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.

a)Talking b)RR 16, oximeter won’t pick up c)HR 64 regular, BP 104/78 d)E4 M6 V5 PERL e)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.

a)Talking b)RR 12/min, oximeter will not register c)HR about 50 irreg irreg, BP 104/64 d)E2 M5 V3 PERL e)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)

Risk Factors for Decreased Thermostability

It does not have to be freezing for hypothermia to develop

Mechanisms of heat loss

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

Physiology

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!DANGER! — Shivering usually stops once the core temperature drops below 30º - 32ºC

Clinical features

The "umbles" stumbles mumbles fumbles grumbles

Physiologic Changes Associated with Hypothermia

Alternative classification I.Clear consciousness with shivering II.Impaired consciousness and/or no shivering III.Unconscious or cardiovascular instability IV.Apparently dead V.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

Management

ABCDE approach (as always)

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, PaCO 2 & ET CO 2. Changes in PaCO 2 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 Rapid / ModerateSlow External Hot bath*Bear hugger ‡ Body to body* Core Moderate: IV fluids (65°), lavage stomach, peritoneum or bladder ‡ Rapid: lavage thorax ‡, CABG IV fluids ‡ (45°) Humidified +/- heated oxygen ‡ * 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)

Slow rewarming kcal/h heat, corresponding to increasing temperature by °C/h AER: warmed blankets (0.9°C/h) ACR: IV solutions heated to 45°C (17 kcal/h); heated, humidified oxygen by mask (30 kcal/h or 0.7°C/h); and heated, humidified oxygen via endotracheal tube (1.2°C/h) If intact, a patient's endogenous physiologic mechanisms (other than shivering) provide similar rates of rewarming (30 kcal/h)

Moderate & rapid rewarming Moderate methods rewarm at approximately 1 - 3°C/h; rapid at > 5°C/h Moderate: warmed gastric lavage (2.8°C/h), IV solutions heated to 65°C (2.9°C/h), and peritoneal lavage with 45°C fluid at 4 L/h (70 kcal/h or 3.0°C/h) (watch for K drop), bladder irrigation Rapid: thoracic lavage at 500 mL/min (6.1°C/h) – right sided, CAVR, cardiopulmonary bypass (400 kcal/h or 18.0°C/h), thoracic lavage at 2 L/min (19.7°C/h), thoracotomy and direct cardiac lavage and warm-water immersion (1500 kcal/h) Endogenous shivering provides rewarming at a rate of 300 kcal/h

Active external rewarming Application of warmth to the surface of the body. Heated blanket, immersion in warm water, bear hugger, heated saline bags in groin or axilla, hot water bottle. What problems might AER induce? Drop in core temperature if too much of the shell (i.e. limbs) are externally warmed.

Active external rewarming AER should be applied to thorax only – take particular care not to warm hands and feet AER should be (usually) applied in combination with active core rewarming

Active core rewarming Humidified and heated oxygen Irrigation of stomach, bladder, pleural space, peritoneal space, CAVR, bypass Warm IV fluids

Active core rewarming How long does it take to microwave 1L saline to 42 C? Depends on microwave power and room temperature - approx. 135 seconds for 650 Watt microwave How much difference would 1L of warm saline make to temp in a patient with temp of 28 C? Increased temp by 0.3 deg C

Active core rewarming Who needs bypass? Cardiac arrest Sudden severe hypothermia (e.g. immersion) Profound hypothermia with deterioration

Cardiac arrest in hypothermia Pulse may be very slow and very weak – use Doppler and assess for 1 minute before declaring pulseless No chest compressions if ANY sign of life Defibrillation may not be successful in profound hypothermia: 1 shock only until temp > 32 Withhold adrenaline until temp > 30 then give every 8 minutes ? Bretyllium has a role (if available); lignocaine not useful; amiodarone less effective in hypothermia