hypothermia

HypothermiaDavid Caro, MD

University of Florida

Emergency Medicine

Introduction

Definition Etiology Epidemiology Physiology Impact on organ systems

Introduction (cont’d)

Response of organ systems Identification Treatment modalities Sequelae Review

Physiology

The hypothalamus is your temperature control center

Core and peripheral receptors provide temperature information

Various responses help maintain temperature homeostasis Cut heat loss; generate heat!

Physiology – Heat Generation Basal Metabolic Rate = 40-60

Kcal/m2

Movement, shivering can increase the BMR 2 to 5 x normal

Physiology: The physics of temperature homeostasis 4 ways heat is lost

Radiation (65%) Evaporation (25%) Convection (5-10%) Conduction (2-5%) (except in

immersion; cold water conductivity 32 times that of air)

Physiology

Our body responds to decreased temperature by attempting to: Decrease heat loss

Vasoconstriction Behavior modification

Increase heat production Shivering

Am J Physiol 1997 Feb;272(2 Pt 2):R557-62

Hypothermia-Definition

A core body temperature below 96.8oF

Mild hypothermia = 92 – 96.8oF Moderate = 88 – 92oF Severe = < 88oF “They’re not dead until they’re

warm and dead.” One case report of recovery after accidental 15.2oC (59.4F) hypothermia.

Hypothermia – Morbidity/Mortality Mild hypothermia – very little Moderate – one study showed a

mortality rate of 21% for moderate hypothermia

Clinical features

Mild: shivering, loss of fine motor control

Moderate: lethargy, confusion Severe: pupillary dilation, coma,

cardiovascular irritability and eventual collapse

HypothermiaEtiology Overview

Increase in heat loss Decrease in heat production Thermoregulatory failure

EtiologyIncreased Heat Loss Exposure to ambient temperature

drop Trauma Alcohol/drugs

Cutaneous vasodilation, impaired shivering, decreased awareness of environment , Wernicke’s encephalopathy (Eur J Appl Physiol 1996;74(3):293-5. Also, Ann Neurol 1981 Oct; 10(4):384-7)

Etiology Increased Heat Loss Exposure (cont’d)

Sports Altitude

Etiology Increased Heat Loss Exposure (cont’d)

Immersion Significant risk begins in water colder than

25 C (72 F). The water temperature of Lake Huron is approximately 4.6 C (40 F) in May and is highest in August at 19 C (66 F).

Hypothermia can occur rapidly during cold-water immersion (one hour or less when water temperature is below 45oF).

Core temp drops 0.3C in 20 min when immersed in 22C (Am J Phys Med Rehabil 1999 Jan-Feb; 78(1):33-8.

EtiologyDecreased Heat Production

Acute Illness Metabolic abnormalities

DKA Hypoglycemia Myxedema coma Hypoadrenalism

EtiologyImpaired Thermoregulation

Sepsis lymphocyte activation IL-1,2,6 (Anesthesiology 1998

Nov; 89(5):1133-40. J Neurotrauma 1999 Mar; 16(3): 225-32)

CVA Autonomic dysfunction

Impact on Organ Systems Respiratory

Altitude will decrease pO2 Temperature of inspired air

decreased Air warmed + humidified by

oral/nasal mucosa prior to hitting lower airways

Expired air rich in warm water – heat lost

Impact on organ systems Hemoglobin increasingly binds O2

as temperature drops (oxyhemoglobin dissociation curve)

Hypoxia may occur due to decrease offloading of oxygen (Cardiovasc Surg 1999 Jun;7(4):425-31

Organ System Response

Acid-base balance 11/18 with decompensated

metabolic acidosis (Coll Antropol 1999 Dec;23(2):683-90

Most texts recommend not correcting for temperature when evaluating ABGs

Impact on Organ Systems Cardiac

Repolarization becomes abnormal with decreasing temperature

Osborn Wave – lead V3 or V4 Increasing cardiac irritability with

decreasing temperature QT prolongation (0.45-0.688 vs.

0.343-0.444; Coll Antropol 1999 Dec; 23(2):683-90)

Osborn Wave

86% of hypothermic patients (Acad Emerg Med 1999 Nov; 6(11):1121-6)

Voltage gradient due to action potential notch in epicardium; epicardium activated later, which manifests as notching or J-point elevation (Circulation 1996 Jan 15;93(2):372-9)

Impact on Organ systems Cardiac

Mild: Tachycardia, hypertension, increased CO

Moderate: Bradycardia, Arrhythmias

Severe: Arrhythmias, hypotension, decreased cardiac output

Below 30C, ventricular fibrillation risk increases

Impact on Organ Systems Neurologic

Decreasing metabolic activity of neurons; therefore, decreased O2 requirement

Linear decrease in CNS function as temperature decreases

Neuron function stops below 20C

Organ System Response

Neurologic Decline in mental status

Mild confusion Delirium Coma

Peripheral anesthesia Ataxia

Impact on Organ Systems Renal

Hypothermia impairs renal concentrating abilities

“Cold-induced diuresis” Potential rhabdomyolysis ATN

Impact on Organ Systems Gastrointestinal

Pancreatitis ± pancreatic necrosis can develop due to HT

Impact on Organ Systems Hematologic

Hemoconcentration Increased blood viscosity Decreased flow in capillaries Potential for thrombosis Potential for DIC

Impact on Organ Systems Musculoskeletal

Temperature extremes can cause crystallization of blood in capillaries of extremities

Cutaneous vasoconstriction occurs in response to lower ambient temperature

Organ System Response

Musculoskeletal Frostbite Gangrene

Frostbite

Grade as burns 1st degree

Erythema, edema, burning Swelling for ten days or more Desquamation Parathesias, aching, and necrosis

of the pressure points of the foot Increased sensitivity to cold,

hyperhydrosis

Frostbite

Grading Second degree

progresses to blister formation, anesthesia, and deep color change

Frostbite

Third degree involves full skin thickness and

extends into the subcutaneous tissue

Subfascial pressure increases; compartment syndromes are common

Frostbite

4th degree Destruction of entire thickness Cyanotic, insensitive; hemorrhagic

blister formation. Severe pain on rewarming Dry gangrene can progress quickly

with mummification. The line of demarcation becomes

obvious at 20-36 days and extends into the bone in 60 or more days.

Trenchfoot

Caused by prolonged exposure of the feet to cool, wet conditions.

The skin is initially reddened with numbness, tingling pain, and itching then becomes pale and mottled and finally dark purple, grey or blue.

If circulation is impaired for more than 6 hours there will be permanent damage to tissue.

If circulation is impaired for more than 24 hours the victim may lose the entire foot.

Hypothermia Identification Thermometry

Most thermometers’ lower temperature limit is 93oF

A special low-temperature-reading thermometer is necessary to read temperatures lower than 93

Evaluation

ABCDEs are the priority Handle patients gently Begin passive rewarming

immediately Cautious ACLS care (coming up)

Evaluation

History is essential Environment/exposure PMH Medications

Exam – be complete! Rectal temperature! Vital signs

Evaluation

Head-to-toe secondary exam Neuro exam important – especially

cranial nerves (Wernicke’s) CV exam Extremities/nose/ears/other end-

arterial places

Evaluation

Testing Cardiac monitor, EKG SaO2, ±ABG Electrolytes, CBC UA

If severe: LFTS, PT/PTT, CK (rhabdo)

Treatment

General Rx for various degrees of hypothermia

Specific Rx for sequelae CV Respiratory ATN/Rhabdo Frostbite/gangrene

Treatment Modalities – Mild hypothermia Warm room Cover with dry, warm blankets Radiant warming Warmed p.o. fluids

Treatment Modalities – moderate hypothermia ABCs – every patient

Airway, Breathing – warm, humidified air by ETT or NRBfm

Circulation – IV access; warmed crystalloid

All of the above Bear Hugger

Treatment Modalities – Severe Hypothermia All of the above Invasive modalities

NG, foley lavage Pleural, peritoneal lavage Dialysis or Cardiac bypass

Treatment Modalities How effective are they?

Reflective Foil – 0.3C/hr Ann Emerg Med 2000 Apr; 35(40):337-45

Warmed IVF – 1.0C/hr J Clin Anesth 1998 Aug;10(5):380-5.

Warm IVF, Warm/humdified oxygen, blankets – 1.4 C/hr Ann Emerg Med 1996 Apr;27(4):479-84

Bear-Hugger – 0.7C/hr Ann Emerg Med 2000 Apr; 35(40):337-45; IVF/humidified O2/BH – 2.4C/hr Ann Emerg Med 1996 Apr 27(4):479-84

Treatment Ideas

Aviat Space Environ Med 1992 Dec;63(12):1070-6 Total immersion in 42C bath – 10.2C/hr Blankets – 0.2C/hr

J Appl Physiol 1998 Nov;85(5):867-8 Subatmospheric pressure to limbs while

applying warm-water blanket increased rewarming 10-fold over WWB alone (13.6C/hr vs. 1.4C/hr)

Treatment modalities-Sequlae Ventricular Fibrillation; MI Renal Failure DIC Frostbite Gangrene Afterdrop

Afterdrop

Paradoxical drop in core temp during rewarming

Due to influx of cold blood from periphery

Can precipitate arrhythmias

Treatment of sequelae

Ventricular fibrillation Cold heart very irritable Will not respond to multiple rounds

of drugs Shock – 3 times, then wait until

warm Bretylium your drug of choice

(ACLS Guidelines)

Treatment of sequelae

Renal Failure Rhabdomyolysis : force fluids;

alkalinization Cold-diuresis : fluids, watch

electrolytes

Treatment of sequelae

Frostbite Narcotics! Warm water immersion – warm,

wet heat is best. Do NOT warm then allow to

refreeze. Better to keep frozen until definitive care is available.

Treatment of sequelae - Afterdrop Try to avoid – aggressive

rewarming Expect arrhythmias, be prepared to

treat

HypothermiaSummary Physiology plays a HUGE role

Etiology Treatment

History is key Rectal temp with low-reading

thermometer Treat temperature aggressively,

but handle patient gently Watch for afterdrop!

Text References

Ann Emerg Med 1993 Feb;22(2 Pt 2):370-7

Wilderness Medicine – Auerbach Rosen’s Principles of

Emergency Medicine