Hypothermia

Table I.

Severity	Core Body Temperature	Clinical Manifestations
Mild	32°-35° C	Confusion, cold, pale skin, shivering and tachycardia
Moderate	28°-31.8° C	Lethargy, reduced or absent shivering, bradycardia, decreased respiratory rate
Severe	<28° C	Obtundation or coma, no shivering, edematous skin, dilated and fixed pupils, bradycardia, hypotension, oliguria
Life Threatening	<25° C	Apnea, asystole

What other disease/condition shares some of these symptoms?

There are no diseases that mimic hypothermia. However, hypothermia may be a sign of multiple diseases or conditions. Environmental conditions, even in warm climates, can lead to hypothermia. A child exposed to wet, cool, or windy conditions is at risk for hypothermia. Trauma and resuscitation efforts may be associated with hypothermia because of environmental exposure, resuscitation with cold fluids, or not protecting the patient during transport.

Sepsis may present with hypothermia, especially in infants or immunocompromised patients. It also may be the presenting sign of child abuse. Submersion in cold water is becoming a more recognized form of child abuse.

What caused this disease to develop at this time?

Depending upon the etiology of the hypothermia, there are many predisposing factors that can be implicated.

Environmental or exposure to cold, wet, or windy weather put children at high risk for developing hypothermia. They have a small body mass to surface area ratio, limited glyogen stores which limit their ability to increase heat production in response to cold, and a lower level of cognitive ability, generally, to recognize and avoid a potentially dangerous environmental condition. Moreover, shivering in children produces less heat that in adults making it an ineffective heat producer in children.

History

If the etiology of the hypothermia is unknown, then pertinent history questions are critical. Specifically, questions related to environmental exposure are important. What was the duration of the exposure, the temperature, the wind speed, was the patient in cold water versus cold air. Cold water transfers heat much faster from the patient to the environment than when the patient is exposed to cold air. Therefore, hypothermia develops much faster if the patient is wet or was submerged in cold water.

Asking about trauma, exposure to infectious diseases, or a history of central hypothermia can also be helpful.

A patient who has a history of hypothalamic dysfunction may be predisposed to hypothermia and that history can help direct the treatment.

Physical Exam

The findings will often depend on the core body temperature. A specialized thermometer that can be placed in the patients esophagus, rectum, or bladder, should be used to record temperatures down to 25°C. A standard thermometer will not record temperatures below 35° C and therefore, may not be useful.

In mild hypothermia, patients may be confused and have findings that are typical of cold adaptation, such as cold pale skin from cutaneous vasoconstriction and brisk shivering.

In moderate hypothermia, shivering may be absent. These patients are often lethargic, bradycardic, bradypneic, with slurred speech, dilated or sluggish pupils. They may also have areas of frostbite. The areas most at risk are the fingers, toes, ears, and face and these should be inspected carefully.

Severe hypothermia may be a difficult diagnosis to make as many of the classic signs of hypothermia are no longer present. The skin may be flushed and the muscles become rigid as the temperature decreases. The muscle rigidity is secondary to the dysfunction of the actin-myosin bundles within the skeletal muscle. The pupils are fixed and dilated and respirations become more depressed. The cardiac output decreases secondary to sludging of the blood, depressed myocardial contractility, and hypovolemia because of vascular leak of fluid into extravascular spaces. This will lead to poor perfusion, weak distal and, eventually, central pulses.

The kidneys also become affected by the extreme low temperature and begin to lose concentrating ability. This leads to diuresis known as cold diuresis, and it contributes to the eventual circulatory collapse. The myocardium also becomes very irritable and can lead to severe or life threatening arrhythmias and, eventually, asystole.

What laboratory studies should you request to help confirm the diagnosis? How should you interpret the results?

Laboratory studies should be tailored to address the suspected cause of the hypothermia. In cases of children who are previously healthy and present with mild hypothermia secondary to environmental exposure, no routine laboratory or imaging studies are necessary.

Moderate, severe and/or life threatening hypothermia can lead to abnormalities in certain blood tests. Routine lab work should include a rapid serum glucose, serum electrolytes, serum blood urea nitrogen (BUN), and creatinine, lipse, a complete blood count (CBC), Prothrombin time (PT), partial thromboplastin time (PTT), and an international normalized ratio (INR). Blood type and crossmatch studies are necessary if the patient may need extracorporeal warming, and finally, arterial blood gas. Capillary gases should be avoided in these patients because peripheral circulation is poor and the gas may not accurately reflect the overall acid base status. For the same reason, peripheral pulse oximetry is not reliable and should not be relied upon for accurate data.

The results of these labs should be carefully reviewed. The serum electrolytes are important, specifically, because hyperkalemia is common in moderate to severe hypothermia. This may be secondary to the rhabdomyolysis from shivering. Serum creatinine may be elevated secondary to cold diuresis, poor cardiac output, or increased creatine kinase from rhabdomyolysis. These can lead to acute renal failure. A generalized coagulopathy may be seen in patients with hypothermia, but the coagulation labs, if run at normal body temperature, may be normal. Alternatively, the arterial blood gas should be run at normal body temperature and it can reveal a metabolic acidosis or a respiratory acidosis, or both.

Laboratory studies also may be indicated if the hypothermia is from an unknown etiology. In a child with multisystem trauma or child abuse, a urine dipstick for blood, liver enzymes, and type and crossmatch for blood products may be indicated. Further, in ill-appearing infants or children with hypothermia, sepsis should be suspected and blood, urine, and cerebral spinal fluid should be sent for culture and cell count. If the patient is at risk for substance or alcohol abuse then a serum or urine toxicology screen can be useful. Finally, a patient who is not responding to rewarming techniques, a thyroxine, thryoid stimulating hormone, adrenocorticoptropic hormone, and serum cortisol should be measured.

Would imaging studies be helpful? If so, which ones?

Routine imaging studies are done based on specific indications. In a previously healthy child with mild hypothermia secondary to environmental exposure, no imaging studies are indicated.

In patients with moderate to severe hypothermia an electrocardiography (EKG) is often indicated because of the risk of cardiac arrhythmias. If there is an abnormality, most patients will have a prominent J wave at the QRS-ST junction. These are called Osborn waves and are not specific for hypothermia. They also do not have any diagnostic or prognostic value in hypothermia. However, because the patient can have any type of abnormal rhythm in moderate to severe hypothermia an EKG is warranted.

In patients with pulmonary findings such as tachypnea, grunting, rales, or a cough, a history of chest trauma, or near drowning, a chest x-ray is indicated. Certainly, patients with multisystem trauma will require imaging and that decision should be based on the type and severity of the injury. Head trauma will often require computed tomography (CT) scan of the head as well as of the abdomen and pelvis if it is a significant trauma.

If child abuse is suspected, a skeletal survey, opthalmologic exam and, possibly, a CT of the head are indicated.

If you are able to confirm that the patient has hypothermia, what treatment should be initiated?

Are there some therapies that should be instituted immediately?

There are special considerations for patients with moderate to severe hypothermia that should be addressed. First, in children with severe hypothermia a pulse may be difficult to detect. The American Heart Association recommends assessing the patient for 30-45 seconds before initiating cardiopulmonary resuscitation efforts. With that in mind, as with any resuscitation, careful attention to the ABCs (Airway, Breathing, and Circulation) is the crucial first step in caring for any patient regardless of circumstances.

In the pre-hospital setting, and upon arrival to the hospital, accurate temperature measurement may be difficult. Removing the patient from the cold environment and placing them in a horizontal position is important to minimize the movement of peripheral blood into the central circulation.

If hypothermia is suspected, the providers should avoid rough handling of the patient, or exertion of the patient because of the extreme degree of myocardial irritability. Everything possible should be done to minimize heat loss during transport and in the hospital. Removal of wet clothing, gentle insulation of the patient with warm blankets, warming of the transport vehicle and warm IV fluids are all helpful. Be cautious in children with moderate to severe hypothermia, however. Active rewarming in transport should actually be avoided in these patients because of the cold afterdrop phenomenon. These patients should be monitored on a cardiorespiratory monitor to observe for any cardiac arrhythmias and abnormal breathing patterns.

Once these children arrive at the hospital, they should be handled very carefully, minimizing movement and keeping them horizontal. The rewarming techniques for moderate to severe hypothermia can take on various methods, but all patients can be given heated, humidified oxygen. The rewarming provided by this technique is limited because in most cases the temperature of the humidifer cannot exceed 41°C without modification. It does, however, help prevent further heat loss at that temperature. Additionally, having intravenous access in children with moderate to severe hypothermia is very helpful.

What about longer term treatment?

There are multiple techniques that can be used for longer term rewarming. Understanding them and when to use them is very important because of the irritability of the myocardium. There are three classes of rewarming methods available:

Passive rewarming: This involves removing wet or cold clothing and applying dry insulation like blankets or sleeping bags in a warm environment. This type of rewarming relies on the patients ability to generate their own heat and does not offer any additional heat to the patient. This can be used in conjunction with active rewarming, even in cases of mild hypothermia.

Active Rewarming can be subdivided into two categories, active external rewarming and active internal rewarming:

-Active external rewarming implies that an external source of heat is being applied to the patient. This can be in the form of forced air rewarming, radiant heat, or direct heat from heating pads or chemical heat packs.

-Active internal rewarming refers to methods used to heat the patient internally. This can be done in several ways. In mild cases of hypothermia, non-invasive techniques like providing warmed oral fluids and heated humidifed oxygen are helpful. Warm intravenous normal saline is also considered a non-invasive technique. Using room temperature fluids may worsen hypothermia and should be avoided. More invasive techniques such as heated saline lavage of the pleural space, bladder, stomach, or peritoneum are used in patients with severe hypothermia or an inadequate response to initial non-invasive forms of rewarming.

Extracorporeal Techniques of Rewarming:

These are reserved for children with severe or life threatening hypothermia with absent circulation or in children for whom other warming techniques are not working. These types of techniques may include cardiac bypass or extracorporeal membrane oxygenation (seeTable II. Rewarming Techniques).