Hypothermia is a widely recognised hazard in trauma care, but recognition does not always consistently translate into effective treatment in the field. In emergency first response, management is often reduced to a single step, applied late and without structure. This approach does not, unfortunately, reflect how hypothermia develops or progresses in a trauma casualty. Treatments for hypothermia must be applied as a logical sequence of interventions, starting early and continuing throughout the care process. The effectiveness of any single measure largely depends on how it is combined with others. Without that structure, heat loss may continue despite attempts to intervene. In this article, we look at passive and active rewarming, and how these support a system-based response to hypothermia management.

What Are Passive And Active Rewarming?

Passive and active rewarming describe two distinct but interdependent approaches within treatments for hypothermia. Passive rewarming focuses on retaining the casualty’s existing body heat and preventing further loss, while active rewarming introduces external heat sources to restore core temperature. Their relevance to hypothermia management lies in their sequence and dependency: passive measures must be established first to stop ongoing heat loss, whereas active rewarming is only effective once that loss is controlled. Applied together, they form the basis of a system-based response, rather than isolated interventions.

The Role Of Ground Insulation

Ground insulation is crucial and must be treated as an immediate intervention in its own right. The Tactical Combat Casualty Care (TCCC) guidelines are explicit: insulation material should be placed between the casualty and any cold surface as a primary step. In most cases, this means elevating the patient away from the ground using a stretcher, blanket, or improvised barrier before applying any further insulation. This can be counter-intuitive in a high stress emergency situation, when the instinct of many first responders is to insulate then elevate. However, wrapping a casualty without separating them from the ground leaves a major source of heat loss untouched. 

Passive Rewarming Starts The Process

Passive rewarming is the first treatment step because it can begin immediately, using simple measures available at the point of contact. Its purpose is not to raise the casualty’s temperature quickly, but to stop the casualty from getting colder. This can include insulating the patient, shielding them from air movement, and creating a vapour barrier that helps retain the heat they are still producing. This early phase establishes the groundwork for everything that follows. If passive rewarming is delayed, the casualty may continue to lose heat while the responders prepare more advanced interventions, and that loss often cannot be fully recovered later.

Active Rewarming Builds On Passive Measures

Once passive rewarming is in place, active rewarming can do what it is intended to do: add heat to a casualty who is no longer losing it completely unchecked. This is where chemical heat packs or warming devices become useful, but only when applied correctly. TCCC guidance specifies placement on the anterior torso and axillae, not directly on the skin and not wrapped around the torso. The reason is a practical one. Incorrect placement increases the risk of burns and produces uneven warming, neither of which improves the overall response. Active rewarming is effective when it is targeted, controlled, and used to build on a stable passive foundation.

Internal Warming Must Match External Warming

A system-based response also has to account for what is happening inside the patient. IV fluids and blood products should be warmed to 38–42°C before administration. If they are not, the treatment works against itself. External warming measures may be underway, but unwarmed fluids can reduce the core temperature from within and deepen the same physiological decline the responders are trying to prevent! Thermal management therefore also includes what enters the patient’s circulation during resuscitation, and not just external active warming interventions such as blankets and foils.

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Treatments for hypothermia are most effective when they are understood as a sequential system rather than a set of individual actions. The point is not to choose between passive and active rewarming as circumstances dictate, but to apply both in the right order and support them with the other steps that make them work effectively.

If you’d like to find out more about field treatments for hypothermia and how TSG can support your operation, please click here to contact us!