Hypothermia

Hypothermia is a core body temperature <35°C which is:

• Due to either:
  • Abnormal thermoregulation
  • Heat loss despite thermoregulation
• Arbitrarily classified by degree:
  • Mild: 32-35°C
  • Moderate: 28-32°C
  • Severe: <28°C

Epidemiology and Risk Factors

Pathophysiology

Hypothermia has pathophysiological effects on all organ systems:

• B
  • ↓ RR
  • ↓ VO₂
  • ↓ PaCO₂
    ↑ CO₂ solubility.
• C
  • ↓ HR and CO
  • ↑ QTc
  • J-point elevation
  • Arrhythmias
    • AF
    • VF
  • ↑ Defibrillation threshold
  • Vasoconstriction
    • Centralisation of blood volume
• D
  • Confusion → obtundation
  • ↑ Seizure threshold
  • ↑ BSL
• F
  • ↓ Vasopressin synthesis
  • ↑ pH
• G
  • ↓ Drug metabolism
• H
  • ↑ Haematocrit
    • ↑ Blood viscosity
  • Neutropaenia
  • Thrombocytopaenia
  • Coagulopathy
• I
  • Shivering

Aetiology

Mechanisms of Hypothermia

↑ Heat Loss	↓ Production	Abnormal Thermoregulation
Radiation ↓ Temperature of external environment. Conduction ↑ With high heat-capacity surrounds, e.g. water. Convection ↑ Effect of conductive losses. Evaporation Vasodilatation Skin barrier disruption Burns. ↑ Surface area:Volume Children.	Endocrine: DKA Hypoadrenalism Hypothyroidism Hypopituitarism Reserve: Hypoglycaemia Malnutrition Extremes of age Immobility	CNS disruption Drug toxicity Uraemia Critical illness Sepsis Malignancy

Clinical Features

• B
  • Apnoea
  • Hypoventilation
  • Pulmonary oedema
• C
  • ↓ BP
  • ↓ HR
  • ↓ Capillary return
  • AF
  • Asystole
• D
  • Obtundation
  • ↑ Tone
  • Abnormal plantar response
  • Fixed, mid-dilated pupils
• F
  • ↑ Followed by ↓ urine output
• G
  • ↓ Bowel sounds

Assessment

History:

Exam:

Investigations

Bedside:

Laboratory:

Imaging:

Other:

Diagnostic Approach and DDx

Investigations

Bedside:

Temperature should be assessed with the most accurate device available and appropriate to the clinical situation. In descending order:

• Intravascular
  
  • PAC
    Gold standard.
  • Jugular bulb
    Approximates PAC except around infusion of IV fluids.
• Oesophageal
  Should be measured at ~T8-9.
• Bladder
  Relies on adequate urine output.
• Nasopharyngeal
• Rectal
• Tympanic

• ABG
  
  • ↓ Core body temperature ↑ solubility of CO₂ and ↓ PaCO₂ in vivo
    Samples will be warmed to 37°C in the analyser, providing a spuriously high result.
• Temperature measurement

Laboratory:

Imaging:

Other:

Management

Good neurological outcome can occur following prolonged resuscitation in patients with severe hypothermia. Changes to ALS in this cohort include:

• Prioritise rewarming
• Brief interruptions of CPR acceptable if <28°C
• Delivery of up to 3 shocks and then withholding until 30°C
• Withholding ALS drugs until 30°C
• Doubling ALS drug interval until 35°C

Resuscitation:

Specific therapy:

Comparison of Rewarming Therapies

Class	Method	Rate of Change (°C/hr)
Passive	Warmed external environment	0.5-1
Active External	Forced-air warmers	1
	Radiant heaters	1-2
	Temperature control pad e.g. Arctic sun.	1.5-3
Active Internal	Humidified warm inspired gas	0.5-1
	Intravascular catheter	2-4
	Body cavity lavage	3-5
	CPB/ECMO	10
	RRT	10

• Pharmacological
• Procedural
• Physical
  • Rewarming
    Note that warming IV fluids is important to prevent further heat loss but ineffective at rewarming.
  • Avoid sudden movements
    May precipitate arrhythmia.

Supportive care:

Disposition:

Preventative:

Marginal and Ineffective Therapies

Anaesthetic Considerations

Complications

• C
  • Arrhythmia
    • During cooling
    • During rewarming
    • With sudden movement or invasive ventilation
      Sudden redistribution of warmed external blood into cold core.
• D
  • ↑ ICP

Prognosis

Key Studies

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References

1. Bersten, A. D., & Handy, J. M. (2018). Oh’s Intensive Care Manual. Elsevier Gezondheidszorg.