Overview of severity of hypothermia

The severity of hypothermia is most accurately defined by core body temperature; however, this is often difficult to measure in the field. An oesophageal probe is the most reliable method of measuring core body temperature, but this is generally only possible in a ventilated patient. Rectal or bladder probes are less reliable alternatives. Standard thermometers often do not measure below 34ºC, and the validity of infrared tympanic thermometers has not been demonstrated in hypothermic patients.

The threshold temperatures for defining categories of hypothermia vary among guidelines. Those used here are thresholds accepted by the Wilderness Medical SocietyDow, 2019.

Clinical assessments other than core body temperature are also important in determining severity and management, and may be more practical in the field; these include level of consciousness, shivering intensity, physical performance, cardiac rhythm and blood pressure; see Indicators of severity of hypothermia for a summary. For more detail on each severity classification, see mild, moderate and severe (profound) hypothermia.
Note: Clinical signs are key to assessing the severity of hypothermia; core body temperature measurement is not the sole determinant.

If the clinical signs of hypothermia are not consistent with the measured core body temperature or the history of cold exposure, always consider concurrent or alternative pathology (eg trauma, hypoglycaemia, drug intoxication).

Table 1. Indicators of severity of hypothermia

[NB1]

Severity of hypothermia

Mild

(32 to 35°C)

Moderate

(28 to 32°C)

Severe

(less than 28°C)

Mental state

may be apathetic but not drowsy or confused

drowsy or confused

usually unconscious

may lack vital signs, but assume alive unless fatal injuries present [NB2]

Movement

impaired [NB3]

impaired [NB3]

not moving

Shivering

yes

reducing

not shivering

Note:

NB1: If the clinical parameters are not consistent with the measured core body temperature or the history of cold exposure, always consider concomitant pathology (eg trauma, hypoglycaemia, drug intoxication).

NB2: A severely hypothermic person may have no respiration, no carotid pulse detected (after assessment for up to 1 minute) and fixed dilated pupils, but may still be alive, unless they have fatal injuries (such as avalanche asphyxiation by snow packing an airway or an ice mask obstructing it).

NB3: A person cannot care for themselves normally; deficits are known as the ‘Umbles’—fumble, mumble, grumble, stumble.