Psychophysiological responses of firefighters to day and night rescue interventions
Introduction
Firefighting is a demanding occupation with high level of psychophysiological stress during rescue interventions (Williams-Bell et al., 2009; Horn et al. 2011, 2019; Smith et al., 2016; Windisch et al., 2017). It is important for firefighters to be physically fit in order to enhance their firefighting performance, which can be quantified by the time required to carry out their duties (Williams-Bell et al., 2009), as well as by the associated physiological responses (Windisch et al., 2017). In France, firefighters work a 24-h shift, so they have to maintain a high level of performance regardless of the time of day or night. However, to date, firefighters’ performance and physiological stress in response to interventions have only been studied during the day (Williams-Bell et al., 2009; Horn et al. 2011, 2019; Windisch et al., 2017) except for the acute response to the alarm signal, which has been compared between day and night conditions (Hall et al., 2016). The authors showed that the increase in heart rate (HR) was similar during the day and night, whereas the increase in cortisol was higher at night than during the day. Several studies conducted with healthy subjects have shown that physical performance is better late in the afternoon than in the early morning (Baxter and Reilly 1983; Thun et al., 2015). However, these results appear to be equivocal, and other studies revealed no effect of circadian rhythm on physical performance (Reilly and Brooks 1982; Deschenes et al., 1998). Moreover, at rest, several physiological functions, such as rectal temperature, HR, plasma lactates, blood pressure (BP) and cortisol levels, follow a circadian rhythmicity (Reilly and Brooks 1982; Deschenes et al., 1998), which can also be found during or after exercise (Harma et al., 1982). These results are controversial since other studies did not show any circadian variations in HR responses with exercise (Reilly and Brooks 1982; Deschenes et al., 1998). Consequently, the general picture of the circadian modulations of performance and physiological variables is unclear. It seems that this variation should be highly dependent upon the type of exercise and the characteristics of the participants. Moreover, beyond the circadian variations, the state (awake/asleep) in which the firefighter is in, at the time of the intervention, can influence his or her psychophysiological. Indeed, HRV and HR seem to be largely under a sleep-wake-dependent process (Viola et al., 2002). Therefore, it would be important for the safety and security of firefighters to measure if the performance of firefighters is modulated by the moment of the intervention (daytime vs night-time).
Moreover, the psychophysiological stress resulting from firefighting activities depends on the interaction of three different phases during an intervention. The first phase, the alarm mobilisation phase, corresponds to the alarm signal followed by the moment when firefighters change into their protective clothing before leaving the fire station. This phase leads to physiological stress with an increase of HR from 20 to 66 beats⋅min−1 in a short period of time (Kuorinka and Korhonen 1981; Karlsson et al., 2011; Hall et al., 2016), and an increase in salivary cortisol concentrations (Hall et al., 2016). The second phase, the fire suppression phase, corresponds to the phase of firefighting activities, such as forcible entry, search and rescue operations or extinguishing a fire (Kales et al., 2003). During this phase, firefighters engage in intense physical activity in psychologically stressful situations with high temperatures. This leads to high, and sometimes maximal, HR, and to an increase in BP, body temperature and psychological stress (Barr et al., 2010; Williams-Bell et al., 2010; Horn et al., 2011; Windisch et al., 2017; Marcel-Millet et al., 2018; Wilkinson et al., 2020). The third phase, the alarm return phase, corresponds to the several hours following a fire suppression. During this period, the psychophysiological stress generated by the intervention remains, and it can sometimes take several hours for the physiological parameters to return to initial levels. After simulated firefighting activities, core temperature continues to increase for many minutes (Selkirk et al., 2004; Horn et al., 2011), and the HR returns to baseline values only after about 80 min (Horn et al., 2011). Furthermore, a significant acute parasympathetic perturbation has been observed after simulated firefighting activities (Marcel-Millet et al., 2018). Indeed, in this study, a greater vagal depression was observed after a simulated emergency than after an incremental running test performed until exhaustion.
The amount of stress during a fire can represent major issues for firefighters on two levels. First, for firefighters with underlying cardiovascular disease, a significant amount of stress can induce pathophysiological changes causing a risk of death (Smith et al., 2016; Kales and Smith 2017). Second, regular exposure to a source of stress could contribute to the long-term development of cardiovascular disease (Steptoe and Kivimäki 2012). Thus, despite a high level of exposure to traumatic risks (Kong et al., 2013), sudden cardiac events are the leading cause of line-of-duty death for firefighters in the United States (Kales et al., 2007; Smith et al., 2016). Consequently, the comparison of a day and night intervention would determine whether or not firefighters’ performance and psychophysiological stress are affected by the time period in which occurs, considering that an increase in cardiovascular stress is associated with adverse outcomes for the health of firefighters (Smith et al., 2016; Kales and Smith 2017).
Finally, the sudden increase in HR and BP in response to the emergency alarm increases the risk of death among firefighters (Kales et al. 2003, 2007). A previous study showed that in comparison to a classical emergency alarm, a gentle awakening carried out by an experimenter mitigated the increase in HR (Hall et al., 2016). Usually, the alarm signal is a sound of at least 85 db(A) emitted by a transmitter carried by the firefighter. Recently, the Regional Fire and Rescue Service offered to some firefighters a modified transmitter that starts by vibrating for 30 s, before emitting a sound signal if the transmitter is not turned off. The objective of this device is to reduce the stress of the firefighters during the alarm. Therefore, it is important to measure its effectiveness in workplace , to guide firefighters in their choice of alarms.
Consequently, the present field study aimed to: (1) compare the daytime and night-time physiological and psychological responses of structural firefighters to a simulated emergency; (2) compare the physiological and psychological responses of structural firefighters to a sound alarm and a vibration alarm, at night. It was hypothesised that the night intervention would exhibit higher physiological response than the day intervention, and that this higher response could be attenuated by a less noisy alarm.
Section snippets
Participants
Sixteen male firefighters participated in this study (Table 1). All the participants had been medically screened by the local firefighter's healthcare institution, and each had received medical clearance to engage in firefighting activities. Prior to testing, all the participants gave their voluntary written informed consent, which indicated the purpose, the benefits and the risks of the investigation and the right to withdraw from participation at any time. This study was conducted in
Performance and physiological parameters
The alarm was triggered at 08:32 h ± 00:07 h for the DaySA condition, at 00:50 h ± 00:17 h for the NightSA condition and at 00:53 h ± 00:16 h for the NightVA condition.
For the three experimental conditions, the CT and HR responses for both the alarm mobilisation phase and the simulated rescue intervention are presented in Table 2. The cardiac parasympathetic indices of the acute recovery phase for the three conditions are also displayed in Table 2. All the cardiac parasympathetic indices were
Discussion
This study aimed to assess the psychophysiological responses of firefighters throughout a simulated rescue intervention performed during the day and at night, in workplace. Furthermore, the study aimed to determine if a modified vibrating alarm has an impact on these psychophysiological responses during a simulated intervention performed at night. The main results revealed that during the alarm mobilisation phase, the HR reactivity was higher during the intervention performed at night than the
Conclusions
In conclusion, the firefighters' cardiac stress in response to the alarm signal was greater during a night-simulated rescue intervention than during the day, and the cardiac parasympathetic reactivation in the post-simulated rescue intervention was more impaired in the night in comparison to the day. Moreover, the firefighters felt sleepier and less self-confident before the night-simulated intervention than before the day-simulated intervention. Nevertheless, the firefighters were equally
Funding sources
The study was part of the European Cross-border Co-operation Programme (Interreg France-Suisse, 2014–2020), and it was supported by a grant from the European Funding for Regional Development (FEDER).
Ethical approval
This study was conducted in accordance with the recommendations of the 1964 Declaration of Helsinki and its later amendments. The research plan was examined and approved by the medical service and the Institutional Review Board of the Regional Fire and Rescue Service of Doubs, and it was approved by the Ethics Committee of the responsible institutional department.
Informed consent
Prior to testing, all participants gave a voluntary written informed consent which indicates the purpose, the benefits and the risks of the investigation and the possibility to stop their participation at any time.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We would like to acknowledge the firefighters for their participation in this study and their technical support. We also thank the institutional board of the Service Départemental d’Incendie et de Secours du Doubs for its support.
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