hypothermia
TRANSCRIPT
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