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Hypothermia
Essentials
- 1
Hypothermia is categorized into mild (32°C to 35°C), moderate (29°C to 32°C) and severe (<29°C) on the basis of a rectal or other core temperature reading.
- 2
Moderate-to-severe hypothermia produces progressive delirium and coma, hypotension, bradycardia and failure of thermogenesis.
- 3
The electrocardiograph will often show slow atrial fibrillation and an extra positive deflection in the QRS (the J or Osborn wave) in leads II and V 3 to V 6 with worsening hypothermia.
- 4
Endotracheal intubation is safe in hypothermia. Ventilation and acid–base status should be manipulated to maintain uncorrected blood gases within the normal range.
- 5
Endogenous rewarming, consisting of drying the patients and placing them in a warm, dry and wind-free environment, should form part of all rewarming protocols.
- 6
In most cases of moderate-to-severe hypothermia rewarming can be achieved with endogenous rewarming plus forced-air rewarming blankets without the need to resort to more aggressive techniques.
- 7
In the arrested hypothermic patient rewarming should be with cardiopulmonary bypass or warm left pleural lavage.
Introduction
Hypothermia is defined as a core temperature of less than 35°C. This can be measured at a number of sites (including oesophageal, right heart, tympanic and bladder). Rectal remains the routine in most emergency departments (EDs), despite concerns at how rapidly it equilibrates to and reflects true core temperature. Conventionally, hypothermia is divided into three groups: mild (32°C to 35°C), moderate (29°C to 32°C) and severe (<29°C) on the basis of measured core temperature. In a field setting, where core temperature measurements may not be possible, moderate and severe are often grouped together as they typically share the clinical features of absence of shivering and altered mental state. These categorization systems can be used both out of and in hospital as a guide to selecting rewarming therapies and prognosis. Mild hypothermia is considered the stage where thermogenesis is still possible; moderate is characterized by a progressive failure of thermogenesis; and severe by adoption of the temperature of the surrounding environment (poikilothermia) and an increasing risk of malignant cardiac arrhythmia. Nevertheless, there are substantial differences between individuals in their response to hypothermia.
Epidemiology and pathophysiology
Hypothermia may occur in any setting or season. True environmental hypothermia occurring in a healthy patient in an adverse physical environment is less common in clinical practice than that secondary to an underlying disorder. Common precipitants include injury, infection, systemic illness, drug overdose and immersion, and are outlined in more detail in Table 24.2.1 . The elderly are at greater risk of hypothermia because of reduced metabolic heat production and impaired responses to a cold environment. Alcohol is a common aetiological factor and probably acts by a number of mechanisms, including cutaneous vasodilatation, altered behavioural responses, impaired shivering and hypothalamic dysfunction.
Table 24.2.1
Hypothermia aetiologies
| Environmental | Cold, wet, windy ambient conditions |
| Cold water immersion Exhaustion |
|
| Trauma | Multitrauma (entrapment, resuscitation, head injury) |
| Minor trauma and immobility (e.g. #NOF, #NOH) | |
| Major burns | |
| Drugs | Ethanol |
| Sedatives (e.g. benzodiazepines) in overdose | |
| Phenothiazines (impaired shivering) | |
| Neurological | CVA |
| Paraplegia | |
| Parkinson’s disease | |
| Endocrine | Hypoglycaemia |
| Hypothyroidism | |
| Hypoadrenalism | |
| Systemic illness | Sepsis Malnutrition |
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