Physiology of structural firefighting

The EMS roles on the fireground that do not deal with civilian victims require a substantial understanding of the physiology of firefighting. This topic is briefly discussed here to assist EMS medical directors in the joint development of rehab and treatment/transport protocols with fire department physicians. Fire department physicians are typically occupational medicine providers who use a list of the essential job tasks associated with firefighting to evaluate medical fitness for duty. NFPA 1582, Standard on Comprehensive Occupational Medical Programs for Fire Departments [11], requires that fire department physicians participate in operational safety matters and that they ^{collaborate with EMS medical directors on procedures for medical support of firefighters at fire incidents.}

Firefighting involves strenuous physical work, sometimes in extreme heat, for variable periods of time. Fire personnel may be exposed to environmental temperatures in excess of 700 °F during structural firefighting [12]. Firefighters depend on their PPE to allow them to function in such temperatures. This includes bunker pants, boots, coat, gloves, hood, helmet, and self-contained breathing apparatus (SCBA). The full protective ensemble may add 50–75 lb of weight [13–15], increasing firefighters’ workloads significantly.

While PPE protects firefighters from heat and burns, it also prevents the physiological cooling that would normally occur through convection and evaporation of sweat. The PPE creates a thermal microenvironment next to the skin that is hot and has 100% relative humidity. The coat and bunker pants comprise multiple layers of composite materials including outer shell, moisture barrier, and thermal barrier. They are rated for total heat loss (THL), which measures evaporative heat transfer or breathability, and thermal protective performance (TPP), which measures thermal insulation as outlined by NFPA 1971, Standard on Protective Ensembles for Structural Fire Fighting and Proximity Fire Fighting [16]. Increasing TPP may result in decreased THL, but any interference with evaporative cooling can contribute to rapid temperature elevations in exercising firefighters [17–20].

Early studies showed that firefighters’ heart rates increase at the time of initial alarm, before any physical activity occurs [13,21]. Heart rate monitoring does not directly correlate with energy expenditure [1,22] or core temperature rise [14]. Part of the increase in heart rate is due to work demand, but much more is due to thermal stress [23]. Increased cardiovascular work and thermal stress are thought to contribute to the substantial cardiovascular morbidity and mortality associated with fire suppression [2,19]. A study looking at cardiovascular effects of repeated, strenuous live-fire drills found that peripheral vasodilation and sweat loss resulted in significant reduction in stroke volume after as little as 20 minutes of performing such drills [24]. Since heart rate is sustained at or near maximum throughout a fire response [1,22,23,25], any decrease in stroke volume immediately translates to decreased cardiac output. This physiology is known as uncompensable heat stress and develops rapidly during fire suppression [20,26].

Firefighters perform many tasks, several of which involve heavy work. A sample list of essential job tasks associated with firefighting appears in NFPA 1582. These tasks represent a combination of aerobic and anaerobic exercise together with requirements for balance, agility, mental acuity, and judgment. Examples of how aerobic, anaerobic, and static physiological demands are combined in firefighting include stair and ladder climbing while carrying 50–75 lb of tools or hose, and use of pike poles to breach walls or ceilings during overhaul [1,27,28].

Live fire exercises induce increased cardiovascular stress when compared to mock fire exercises [29]. Of the typical tasks associated with fire rescue services, interior structural firefighting while wearing SCBA demands the most energy [30]. While wearing SCBA protects firefighters from inhaling carbon monoxide and other toxic products of combustion [31,32], it adds weight and restricts movement and peripheral vision [33].

After performing strenuous work such as search and rescue and initial knockdown of a structure fire, firefighters begin “overhaul” [34]. The use of axes and pike poles to search for smoldering fire within walls and ceilings requires further disproportionate upper body exertion, which is associated with greater cardiovascular stress [35] when the firefighter may already be fatigued, hot, and dehydrated. SCBA use is variable during overhaul. Lack of SCBA use while sifting through piles of smoldering material may increase firefighters’ exposure to CO. Non-flaming combustion produces more smoke than flaming combustion; the dominant dangerous gas in smoke is CO [36]. Increased heart rate [23] and exposure to CO [37] during exercise result in changes in ST segments on ECGs. Fire suppression activities (compared to non-emergency duties) may cause much higher chances of death from coronary heart disease in firefighters [2,38].

Core temperatures of firefighters continue to rise after completion of firefighting tasks in heat [22,39–44].

The cumulative evidence underscores the need for rehab areas at fire scenes and incorporation of cooling in rest cycles, as advocated by others [22,23,42], and perhaps to preemptively evaluate firefighters for their ability to tolerate heat stress [44].