Approach to Hypothermia

Definition: Core body temperature <35 °C 

Normally, the body's autonomic system preserves the core temperature to 37 +/- 0.5 °C. In response to a cold stress, the hypothalamus attempts to stimulate heat production through shivering and increased thyroid, catecholamine, and adrenal activity. Sympathetically mediated vasoconstriction minimizes heat loss by reducing blood flow to peripheral tissues. In some cases, our body's regular mechanisms are overcome, or behavioural adaptations such as clothing and shelter are not available to protect against hypothermia.

Staging of Hypothermia
Mild hypothermia – Core temperature 32 to 35°C (90 to 95°F)
Moderate hypothermia – Core temperature 28 to 32°C (82 to 90°F)
Severe hypothermia – Core temperature <28°C (82°F)

Symptoms (presented in order of increasing severity)

• shivering,  tachypnea, tachycardia, initial hyperventilation, ataxia, dysarthria, impaired judgment, shivering, and "cold diuresis."
• reductions in pulse rate and cardiac output, hypoventilation, central nervous system depression, hyporeflexia, decreased renal blood flow, and loss of shivering at the lower end of the moderate hypothermia core temperature range. Paradoxical undressing may be observed. Atrial fibrillation, junctional bradycardia, and other arrhythmias can occur. The pupillary light reflex is depressed through slowing of both constriction and dilation. Dilated pupils are seen below a core temperature of approximately 29°C
• pulmonary edema, oliguria, areflexia, coma, hypotension, bradycardia, ventricular arrhythmias (including ventricular fibrillation), and asystole. Loss of corneal and oculocephalic reflexes may be noted.
• death

Differential Diagnosis

Broadly, hypothermia can be divided into environmental and non-environmental. Common environmental causes can include submersion in water and prolonged exposure to cold environments. 

Non-environmental causes of hypothermia include many illnesses.

• Decreased body heat generation (hypothyroid, adrenal insufficiency, hypoglycaemia, malnutrition)
• Impaired thermoregulation (CVA, CNS trauma, MS)
• Drugs/toxins (general anesthetics, ethanol, phenothiazines, barbiturates, antidepressants and organophosphate)
• Trauma (burns)
• Infection (sepsis)
• Other (vascular insufficiency, uraemia, iatrogenic, prolonged cardiac arrest)

Treatment

1. Resuscitation

Start with emergency management principles, including assessment of airway, breath and circulation. Endotracheal intubation is performed in patients with respiratory distress or those who cannot protect their airway. CPR may be required. Note peripheral pulses can be difficult to palpate in a vasoconstricted bradycardic patient. It is best to check for a central pulse for up to a full minute using a continuous-wave Doppler, if available. Alternatively, a focused bedside echocardiogram can be performed.

*Note: rough handling of the patient may precipitate arrhythmias, including ventricular fibrillation. Take care to avoid jostling the patient during the physical examination or the performance of essential procedures.

2. Rewarming 

Core temperature, ideally esophageal temperature in intubated patients, should be monitored closely to assess the adequacy of therapy and to prevent iatrogenic hyperthermia. Rewarming should begin as soon as possible. Rewarming techniques are divided into passive external rewarming, active external rewarming, and active internal core rewarming, and are used depending on severity of hypothermia. 

Passive external: after wet clothing is removed, the patient is covered with blankets or other types of insulation.

Active external:  warm blankets, heating pads, radiant heat, warm baths, or forced warm air is applied directly to the patient's skin.

Active internal: Endovascular temperature control catheters are preferred. If not available, irrigation of the peritoneum or the thorax (via the pleural space) with warmed isotonic crystalloid can be done.

3. Other

For patients who fail to rewarm appropriately despite aggressive rewarming measures, it is recommended to treat with empiric broad-spectrum antibiotics and a single dose of glucocorticoid (eg, dexamethasone 4 mg IV or hydrocortisone 100 mg IV). 

Concurrently, work-up should be completed to investigate underlying causes of hypothermia if unclear.

References

https://www-uptodate-com.myaccess.library.utoronto.ca/contents/accidental-hypothermia-in-adults?search=hypothermia%20treatment&source=search_result&selectedTitle=1~150&usage_type=default&display_rank=1

https://blackbook.ucalgary.ca/schemes/general/hypothermia/

A.L.