Hypothermia- Definition, Types and the Predisposing Factors by Valerie

Hello, my dear group G members. Here I've included a summary of my part on hypothermia, which is the definition and predisposing factors. Enjoy!

DEFINITION

• Hypothermia is defined as a reduction of the core body temperature to 35'C.
• Shifts the oxygen-dissociation curve to the left, decreasing oxygen release from hemoglobin to tissues
• The normal range of body temperature is 36.2- 37.2'C. Body temperature is regulated by the preoptic interior hypothalamus in the brain.
• The basis of thermoregulation- It is activated by cold exposure, producing a reflex of vasoconstriction and stimulating the hypothalamic nuclei in the brain.

• When heat loss exceeds heat production, the brain tries to compensate for the disparity in a number of ways:

Heat production mechanisms-

• shivering -involuntary muscle contraction speeds up metabolism to produce heat
• increasing tyrosine and epinephrine production
• autonomic and endocrinologic responses
• adaptive behavioral responses

Heat conservation mechanisms- Peripheral vasoconstriction and behavior responses

• However, as the body remains exposed to cold, shivering stops, oxygen consumption declines and the heartbeat becomes irregular as hypothermia takes its toll.

MAIN CAUSES

Increased heat loss
Conduction: Direct contact with surfaces such as wet clothing or cold sleeping surface

Convection: Heat transfers by motion such as wind blowing layers of warm air on skin

Radiation: Temperature gradient between body and air

Wind chill: Effective temperature on wet skin

Wet chill: Heat loss to wet skin

Respiration

Evaporation

Decreased heat production

Due to hypopituitarism, hypoadrenalism, hypothyroidism, malnutrition and hypoglycemia.

Impaired thermo regulation

Hypothalamus unable to regulate body temperature.

Hypothermic patients have these criteria in common:
Exposure to windy and wet weather with moderate to cold temperatures

Moisture- perspiration

Poor clothing choices- cotton, no hat

Dehydrated, have not had much to eat and lethargic

STAGES OF HYPOTHERMIA- according to core temperature

Mild (32-35'C)

Generalized increase in muscle tone and shivering to increase heat production.

Pulse rate increases, causes lethargy, ataxia (failure of muscular coordination), impaired coordination, apathy and tachycardia.

Peripheral vasoconstriction- minimize heat loss from skin

Moderate (29-32'C)

Level of consciousness is progressively reduced, the body slows down, shivering stops.

Severe (below 29'C)
Inactivity of endocrine and autonomic nervous system, reduced cerebral blood flow, lower metabolism and oxygen utilization.

Insulin loses effectiveness- leads to hyperglycemia.

TYPES OF HYPOTHERMIA

It is classified as accidental or intentional, primary or secondary, and by the degree of hypothermia.

Accidental

Unanticipated exposure in an inadequately prepared person (Examples: inadequate shelter for a homeless person, a person caught in a winter storm, or in Sima's case- a person lost in a jungle terrain.

Intentional

Therapeutic hypothermia where the patient is in an induced state for the purpose of neuroprotection usually after cardiac arrest.

Primary

Due to environmental exposure- with no underlying medical condition causing disruption of temperature regulation.

Secondary

Low body temperature due to a medical illness impairing the heat-regulation mechanism.

Examples: Diabetes, strokes, certain medications

More common among the elderly ( Initiation of vasoconstriction and shivering- slower)

PREDISPOSING FACTORS

The elderly

Due to neuromuscular inefficiency, less physical activity, and impaired shivering mechanism. Thus, they are less adept at increasing heat production and respiratory quotient when needed.

Lack of cold awareness

Impaired thermal perception and blood flow, poor adaptive behavior.

Malnourished

Less insulative subcutaneous fat layer, altered thermoregulation and poor nutrition- less fuel reserves.

Endocrinologic deficiency

Hyperthyroidism, hypoglycemia, adrenal insufficiency, hypopituitarism.

Affects heat production mechanisms.

Alcohol
Increase heat loss, impairs thermoregulation mechanisms by interacting with their respective neurotransmitters.

Cause cutaneous vasodilation- radiative heat loss.

Impair shivering mechanism.

Neuropathy and diabetes mellitus
More common in older adult diabetics

Abnormal plasma osmolality interferes with hypothalamic function.

Pharmacologic factors
Several medication- impair centrally mediated thermoregulation and vasoconstriction

Examples: Barbiturates, tricyclic antidepressants, phenothiazines and benzodiazepines (suppresses shivering)

Lithium- decreases core temperature

Prescribed pain killers- narcotics- dulls perception and decision making abilities.

Anti hypertensives cause vasodilation of capillary beds at dermis layer

Pathologic lesions
Neoplasms, congenital anomalies, Parkinson's disease

Causes neurologic impairment- behavior risks

Wet clothing
Wind and water significantly reduces material's insulating capacity

Cotton clothing absorbs perspiration, keeps it close to the body, causing further heat loss.

90% Heat loss from head unless person is wearing a cap/ hat.

Physical trauma
Trauma leads to- arrhythmias, (heart), acidosis and coagulopathies.

Hypotension (from hemorrhage) and hypovolemia may jeopardize thermostability.