Hypothermia



Background

Hypothermia = core temperature of <35°C

Heat loss through: Radiation, Conduction, Convection, Evaporation.

  • Hypothermia results in multiple organ failure. 
  • Shivering increases metabolic rate (2-5 times) but only while glycogen stores last and above 30°C.
  • Vasoconstriction fails at temperatures below 24°C
  • Tachycardia is followed by bradycardia, hypotension and fall in cardiac output.
  • Arrhythmias are common, may be refractory to treatment. Asystole and VF may occur spontaneously < 25°C.
  • Cerebrovascular autoregulation is lost at 24°C. Cerebral metabolism falls & EEG becomes flat at 20°C when the patient appears dead.
  • Resp rate stimulation is followed by depression as metabolism slows.
  • In severe hypothermia CO2 retention respiratory acidosis indicate disturbance of normal respiratory responses.
  • Renal blood flow falls. There may be a large diuresis due to initial vasoconstriction.

Aetiology/causes

General

  • Young and old
  • Systemic illness
  • Sepsis
  • Malnutrition

Environmental

  • Cold, wet, windy conditions
  • Cold water immersion
  • Exhaustion
  • Marathon runners

Trauma

  • Multiple trauma
  • Minor trauma, immobility (e.g. # NOF)
  • Major burns

Drugs

  • Ethanol
  • Sedatives (e.g. bdz, TCAs, opioids OD)
  • Phenothiazines (impaired shivering)

Neurological

  • CVA
  • Paraplegia
  • Parkinson's disease

Endocrine

  • Hypoglycaemia and diabetes
  • Hypothyroidism
    • Hypoadrenalism

Severity

35-32°C

Mild

Increased basal metabolic rate,  Maximum shivering thermogenesis

Amnesia, dysarthria, ataxia, apathy,  Maximum respiratory stimulation, tachycardia

Normal blood pressure

32-29°C

Moderate 

Stupor.  Shivering stops, muscular rigidity

Atrial fibrillation and other dysrhythmias,  Pulse and cardiac output 2/3 of normal = low BP

Insulin ineffective

Progressive loss of consciousness, pulse and respiration, pupils dilated at temperature

Susceptible to VF

O2 requirements 50% of normothermics at temperature 28°C

<29°C

rectal or oesophageal
low reading thermometer
to 15cm

Severe

Clinical signs of life may become almost undetectable.

Pupils fixed and dilated.

Loss of reflexes and voluntary motion (knee jerk the last reflex to be lost and first to return)

Major acid-base disturbance

Cerebral blood flow 1/3 normal and cardiac output (CO) 45% normal at 25°C

Risk of pulmonary oedema

Significant hypotension

Absent corneal and oculocephalic reflexes at 23°C

Maximum risk of VF at 22°C

Flat EEG at 19°C

Asystole at 18°C

Lowest successful survival from accidental hypothermia is 16°C

TM temperature is probably accurate when used in ED on patients who have cooled slowly

In the pre-hospital setting, moderate and severe hypothermia are grouped together as 'profound' hypothermia


Laboratory Investigations

J wave A

U&E

  • Hypo- or hyperkalaemia - rhabdomyolysis may occur.
  • Raised urea and creatinine

Glucose

  • Hypo- or hyperglycaemia

Hyperamylasaemia

  • Pancreatitis is common

CK

  • May be elevated

FBC

  • Increased haematocrit due to intravascular volume depletion
  • Thrombocytopaenia

Clotting

  • Coagulopathy including DIC is common
J Wave B

ABG

  • Hypothermia is protective against hypoxia by shifting 02 - Hb curve to the left
  • Initial respiratory alkalosis followed by a respiratory and metabolic acidosis
  • Use measured values not " corrected" for temperature

ECG

  • Bradycardia and AF (physiological arrhythmias of hypothermia)
  • Systole prolonged > diastole
  • Increased conduction time > relative refractory period predisposes to re-entrant arrhythmias
  • Prolonged PQ, QRS and QT intervals Osborne wave J wave) - most common in II and V3-V6 image

Management

General Measures

  • ABC
  • Intubation as required, as gently as possible
  • Warmed, humidified oxygen at 40-46 ° C
  • Remove wet clothing and insulate to prevent further heat loss
  • Gentle handling at all times - may precipitate dysrhythmias
  • Consider co-existent pathology
  • IV, Urinary and gastric catheters if necessary
  • Temperature and cardiac monitoring
  • Warmed Fluid resuscitation (Dehydration is frequently present)
  • Dextrose containing fluids will also provide energy substrate
  • Avoid Ringer's lactate
  • Avoid central lines and Swan Ganz catheters

Endogenous Rewarming

  • Ideal for mild hypothermia
  • A part of all rewarming protocols
  • Requires some endogenous thermogenesis
  • Warm environment, warm clothing and insulation

External exogenous warming

  • External application of heat
  • Required at temperatures <32 ° C
  • Purported link with peripheral vasodilation, hypotension and core temperature after- drop
  • Immersion is not recommended
  • Forced air warming blanket (Bair Hugger)
  • Should achieve warming rate of 2 ° per hour

Core Exogenous Rewarming

  • Application of heat to the core
  • Warmed humidified inhaled oxygen
  • Warmed IV fluids provide little heat but prevent ongoing loss
  • Insulate IV lines
  • Blood warmer for blood and IV fluids
  • Cardiopulmonary bypass or left pleural lavage is life saving measure in arrested hypothermics
  • Most other methods are less effective

Frostbite and other injuries may require treatment.


Arrhythmias

  • VF may occur spontaneously or be precipitated by rough handling with temperatures <29° C
  • Sinus bradycardia and AF with slow ventricular response are common and should be regarded as physiological arrhythmias of hypothermia
  • AF usually reverts spontaneously during rewarming  (Other atrial arrhythmias should be regarded as innocent)
  • Transient ventricular arrhythmias should be ignored
  • Magnesium may be effective   (Lignocaine appears to be less effective, Procainamide should be avoided)

CPR in hypothermia

  • Misdiagnosis of cardiac arrest is a hazard in the pre-hospital setting
  • Chest wall elasticity and myocardial compliance are reduced and may make chest compression difficult
  • CPR rate as for normothermic victims
  • Drug metabolism is reduced and accumulation 
    • Epinephrine and other drugs are often withheld until core body temperature is >30°C
    • Above 30°C intervals between doses should be doubled and the lowest doses recommended used
  • Ventricular fibrillation may not respond to defibrillation if the core temperature is <30°C
    • If no response to 3 initial shocks, subsequent shocks should be delayed until the core temperature >30°c
  • Initiate CPR in accidental hypothermia unless Guidelines of the Wilderness Medical Society, USA)
    • Not for resuscitation status is documented and verified
    • Obvious lethal injuries are present
    • Chest wall decompression is impossible
    • Any signs of life are present
    • Rescuers are endangered by evacuation delays or altered triage conditions
  • Bypass is better that external rewarming in hypothermic cardiac arrest. [Bestbets]

Prognostic Factors in Hypothermia

No strong indicators to predict death or permanent neurological dysfunction in patients with significant hypothermia
No definitive indicators to suggest which patients can or cannot be resuscitated successfully

Parameters that may identify the non-salvageable patient

  • Elevated serum potassium >10 mmol/l
  • Core temperature <6-7°C
  • Core temperature <15°C if there has been no circulation for >2 hours
  • Venous pH <6.5 - blood gas measurements should not be corrected for temperature
  • Severe coagulopathy
  • Clots within the heart on thoracotomy
  • Failure to obtain venous return during cardiopulmonary bypass


Content by Dr Jon Dallimore, Dr Íomhar O' Sullivan 10/02/2003. Reviewed by Dr ÍOS 20/02/2004, 15/05/2005, 28/04/2007. Last review Dr IOS 29/11/19.