Prevention and management of hypothermia is an essential feature of tactical combat casualty care (TCCC). Core (central) hypothermia is a significant risk factor following trauma as many physiological body functions are temperature reliant. Core hypothermia is therefore associated with multiple secondary complications such as cardiac dysfunction, altered liver metabolism, immune reduction, and impaired blood clotting capacity.
Even in warmer environments, post-traumatic hypothermia is a relatively common problem due to a combination of environmental heat loss (convection, conduction, evaporative, radiation) and the heat loss associated with the delivery of health care measures (intravenous fluid administration, surgical interventions). Universal measures to mitigate heat loss are therefore instituted early within TCCC protocols.
TCCC environmental temperature management typically includes the minimisation of heat loss by use of a hypothermia wrap. A hypothermia wrap may be as simple as using a space blanket or may alternatively involve the sequential layering of multiple components such as blankets, chemical active heating devices, and vapour control barriers (known as a “thermal burrito”).
Hypothermia wraps are especially important when the likelihood or consequences of core hypothermia are elevated, such as in association with severe traumatic injury, large body surface area burns, prolonged pre-hospital management time duration, cold or wet environments, and the use of rotary wing aeromedical evacuation assets.
As the reversal of established core hypothermia within austere environments is typically very difficult or impossible to achieve, hypothermia prevention by limitation of heat loss is therefore the primary aim of environmental management in TCCC.
Whilst core hypothermia results in detrimental outcomes for trauma casualties there are some situations where selective temperature reduction (cryotherapy) techniques are beneficial. A common example of this is illustrated in the first aid management of finger amputations. As the oxygen consumption and metabolic activity of living tissue decreases with lower temperatures, an amputated finger completely devoid of blood supply may be kept alive using an ice slurry and successfully reattached even 24 hours after injury.
Arterial tourniquets when utilised in accordance with TCCC guidelines represent a highly effective life saving measure in the management of catastrophic haemorrhage. Vascular trauma and limb tourniquet application is however associated with impaired functional recovery and decreasing limb viability, particularly when surgical management for the restitution of blood supply is delayed.
The Tourniquet Traffic Light (TTL) provides a simplified framework for the common understanding of the time dependent outcomes associated with limb tourniquet application. Limb viability after vascular injury and tourniquet application is however not solely a time dependent phenomenon as multiple modifying factors may also influence limb salvage rates and functional outcomes.
Such modifying factors include the pattern of limb wounding, overall condition of the casualty, effectiveness of resuscitation efforts, and the type of interventions undertaken during the initial surgical management of the traumatised extremity. In this regard, the time-based recommendations described in the TTL relate to “warm ischaemia” time. Warm ischaemia time refers to the duration that the limb remains at or near body temperature after the blood supply has been lost by injury or tourniquet application.
Similar to management of amputations, however, the application of limb cryotherapy after tourniquet application will reduce tissue metabolic rate and oxygen consumption with the potential to increase limb function and viability for any given duration of vascular compromise.
Limb cryotherapy in an austere environment is relatively easy to achieve. As an effective tourniquet eliminates blood circulation below the level of application, the injured extremity is thus disconnected from vascular rewarming and therefore able to spontaneously cool to ambient room temperature over time. TCCC management of a casualty with an arterial tourniquet applied should therefore be directed towards facilitating ambient limb cryotherapy. The Limb Exclusion Hypothermia Wrap (LEHW) achieves simultaneous core temperature environmental protection whilst enabling ambient limb cryotherapy after arterial tourniquet application.
LEHW techniques may be applied with any hypothermia wrap construct. In simplest form the LEHW simply excludes the vascular compromised limb from the hypothermia wrap (Figures 1-2). There are several advantages to this type of construct (Table 1). From a limb perspective, any degree of ambient environmental cryotherapy is of potential benefit in comparison to limb rewarming by inclusion within the hypothermia wrap.
Having the limb exposed facilitates surveillance and management of the limb for rebleeding in association with active resuscitation. From the perspective of core thermal protection in an austere environment, having the injured limb outside the hypothermia wrap means body heat is preferentially distributed to core rewarming rather than to the injured extremity.
Furthermore, as interventions upon the injured limb such as tourniquet adjustment or wound care can be undertaken without disturbing the hypothermia wrap, unwanted central body heat loss is minimised.
Figure 1: Limb Exclusion Hypothermia Wrap using a Lightweight Casualty Blanket (Frontline Safety Australia; Newcastle, Australia).
Table 1: Benefits of LEHW.
The ambient environmental cryotherapy achieved by LEHW techniques may also be augmented by active cryotherapy. Active cryotherapy techniques may include wetting the limb to increase the rate of evaporative heat loss or by the application of an ice slurry. Care must be taken to avoid freezing the tissues (frostbite) by the direct application of ice or snow. In this regard an ice-water slurry is recommended for active cryotherapy management.
Risks of LEHW application relate to both the local and systemic effects of hypothermia achieved within the injured extremity. A severely injured limb with a tourniquet in place is at elevated risk of freezing injury (frostbite) as the normal thermal protective mechanisms are impaired. Particularly in an alpine environment when the ambient temperature is below freezing point, the excluded limb may still require some degree of thermal protection for the prevention of frostbite, especially if extended evacuation times are anticipated.
Within the initial surgical care facility and once restoration of blood supply has been achieved, extra vigilance in core temperature management will be required as the cooled extremity will act as a heat sink and cause a thermal after-drop upon return of cooled venous blood into the central circulation.
The LEHW represents a simple technique to achieve core thermal protection measures whilst potentially improving outcomes for a severely injured limb after an arterial tourniquet has been applied. While reducing oxygen consumption and metabolic rate of the effected tissues may be associated with reduction in limb associated morbidity and the systemic metabolic consequences after reperfusion, LEHW techniques do not substitute for rapid evacuation and expeditious surgical management. In this regard the principles of the Tourniquet Traffic Light remain unchanged – the optimal standard of care for vascular compromised limb is for revascularisation within 2 hours of injury.