Chapter 169

Hypothermia is defined as a core body temperature below 35 °C. The normal physiologic thermoregulatory responses start to fail once the core body temperature reaches this level, leading to the body’s inability to generate enough heat to maintain bodily functions. Hypothermia can be subdivided into primary and secondary hypothermia.1 Primary accidental hypothermia occurs when a previously normal individual is subjected to an environmental stress. Secondary accidental hypothermia occurs when a predisposing factor leads to disruption of temperature homeostasis and increases the individual’s susceptibility to lesser environmental stresses (e.g., drug intoxication, trauma, and endocrine disorders). Drug intoxication and trauma are acquired conditions that are highly associated with the development of hypothermia.1–6

Hypothermia can progress after arrival in the Emergency Department. Studies in the trauma population have reported a significant percentage of patients to have a decrease in their core body temperature during their stay in the Emergency Department.2 This recognition has led to the development of multidisciplinary approaches to maintaining normothermia as the patient moves through the hospital.3The importance of continuity and communication in dealing with this problem cannot be overstated.

Hypothermia can be characterized as mild, moderate, and severe. Mild hypothermia is defined as a core temperature of 32.2 to 35 °C or 90 to 95 °F. Moderate hypothermia is defined as a core temperature of 28 to 32 °C or 82.4 to 90 °F. Severe hypothermia is defined as a core temperature of less than 28 °C or 82.4 °F. The scale can be amended in the trauma patient to 34 to 36 °C or 93.2 to 96.8 °F for mild hypothermia, 32 to 34 °C or 89.6 to 93.2 °F for moderate hypothermia, and < 32 °C or < 89.6 °F for severe hypothermia.1

Mild hypothermia causes the body to increase heat production by shivering and increasing its metabolic rate. The heart rate increases and the patient may become tachypneic. Peripheral vasoconstriction may result in acrocyanosis. Neurologic symptoms may include confusion, dysarthria, and impaired judgment.

Moderate hypothermia is associated with a further decrease in the mental status. This includes lethargy, hallucinations, and loss of the pupillary reflex. The heart rate changes from tachycardia to bradycardia.4 The cardiac rhythm commonly converts from normal sinus to atrial fibrillation.4 The respiratory pattern becomes depressed, with a decreasing respiratory rate and tidal volume. The patient stops complaining of “feeling cold” and shivers less.

The patient is usually comatose by the time severe hypothermia is present, often with evidence of cardiorespiratory collapse. Ventricular irritability becomes evident. Ventricular fibrillation becomes refractory to conversion below 28 °C. Asystole occurs at 20 °C. The characteristic electrocardiographic (ECG) finding of an Osborne J wave occurs at approximately 32 °C (Figure 169-1). This is a positive deflection between the QRS and the ST segment.4–6 It is often best seen in ECG leads aVL, aVF, and the left chest leads.4–6

###### Figure 169-1

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