Hypothermia Dr R D Hardern
Scenario 1 A 19 year old student is rescued from the Wear; he was in the water for 25 minutes after slipping on ice on the riverside footpath. He was unable to get out himself because of steep banks where he fell in so clung to a tree root.
Talking • RR 16, oximeter won’t pick up • HR 64 regular, BP 104/78 • E4 M6 V5 PERL • TM temperature 31.3° C How will you rewarm this patient?
Scenario 2 84 year old lady found on floor when neighbour when to investigate why not seen for 2 days. Old notes include admissions for angina.
Talking • RR 12/min, oximeter will not register • HR about 50 irreg irreg, BP 104/64 • E2 M5 V3 PERL • TM temp 32.3°C, right leg short and rotated • Is she sicker than the previous patient? • If so how? • How will you management differ? • If she arrests, will ALS differ from usual?
Classification of hypothermia • Primary: casualty has normal thermoregulatory response but these are overwhelmed by environment • Secondary: casualty has impaired thermoregulation (so greater problems for given environment) • Iatrogenic: e.g. for CABG
Contents • Definition • Factors leading to hypothermia • Physiology of response to cold • Clinical features • Management
Definition • Core temperature below 35° C • Tympanic thermometers are not accurate (despite what BMJ article says)
Convection • Fluid with a lower temperature than the body contacts the skin and then moves on • You use convection when you blow on hot food or liquids to cool them • Amount of heat lost depends on the temperature difference between your body and the environment, plus the speed with which the air or water is moving • Air in motion takes away a LOT of heat • With air in motion, the amount of heat lost increases as a square of the wind’s speed
Conduction • Transfer of heat away from the body to objects or substances it comes into contact with • Air conducts heat poorly — still air is an excellent insulator • Water conductivity is much greater than that of dry air
Evaporation • Responsible for 20% - 30% of heat loss in temperate conditions • Wet clothing enhances heat loss • Respiratory heat loss can be significant
Radiation • Direct emission of heat • Heat radiates from the body to the clothing, then from the clothing to the environment • The greater the difference between body and environmental temperatures, the greater the rate of heat loss
Energy • Heat is generated in the muscles and by metabolic chemical reactions (mainly in the liver) • With rapid cooling (e.g. immersion) energy reserves are relatively intact and casualty will often rewarm once removed from cold • With less severe cold, body temp only falls when energy reserves are depleted (e.g. elderly in cold house); worse prognosis
Fluid balance • Cold induced vasoconstriction increases fluid kidneys “see”; they respond with diuresis • Cold reduces tubules’ ability to reabsorb water and reduces sensitivity to ADH; further increases diuresis • Fluid shifts from intravascular → extravascular → intracellular space; this reverses on rewarming so intravascular space increases by up to 30% above normothermic volume
Vascular responses • Cold induced vasoconstriction (CIVC) – cool outer shell, warmer core protected • Removal from cold relaxes CIVC; vascular volume increases • Removal from water relaxes “squeeze”; vascular volume increases but fluid volume does not – “distributive shock” • Distal half of limbs may show CIVD – Hunter reflex. Reduce risk of cold injury but increase rate of heat loss
Speed of cooling • Acute – typically immersion. • Energy reserves intact, very little fluid shift • Rewarm if can be removed from cold • Subacute (exhaustion) – e.g. lost hill walker • Cold is less severe, hypothermia does not develop until energy reserves deplete • Spontaneous rewarming less likely to be effective
Speed of cooling • Subchronic (urban) • Cold is not extreme but is prolonged • Fluid shifts extensive • Loss from vascular space may have been replaced with oral intake • Rapid rewarming may reverse fluid shifts more quickly than kidneys can manage, resulting in pulmonary oedema
Shivering • Involuntary shivering begins in response to a drop in the body core temperature • Can last for hours if sufficient energy reserves • Heat production roughly equal to that of a brisk walking pace: 5 times resting metabolic rate • DANGER!— Shivering usually stops once the core temperature drops below 30º - 32ºC
The "umbles" • stumbles • mumbles • fumbles • grumbles
Alternative classification • Clear consciousness with shivering • Impaired consciousness and/or no shivering • Unconscious or cardiovascular instability • Apparently dead • Death due to irreversible hypothermia
II • Lethargy • Loss of fine motor coordination • Cold diuresis • As the temperature drops below 34°C, a patient may develop altered judgment, amnesia, and dysarthria. • Respiratory rate may increase • At 33°C, ataxia and apathy are seen typically. • Patients generally are stable haemodynamically and able to compensate for the symptoms.
III • Oxygen consumption decreases, and the CNS depresses further • Patients become at risk for significant arrhythmias.
IV • Very cold skin • Unresponsive • At 28°C, the body is markedly susceptible to ventricular fibrillation and further depression of myocardial contractility • Rigidity • Apnoea • No pulse • Areflexia • Unresponsiveness • Fixed pupils
Airway • Must be patent • Be gentle with airway manoeuvres • If comatose consider cuffed tracheal tube (and gastric tube); despite potential risk of precipitating VF (risks overstated) the indications for intubation do not alter in hypothermia • Humidified & warmed air/oxygen is ideal
Breathing • Warm humidified oxygen if available • Oxygen if at all possible – ODC shifts to left • If very slow / shallow breaths assist ventilations; avoid hyperventilation – monitor pH, PaCO2 & ET CO2. • Changes in PaCO2 have a greater effect on pH in hypothermia • Pulse oximeter unreliable when skin shut down • ABGs – don’t correct for temperature
Circulation • Fluids should be warm (41) – microwave can be used for saline • But heat in the fluids will NOT be enough to rewarm the patient • Saline or isotonic dextrose • 10ml/kg bolus • 5ml/kg/hour infusion • Don’t overload (consider CVP line if cardiopulmonary comorbidity) • e.g 700ml saline in first hour then 350ml/hour • Avoid Hartmann’s – cold liver cannot metabolise lactate
Circulation • Monitor fluid status carefully • Invasive monitoring if fluid shifts likely or if pre-existing cardiorespiratory problems • Supplemental K may be needed – check U&Es • Inotropes usually avoided; try to avoid catecholamines in patient with frostbite
Disability • If GCS down (and it will be in profound hypothermia) rule out / treat hypoglycaemia • In acute hypothermia glucose not low, if hypothermia occurs after exhaustion of energy hypoglycaemia will be present
Other things to consider • D also stands for “degrees” • Measure core temperature • Are they shivering? • E stands for exposure • Remove cold wet clothes • But expose only one bit at a time and keep covered up as much as possible • Keep the room as warm as possible • K stands for potassium • Often low
Rewarming in hypothermia • What types of rewarming can be used? • When should each type of rewarming be used?
Methods of rewarming • Passive • Active
Passive external rewarming • Often sufficient for stage I hypothermia may work for stage II • Remove cold / damp clothes • Wrap in warm blankets (including head) • Nurse in warm environment • Provide hot SWEET drink if airway safe • Keep feet and hands cool (to maximise stimulus to produce heat) • Consider HME filter to reduce respiratory heat loss
Passive external rewarming • How hot should the room be in which a hypothermic patient is managed? • > 21 C (and minimise air flow) • What reflex needs to be preserved if PER is to be used? • Shivering • What is silver foil useful for? • Maintaining temperature
Active warming What are the indications for active warming? • Cardiovascular instability • Stages III and IV (? Stage II) • Failure of PER to rewarm • Impaired thermoregulation (drugs, toxins, CNS problem, diabetes, acute cord lesion) • Decreased heat production (endocrine insufficiency, malnutrition/chronic liver disease, hypoglycaemia, impaired shivering: trauma or drugs)
Active rewarming • How is active warming subdivided?
Active rewarming * not used in hospital; ‡methods of choice at UHND
Fast or slow rewarming? • Slow if greater fluid shifts expected; cautious rewarming and close monitoring • Fast if little fluid shift expected or if arrested • Hct may provide guide to fluid shifts (rises as temp decreases because of decreased blood volume)