are done at 37ºC, so there may be a clinically evident coagulopathy, but normal coagulation studies • Can see leukopenia and thrombocytopenia • Lactate levels may be very high but not necessarily correlated with mortality Danzl 2010

  31. Hypothermia • Epidemiology and Definitions • Pathophysiology • Clinical features • General approach • Advanced cardiac life support • Rewarming

  32. What are some clinical features of hypothermia? • Confusion • Dizziness • Dyspnea • Uncoordination • Apathetic • Paradoxical undressing • Decreased level of consciousness Danzl 2010

  33. Hypothermia Swiss Staging System • Stage 1 - Conscious, shivering • Usually 32-35°C • Stage 2 - Impaired consciousness, not shivering • 28-32°C • Stage 3 - Unconscious, not shivering, vital signs present • 24-28°C • Stage 4 - No vital signs • < 24°C Brown 2012

  34. General Principles of Management • Prevent further cooling • Accurate temperature measurement • ABCs • Advanced cardiac life support • Rewarming techniques • Identify and treat precipitants

  35. Prehospital initial management • Remove all wet clothing • Rewarm patient with blankets and possibly reflective foil wrapping • Accurate core temperature measurement may be difficult in the field • Be careful moving patients – cold heart is prone to VF • Insulate patient from further heat loss

  36. When to withhold (not start) resuscitation • Obvious lethal injuries • Frozen head and mouth and blocked with ice • Frozen and CPR not possible Danzl 2010

  37. ABCs • Intubation • Theoretical risk of inducing VT - but this is not seen in prospective observational studies • May be very difficult if pt is extremely cold/rigid • Pulse check for 30-60s (pulse and respirations can be difficult to detect in the hypothermic patient) • If possible, give warmed humidified O2 during bag-mask-valve • CPR/ defibrillation

  38. Advanced cardiac life support • ‘The temperature at which defibrillation should occur, and the number of attempts has not been established’ Vanden Hoek 2010

  39. Advanced cardiac life support (ACLS) • ACLS medications may not work in the hypothermic heart • Drug metabolism is theoretically decreased and may build up to toxic levels Vanden Hoek 2010

  40. Advanced cardiac life support • Oral medications won’t work effectively because of decreased GI motility • Intramuscular medications won’t work effectively because of peripheral vasoconstriction Danzl 2010

  41. Medications • Most dysrhythmias convert with rewarming • Defibrillation attempts are usually ineffective below 28-30ºC Danzl 2010

  42. Dysrhythmias • Bradycardias – usually physiologic • Pacing not needed unless hemodynamic compromise persists after rewarming • Atrial fibrillation usually converts with warming • Ventricular fibrillation • Lidocaine and procainamide don’t work very well in hypothermia • Evidence is limited for medications in VF Vanden Hoek 2010 Soar 2010

  43. Current recommendations • For patients < 30°C, it is reasonable to shock if in VT/VF • Further shocks are ‘reasonable’ along with warming strategies • Current benefit of medications in humans under 30°C not known, although animal studies suggest a benefit Vanden Hoek 2010

  44. Comparing different guidelines • American Heart Association Guidelines • The value of more than 1 shock if < 30°C is not known • May be reasonable to perform further defibrillation attempts • European Resuscitation Council Guidelines • Consider 3 defibrillation attempts if < 30°C • If not successful, consider withholding further until temp > 30°C Brown 2012

  45. Overview of accidental hypothermia Cardiac arrest (usually VF or asystole) Signs of life Rewarming techniques CPR Defibrillate if indicated Rewarming techniques Goal: optimize heart for return of spontaneous circulation and normal sinus rhythm Post-warming care Identify and treat underlying precipitants

  46. IV fluids • Patients are usually hypovolemic and should receive warmed saline solution, heated to 40ºC • Can microwave IV bags – 2 mins on high power for 1L • There is significant loss of heat through IV tubing • Ringer’s lactate is inefficiently metabolized by the liver Danzl 2010

  47. Measuring temperature • Continuous core temperature is important (rectal temperature can lag behind core temperature changes, and difficult if ongoing CPR) • Tympanic temperature equilibrates most rapidly with core temperature, but is less accurate, and has more variability • Esophageal probe is ideal, but rarely available in the prehospital setting or low resource setting

  48. Core temperature afterdrop • Further drop in patient’s core temperature after removal from cold • Caused by countercooling of blood, and rewarming of extremities Danzl 2010

  49. Rewarming definitions • Passive external rewarming • Letting the patients warm themselves • Active external rewarming • Applying heat to the patient externally • Remove wet clothing • Apply dry blankets Conscious, shivering Impaired consciousness, unconscious, or no vital signs • Forced air heating systems or warm blankets Brown 2012

  50. Warm air device – ‘Bair Hugger’