Hypothermia

Hypothermia is a common and often overlooked factor in critical care transport. Hypothermia may be the cause of the patient's critical illness or a complication of treatment and transport. Removing clothing, giving IV fluids, and transport in an ambulance or helicopter can all cause or worsen hypothermia. Hypothermia can worsen shock, bleeding, and multiple other medical conditions. This section will review the details surrounding hypothermia in the critical care setting.

 

Response to cold temperatures

Whenever the body is exposed to a cold environment, adaptation begins instantaneously. Initially, this response focuses on reducing heat loss and increasing heat production. If heat loss continues, the metabolism begins to fail as various physiologic processes break down.

Hyper-metabolism

The initial state of hyper-metabolism and the steps taken to conserve heat are commonly felt by all of us.

• Constriction of capillaries in the skin is instant. This slows heat loss from the skin and makes the skin cool to the touch.
• The blood supply to skeletal muscle also decreases. This can make the digits and limbs feel "stiff" or slow to react.
• Shivering begins, and the muscles rapidly vibrate to generate heat—this response is limited by glycogen stores in the muscles.

After these initial steps if hypothermia does not resolve the following occurs:

• The cardiac output increases to move warm blood to the vital organs and increase heat production by the cardiac muscle.
• The cells of the body increase their consumption of energy to increase heat production.

These advanced adaptive steps may cause worsening hypoxia, cardiac ischemia, and cellular ischemia in patients who are already critically ill and on the verge of multiorgan failure. 

Hypo-metabolism

If heat loss continues despite adaptations to increase the metabolism, the rate of metabolism will gradually decrease as the core temperature drops. This is primarily due to the physics of temperature, as cold decreases the rate of chemical reactions.

• Shivering will terminate if the muscle tissue reaches 32 degrees Celsius.
• The blood glucose increases as insulin cannot work at low temperatures.
• Respiratory drive decreases as CO2 production by the cells decreases.
• Cardiac output decreases and systemic vascular resistance increases to compensate.
• Lactic acidosis begins to set in as cells receive less oxygen than they require, even in their suppressed metabolic state.
• Ultimately capillaries begin to increase in permeability due to changes in the shape of proteins holding them together.
  • This causes fluids to shift out of the circulation and into the intracellular space, potentially worsening hypotension and limiting the response of hypotension to fluid resuscitation. 

The effective energy and oxygen utilization rate by cells is cut in half for every 10 degrees Celsius drop in body temperature. This is why the phrase "they aren't dead until they are warm and dead" is commonly used. Individuals who sustain cardiac arrest in very cold temperatures can have fully neurologically intact recovery hours later due to reduced oxygen demand by the brain.

 

Stages of Hypothermia

The three stages of hypothermia are mild, moderate, and severe. They are defined by the core temperature in Celsius but can also be estimated based on the patient's signs and symptoms.

Mild (36-32 Degrees Celsius)

• Increased basal metabolic rate as described above.
• Increased heart rate and contractility.
• Decrease in heart rate and metabolism begins at 32 degrees.

Moderate (32 to 29 Degrees Celsius)

• At 32 degrees, the body attempts to conserve and produce heat dramatically slow.
• Shivering stops
• The level of consciousness begins to decline
• Systemic vascular resistance begins to fall
• Hyperglycemia and mixed metabolic/respiratory acidosis is seen.

Severe (Less than 28 Degrees Celsius)

• Hypotension due to the combination of low cardiac output and low systemic vascular resistance is the key sign of severe hypothermia.
• ECG changes are common.
  • Prolongation of the PR interval, QRS complex, and QT interval.
  • Ventricular fibrillation risk is high; risk peaks at 22 degrees Celsius.
• Coagulation is severely inhibited: PT and PTT are increased by over 50%, and platelet activity is decreased by 40%

 

Treatment of Hypothermia

The treatment of hypothermia is focused on rewarming. The primary decision points are whether to use active vs. passive warming; and when to give medications.

• Bring all hypothermic patients into a controlled environment, remove all clothing, and wrap them in blankets as soon as possible.
• If the altered mental status is present, control the airway as soon as possible.
• In patients with moderate or severe hypothermia, initiate cardiac monitoring immediately.
• Defer ALL medications until the core temperature is over 30 degrees Celsius. This includes epinephrine in code situations.

Passive Re-warming

Patients with mild hypothermia can be brought into a warm environment, have wet clothing removed, and be covered with blankets. The absence of altered mental status and the presence of shivering are signs that this level of intervention is sufficient.

Active External Re-warming

Patients with moderate hypothermia should receive active external re-warming. In cold climates, specialized heating pads are likely to be available. Chemical hot packs are also appropriate if padded with a layer of cloth to prevent burns. The presence of altered mental status or absence of shivering should prompt this intervention. The presence of hypotension or ECG changes should also lead you to consider internal rewarming.

Active Internal Re-Warming

Severe hypothermia, defined by ECG changes or unresponsiveness, should be treated with active rewarming. Moderate hypothermia in trauma or severely ill patients should also prompt internal rewarming. There are several methods for active internal rewarming.

• Heated IV fluids
• Hemodialysis
• Gastric lavage with warmed fluids
• Rectal lavage with warmed fluids
• Active ECMO Treatment

The method that is likely to be used will depend on your area of practice and what other treatments the patient requires. Heated IV fluids are the most common, readily available, and safe option for the majority of patients. Caution is required in those with a history of heart failure or signs of fluid overload. In these situations, consider delaying internal rewarming until completion of transport, or consultation with medical command, if at all possible.