Page 1843 - Hall et al (2015) Principles of Critical Care-McGraw-Hill

1312 PART 11: Special Problems in Critical Care

factors, clinical diagnosis, and management of hypothermia. The use TABLE 131-1 Causes of Hypothermia
of hypothermia as a treatment, such as after cardiac arrest, is covered
in Chap. 26. Mechanism Clinical Disorder
Decreased heat production Endocrinologic failure
REGULATION OF BODY TEMPERATURE Hypopituitarism
Body temperature is closely regulated through a balance between heat Hypoadrenalism
production and heat dissipation. The majority of endogenous heat pro- Hypothyroidism
2
duction results from metabolic activity in the heart and liver. The skin
accounts for 90% of heat loss and the lungs contribute the rest. Radiation Insufficient fuel
cooling (heat loss in the form of infrared radiation) is the primary Extreme exertion
method of thermal load dissipation and accounts for approximately 60% Hypoglycemia
of heat loss. Conduction (direct transfer of heat to a cooler object) and
convection (heat removal by air currents) account for 10% to 15% of Malnutrition
heat loss while evaporation from skin and the respiratory tract accounts Neuromuscular inefficiency
for 25% to 30% of heat loss. Conduction is an important mechanism of Age extremes
heat loss in immersion accidents because thermal conduction of water Impaired shivering
is approximately 30 times that of air. Convection is important in windy
conditions by removing the warm insulating layer of air around the Inactivity
body. The preoptic nucleus of the anterior hypothalamus is the thermal Increased heat loss Environmental exposure
control center, which maintains body temperature at a given set value. Induced vasodilation
In response to a decrease in core body temperature, the hypothalamus
initiates mechanisms to conserve heat by cutaneous vasoconstriction Drugs, including alcohol
and produce heat by stimulation of muscular activity in the form of Toxins
shivering. Nonshivering thermogenesis occurs via increased activity Skin disorders
2,3
of thyroxine and catecholamines. In a conscious individual, the appre-
4,5
ciation of cold induces the individual to exercise, wear more clothes, or Burns
move to a warmer environment. Psoriasis
As the core temperature decreases below 35°C (95°F), the coordi- Exfoliative dermatitis
nated systems responsible for thermoregulation begin to fail because the
physiologic responses to minimize heat loss are very limited. Primary Iatrogenic
6
hypothermia (accidental hypothermia) refers to a spontaneous decrease Cold infusion
of core body temperature, usually as result of exposure to cold envi- Emergent deliveries
ronments without adequate protection. Environmental hypothermia
results from a combination of heat loss by convection (degree of wind Impaired thermoregulation Peripheral failure
exposure), conduction, and radiation to the surrounding ambient air. Neuropathies
Secondary hypothermia represents a complication of an underlying Spinal cord transection
disorder. Some of the disorders and conditions that may predispose an Diabetes
individual to hypothermia by decreasing heat production, increasing
heat loss, or interfering with the central or peripheral control of thermo- Central failure
regulation are listed in Table 131-1. 1-3,6-10 Metabolic
Drugs: barbiturates, tricyclic antidepressants, sedatives, alcohol
EPIDEMIOLOGY Trauma

From 1979 through 2002, 16,555 deaths attributed to excessive natural Cerebrovascular accident (CVA), subarachnoid bleed
cold were reported in the United States (an annual average of 689 deaths). Parkinsonism
11
Approximately half of the deaths from hypothermia occur in persons
older than 65 years and males account for the majority of deaths. In Hypothalamic dysfunction
addition, for persons older than 65 years, the death rate for men and Multiple sclerosis
women of black and other races was much higher than that for white Anorexia nervosa
men and women. Race-specific differences may reflect differences in
12
socioeconomic determinants for factors that are important in the pre- Miscellaneous Sepsis
vention of hypothermia, such as access to protective clothing, shelter, Pancreatitis
and medical care. Hypothermia may occur in any climate and during Carcinomatosis
any season of the year. Hypothermia is most common among the elderly,
the homeless or mentally ill, trauma victims, outdoor workers, children, Uremia
and individuals with certain medical conditions as mentioned above. Vascular insufficiency

CLINICAL PRESENTATION thermometer capable of measuring temperatures as low as 25°C (77°F).
■ DIAGNOSIS Bladder catheters with thermistors provide readings similar to intra-
vascular devices. A rectal thermistor probe is often most practical even
When exposure to cold is obvious by history, the diagnosis is simple. though measurements may lag behind core changes. The probe should
Measurement of core body temperature is important in diagnosis of be inserted to an adequate depth but avoid cold fecal material. An
less overt presentations and in determining the severity of hypothermia. esophageal probe is an alternative, but measurements may be falsely ele-
Standard thermometers do not measure temperatures below 35°C vated in an intubated patient receiving heated oxygen. The reliability of
(95°F) and core temperature is best measured by using a cold-recording tympanic temperature devices has not been established in hypothermia. 7

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