Journal of Affective Disorders 162 (2014) 128–133
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Research report
Resilience buffers the impact of traumatic events on the development of PTSD symptoms in firefighters Jong-Sun Lee a, Yeon-Soon Ahn c, Kyoung-Sook Jeong c, Jeong-Ho Chae d, Kyeong-Sook Choi b,n a
Graduate School of Medical Science and Engineering, KAIST, Daejeon, Republic of Korea Department of Neuropsychiatry, Eulji University School of Medicine, Eulji University Hospital, 95 Dunsanseo-ro, Seo-gu, Daejeon 302-799, Republic of Korea c Department of Occupational and Environmental Medicine, Dongguk University, Ilsan Hospital, Goyang, Republic of Korea d Department of Psychiatry, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 October 2013 Received in revised form 18 February 2014 Accepted 19 February 2014 Available online 19 March 2014
Background: Resilience is considered to be a powerful protective factor in buffering the detrimental impact of traumatic stress on the development of posttraumatic stress disorder (PTSD). However, there is a striking lack of research concerning the development of a model of resilience, especially one including both risk and protective factors. The aim of this study was to investigate the possible mediators and moderators influencing the relationship between traumatic stress and PTSD using a moderated mediation analysis. Methods: Study participants included 552 Korean firefighters from four large cities. The subjects completed a series of self-report measures including the Life Event Checklist, the Perceived Stress Scale, the Occupational Stress Scale, the Impact of Event Scale-Revised, and the Connor–Davidson Resilience Scale. Results: Traumatic stress had both a direct and an indirect, via perceived stress, impact on the development of PTSD symptoms. Additionally, the mediation of the association between traumatic stress and PTSD symptoms via perceived stress was moderated by individual resilience. In particular, under the same level of traumatic stress, firefighters with high levels of resilience (scores Z 75, upper 25th percentile or Z90, upper 10th percentile) were protected from both the direct and indirect impacts of traumatic stress relative to those with lower levels of individual resilience. Conclusions: The current findings provide a comprehensive picture of individuals who should be considered at high risk for the development of PTSD symptoms following traumatic stress and identify the factors that should be targeted by efforts to prevent PTSD. & 2014 Elsevier B.V. All rights reserved.
Keywords: Traumatic stress PTSD Perceived stress Resilience Firefighters Moderated mediation
1. Introduction According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR; APA, 2000), exposure to traumatic events is the first criterion for the development of posttraumatic stress disorder (PTSD). Rescue workers, such as firefighters, constitute a high-risk group relative to the general population because they are repeatedly exposed to traumatic events on a daily basis. Notably, the estimated prevalence of PTSD among firefighters ranges from 17% to 22% (Vieweg et al., 2006; Wagner et al., 1998), whereas that of the general population ranges from 1% to 8% (Hauffa et al., 2011; Kessler et al., 1995). Despite this discrepancy, there is a striking lack of research
n
Corresponding author. Tel.: þ 82 42 611 3443; fax: þ 82 42 611 3445. E-mail address: [email protected] (K.-S. Choi).
http://dx.doi.org/10.1016/j.jad.2014.02.031 0165-0327/& 2014 Elsevier B.V. All rights reserved.
concerning PTSD in firefighters. The identification of risk and protective factors and an understanding of the interactive mechanisms underlying the association between exposure to traumatic events and the development of PTSD symptoms have significant implications for elucidating the factors that should be targeted for the prevention of PTSD and identifying the groups that should be considered at high risk for this disorder. One of the risk factors for the development of PTSD symptoms is a maladaptive cognitive appraisal in response to a traumatic event (Bryant and Guthrie, 2007). Individuals with previous or ongoing exposure to trauma are more likely to perceive stressful events as threatening and uncontrollable due to their susceptibility to feelings of pervasive helplessness (Hammack et al., 2012; Maier, 2001) and their limited coping resources (Gilbar et al., 2010; Pietrzak et al., 2010). Evidence of the mediating effect of perceived stress in the relationship between attachment anxiety and PTSD symptoms has been found in individuals who are exposed to
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7 years of ongoing traumatic events (Besser et al., 2009). However, there is a relative lack of research investigating whether perceived stress can mediate the association between exposure to traumatic events and PTSD symptoms in firefighters. Similarly, there is a dearth of research regarding protective factors against the development of PTSD. Recent studies have noted that not all individuals exposed to trauma develop PTSD symptoms and that some are able to manage and cope with adversity; such findings indicate the importance of resilience as a protective factor (Agaibi and Wilson, 2005; Hoge et al., 2007). Resilience can be defined as the ability to adapt and successfully cope with acute or chronic adversity (Connor, 2006; Luthar et al., 2000). Resilience reduces susceptibility to depression and suicide in individuals with childhood trauma (Roy et al., 2011; Wingo et al., 2010) and in veterans (Pietrzak et al., 2009). Therefore, it is important to investigate whether the moderating effects of resilience also protect firefighters who have been exposed to traumatic events from developing of PTSD. Recent models of resilience include risk as well as protective factors that may interact to reduce negative consequences and facilitate positive ones (Fergus and Zimmerman, 2005). In this context, the present study aimed to investigate risk and protective factors in the relationship between traumatic stress and PTSD symptoms, using a moderated mediation analysis. This approach allows for the evaluation of mediation and moderation in the same model as well as for the identification of the ranges within which the moderators have a statistically significant conditional indirect effect (Preacher et al., 2007). This method offers a more comprehensive picture by elucidating specific mechanisms by which exposure to traumatic stress leads to the development of PTSD symptoms and by identifying the individuals who may be more susceptible to such symptoms when exposed to similar experiences. The present study hypothesizes that (1) traumatic stress will increase the risk of developing PTSD symptoms via heightened perceived stress and work-related stress, and that (2) firefighters with high levels of individual resilience will be protected from the detrimental impact of traumatic stress on the development of PTSD. Given that the subjects participated in the present study were firefighters, two types of perceived stress measures were included – one is general and one is job-specific. To examine the above hypotheses, this study first investigated whether perceived stress and work-related stress mediate the association between traumatic stress and PTSD symptoms. This study then examined whether the indirect effect of traumatic stress on PTSD via perceived stress and work-related stress is moderated by individual resilience. This is the first study to evaluate a model of resilience by investigating the interaction between risk and protective factors related to PTSD symptoms using a moderated mediation analysis.
2. Methods and materials 2.1. Participants and procedure A total of 679 firefighters from four large cities in Korea participated in the present study. However, 16 firefighters with previous psychiatric histories and 19 firefighters with family histories of psychiatric illness were excluded from the final analyses. An additional 92 firefighters were also removed from the final analyses due to missing data or low reliability (e.g., the same response on all questionnaires). Thus, the final analyses included a total of 552 firefighters, for whom demographic information is presented in Table 1. Following the completion of informed consent forms, participants filled in a series of questionnaires including the Life Event Checklist, the Perceived Stress
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Table 1 Characteristics of the sample (N ¼522). Characteristics of the sample
%
N
Age 20–29 30–39 40–49 50–59 60–69
15.9 44.2 30.6 9.1 0.2
88 244 169 50 1
93.3 6.7
515 37
years years years years years
Gender Male Female Marital status Married Single Divorced Bereaved Education Middle school High school College/university Graduate school
61.1 33.7 0.9 4.3
337 186 5 24
1.3 34.4 57.1 7.2
7 190 315 40
LEC IES-R PSS KOSS-SF CD-RISC
1.22 10.26 12.54 48.33 64.52
M
SD 1.63 12.92 6.20 7.18 17.08
LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale.
Scale, the Occupational Stress Scale, the Impact of Event ScaleRevised, and the Connor–Davidson Resilience Scale. All data were assigned numeric identifiers to ensure the confidentiality of participants and stored accordingly. Total time for completion of the measures ranged from 40 to 50 min. This study was approved by the Institutional Review Board of Eulji University Hospital and the Institutional Review Board (IRB) of Dongguk University Ilsan Hospital.
2.2. Assessments Demographic information such as gender, age, level of education, and marital status was obtained using a self-report questionnaire. Traumatic events were identified using the validated Korean version of the Life Event Checklist (Bae et al., 2008), which was originally developed by Blake et al. (1995) at the National Center for PTSD using the Clinician-Administered PTSD Scale (CAPS). The Life Event Checklist includes 17 domains of potentially traumatic events, such as disasters, accidents, physical and sexual assaults, and combat-related events, which were identified using multiplechoice responses to specify different levels of exposure to these events (“experienced,” “witnessed,” “learned of,” “not sure,” and “doesn't apply”). The number of traumatic events experienced was summed and included in the final analyses. The original and Korean version of the Life Event Checklist has been shown to possess good psychometric properties (Bae et al., 2008; Gray et al., 2004). Perceived stress was assessed by the validated Korean short version of the Perceived Stress Scale (Park and Seo, 2010), a selfreport questionnaire originally developed by Cohen et al. (1983b). This measure is widely used to identify the extent to which respondents feel that their stress is unpredictable, uncontrollable, and overwhelming (Cohen et al., 1983a). It is a 10-item self-report questionnaire using a five-point Likert scale ranging from 0 (never)
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to 4 (very often). Total scores range from 0 to 40, with higher scores indicating greater perceived stress. Job specific perceived stress was measured using the Korean Occupational Stress Scale- Short Form (Chang et al., 2005). This questionnaire was developed and validated using a nationwide epidemiological study to assess the work-related stress of Korean employees with regard to the physical environment, job demands, job-related control, interpersonal conflict, job insecurity, organizational system, rewards, and occupational climate. This measure contains 24 items and uses a four-point Likert scale ranging from 1 (never) to 4 (always). Total scores range from 24 to 96, with higher scores indicating greater workplace stress. PTSD symptoms were measured using the validated Korean version of the Impact of Event Scale-Revised (Lim et al., 2009). This measure was originally developed by Weiss (2007) to assesses the experience of posttraumatic symptoms such as intrusion, avoidance, and hyper-arousal and consists of 22 items rated on a fivepoint Likert scale ranging from 0 (not at all) to 4 (extremely). Total scores range from 0 to 80, with higher scores indicating more posttraumatic symptoms. Individual resilience was measured using the validated Korean version of the Connor–Davidson Resilience Scale (Baek et al., 2010), which was originally developed by Connor and Davidson (2003). This 25-item scale uses a five-point Likert scale ranging from 0 (not true at all) to 5 (true nearly all of the time) to assess the ability of individuals to tolerate challenges, seek social networks, and adapt to changes. Total scores range from 0 to 100, with higher scores indicative of greater resilience. This measure demonstrates high internal consistency, test–retest reliability, and convergent validity in a general clinical population (Connor and Davidson, 2003). 2.3. Statistical analyses Correlational analyses of five variables (traumatic events, perceived stress, job-related stress, posttraumatic symptoms, and individual resilience) were performed using SPSS 21.0 Software. Moderated mediation analyses were performed on SPSS macro utilizing PROCESS, which was provided by Hayes (2013). The PROCESS analysis in this study included one independent variable (traumatic events), two mediators (perceived stress and workrelated stress), one moderator (individual resilience), and one dependent variable (PTSD symptoms). Gender (male/female) was treated as a covariate, and the variables were mean-centered prior to the analysis. The number of bootstrap samples for the biascorrected bootstrap confidence intervals (CIs) was 5000. The normality of the data for each variable was checked using skewness, kurtosis values, and visual inspection of histograms and box plots. To test the moderated mediation analysis (Model 15), the following conditions were considered: (1) if the effect of the mediator on the dependent variable depends on the moderator, then the effect of the independent variable on the mediator should be significant, and (2) that the conditional indirect effect of the independent variable on the dependent variable via the mediator depends on the presence of a certain range of the moderator. To demonstrate moderated mediation, the second condition is essential (Preacher et al., 2007).
3. Results 3.1. Demographic characteristics of the sample Study participants ranged from 24 to 60 years old (M ¼37.79, SD ¼7.90); 93.3% (n ¼515) were male and 6.7% (n ¼ 37) were female. A total of 61.1% (n ¼337) were married, 33.7% (n ¼186)
were single, 0.9% (n ¼ 5) were divorced, and 4.3% (n ¼ 24) were widowed. The educational experience of participants ranged from 9 to 20 years. Further details concerning the characteristics of the sample are presented in Table 1. 3.2. Preliminary correlation analyses Correlations of the five variables are presented in Table 2. Traumatic events were positively associated with PTSD symptoms, perceived stress, and work-related stress. PTSD symptoms were significantly correlated with perceived stress and inversely correlated with individual resilience. Perceived stress was positively related to work-related stress. Finally, work-related stress was significantly associated with individual resilience. An inspection of correlation values revealed that all the variables used in the present study had significant associations with each other. Therefore, there was no unnecessary factor for further analysis. 3.3. Moderated mediation First, the mediator variable model (Table 3) indicated that the number of traumatic events the firefighters experienced was positively correlated with their level of perceived stress (p o0.001). A similar pattern was observed in the association between traumatic events and work-related stress in that the number of traumatic events experienced by the firefighters was positively associated with the amount of work-related stress (p ¼0.046). Second, the dependent variable model indicated that PTSD symptoms were positively influenced by perceived stress (po0.001) and significantly affected by traumatic stress (po0.001). However, workrelated stress did not affect PTSD symptoms. These results suggest that perceived stress, but not work-related stress, partially mediates the relationship between traumatic events and the development of PTSD symptoms. A bootstrapped 95% CI confirmed that the indirect effect of perceived stress was significant (LLCI: 0.5927, ULCI: 1.7883). The interaction between perceived stress and individual resilience (Interaction 1) and the interaction between traumatic events and individual resilience (Interaction 3) were significant, but the interaction between work-related stress and individual resilience (Interaction 2) was not. Importantly, the conditional indirect effect of traumatic events on PTSD symptoms via perceived stress differed depending on the degree to which a subject was resilient (Table 3). More specifically, the conditional indirect effect was weaker in the upper 25th percentile of firefighters (scoresZ75), whereas it was significant up to the 50th percentile of firefighters (a bootstrapped 95% CI did not include zero). These results imply that the greater the number of traumatic events experienced by the firefighters, the greater the level of their perceived stress; the latter, in turn, increases their vulnerability to developing PTSD symptoms. However, firefighters with a high level of individual resilience (upper 25th percentile) Table 2 Mean, Standard deviation, correlation between main variables (N ¼522). Variables
1
1. LEC 2. IES-R 3. PSS 4. KOSS-SF 5. CD-RISC
1 0.28nnn 0.27nnn 0.09n 0.04
2
3
4
5
1 0.26nnn 0.05 0.23nnn
1 0.28nnn 0.003
1 0.16nnn
1
LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale. nnn n
po 0.001. p o 0.05.
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Table 3 Moderated mediation analysis (N ¼ 522). Outcome variable: PSS
Mediator variable model LEC
Outcome variable: KOSS-SF
B (SE)
t
P
B (SE)
t
P
0.98 (0.16)
6.31
o 0.001
0.38 (0.19)
2.00
0.046
Outcome variable: PTSD symptoms B (SE) Dependent variable model PSS KOSS-SF IES-R CD-RISC Interaction 1 Interaction 2 Interaction 3 CD-RISC
0.41 0.02 1.60 0.17 0.02 0.00 0.04
t
(0.09) (0.08) (0.32) (0.03) (0.00) (0.00) (0.02)
Effect (SE)
P
o 0.001 NS o 0.001 o 0.001 o 0.001 NS 0.03
4.70 0.21 5.06 5.86 4.24 0.10 2.16 t
LLCI
Conditional direct effect of traumatic life events on PTSD symptoms at values of the individual resilience 18.52 (10th percentile) 2.3988 (0.4673) 5.13nnn 12.52 (25th percentile) 2.1383 (0.3868) 5.53nnn 0.52 (50th percentile) 1.6174 (0.3146) 5.14nnn 10.48 (75th percentile) 1.1399 (0.3929) 2.90nn 25.48 (90th percentile) 0.4888 (0.6220) 0.79 Mediator
Moderator: CD-RISC
Indirect effect (SE)
ULCI
1.4807 1.3785 0.9994 0.3681 0.7330
LL 95% CI
3.3168 2.8982 2.2354 1.9117 1.7106 UL 95% CI
Conditional indirect effect of perceived stress in the relationship between traumatic life events and PTSD symptoms at specific levels of individual resilience PSS 18.5174 (10th percentile) 0.7671 (0.2036) 0.3814 1.1712 12.5174 (25th percentile) 0.6484 (0.1704) 0.3471 0.9942 0.5174 (50th percentile) 0.4109 (0.1191) 0.1966 0.6737 10.4826 (75th percentile) 0.1933 (0.1103) 0.0479 0.4002 25.4826 (90th percentile) 0.1035 (0.1654) 0.5189 0.1540 Interaction 1: PSS CD-RISK; Interaction 2: KOSS-SF CD-RISK; Interaction 3: LEC CD-RISK. LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale. nnn nn
p o 0.001. p o 0.01.
may be protected from the risk of developing PTSD symptoms (Fig. 1). The conditional direct effect of traumatic events on PTSD symptoms also differed depending on the amount of a subject's individual resilience. The conditional direct effect on the upper 10th percentile of firefighters (scores Z90) was not significant, whereas the direct effect of traumatic stress events on PTSD symptoms on firefighters with scores up to the 75th percentile was significant (Table 3). This result indicates that firefighters with high levels of individual resilience (upper 10th percentile) are better protected from the risk of developing PTSD symptoms compared with those with lower levels of individual resilience (up to the 75th percentile). The final model of moderated mediation in the present study is presented in Fig. 2.
4. Discussion The present study aimed to identify the mechanisms underlying interactions among the risk and protective factors related to PTSD symptoms using a moderated mediation analysis. This study yielded two key findings. First, perceived stress partially mediates the relationship between traumatic stress and PTSD symptoms. This implies that exposure to traumatic events directly and indirectly, via the amplification of perceived stress, influences the development of PTSD symptoms. Second, the mediating pathway of the association between traumatic stress and PTSD symptoms through perceived stress was moderated by individual
resilience. Resilience also buffered the direct pathway of traumatic stress on PTSD symptoms. Although the relationship between exposure to trauma and PTSD is well-known and frequently studied, the specific mechanisms by which PTSD symptoms develop following such experiences have yet to be fully established. A stressor tends to trigger a stress reaction if that stressor is perceived as threatening and overwhelming. In this manner, the transactional model of stress highlights the importance of the perception of stress in the association between stressful events and psychiatric symptoms (Lazarus and Folkman, 1984). The present findings suggest that firefighters with high levels of traumatic stress may be more likely to perceive their stressors as uncontrollable and threatening which, in turn, results in a vulnerability to the development of PTSD symptoms. These findings is in line with prior research showing that high levels of perceived stress are highly predictive of the severity of depressive symptoms (Lee et al., 2013; Pedrelli et al., 2008). The present study expands on previous findings by demonstrating that trauma exposure in firefighters has a significant impact on PTSD symptoms via perceived stress. The fact that trauma exposure directly influences PTSD symptoms apart from indirect links via perceived stress indicates that there may be additional relevant variables mediating the association between trauma exposure and PTSD symptoms. Thus, future research should explore a greater variety of mediators to provide a better understanding of the specific mechanisms operating in the relationship between trauma exposure and the development of PTSD symptoms.
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However, the mediating effect of work-related stress in the relationship between traumatic stress and PTSD symptoms was not supported. The present results demonstrate that the greater the number of traumatic events a firefighter has experienced, the higher the level of work-related stress the firefighter will have. However, this does not necessarily lead to the development of PTSD symptoms. As Cohen et al. (1983b) indicated, the fact that perceived stress, but not work-related stress, mediates the relationship between trauma exposure and PTSD symptoms implies that psychological symptoms can be influenced by an individual's perception of global stress levels rather than specific stress levels. It is important to note that not all firefighters with high levels of trauma exposure develop PTSD symptoms via an increase of perceived stress. Some firefighters with high levels of resilience (upper 25th percentile; Z 75) appeared to overcome adversity and were protected from the risk of developing PTSD symptoms, whereas those with lower levels of individual resilience (up to the 75th percentile; o75) were more susceptible to the development of PTSD symptoms via the escalation of perceived stress (conditional indirect effect). Furthermore, firefighters with high levels of individual resilience (upper 10th percentile; Z90) can escape from the detrimental impact of traumatic stress on PTSD symptoms (conditional direct effect) compared with those with scores up to the 90th percentile who experienced the same level of
A B C D E
trauma stress. These findings illustrate the importance of individual resilience as a protective factor against PTSD symptoms in firefighters with high levels of traumatic stress and/or perceived stress. The finding that resilience is not significantly correlated with traumatic stress events suggests that resilience might be a trait rather than a state. The current results further underscore individual resilience as a key factor when evaluating fitness for duty in firefighters frequently exposed to traumatic events. This study has several limitations that should be noted. The cross-sectional design of this study limits its ability to confirm a causal relationship between the variables that were investigated. Thus, the results of the present study should be interpreted cautiously, and future research using longitudinal and experimental designs is needed to clarify the direction of any causal relationships between the observed variables. Secondly, the present research relied solely on self-report questionnaires. Even though all the scales have been validated, future studies utilizing standardized interviews or observational methods would provide more accurate and detailed information regarding the complex mechanisms underlying the relationship between traumatic stress and the development of PTSD symptoms. Thirdly, although the life events checklist used in the present study included traumatic events that are often experienced by the firefighters, future research focusing on firefighter-specific traumatic events would provide more
A
B
A B C D E
A B
C C D
D
E
E
Fig. 1. The number of traumatic stress events (left) and the level of perceived stress (right) as a function of individual resilience. This interaction predicts PTSD symptoms. The slope indicates that as the number of traumatic events (left) or the level of perceived stress (right) increases PTSD symptoms also increase. However, the slope is increasingly reduced as the level of individual resilience increases.
Individual resilience Global Perceived stress Job specific perceived stress
Traumatic stress events
PTSD symptoms
Fig. 2. The final moderated mediation model. The indirect link associating traumatic stress and PTSD symptoms via perceived stress is moderated by individual resilience. The direct link associating traumatic stress and PTSD symptoms is moderated by individual resilience. The dotted line indicates a non-significant pathway in the association between traumatic stress and PTSD symptoms.
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relevant information in understanding firefighters' traumatic experiences. Finally, the present study included only a few female participants. For this reason, future research with a large number of female participants is needed to determine whether the results of the present study can be generalized to women. Despite these limitations, the present study is the first to use a moderated mediation analysis to investigate the possible mediators and moderators of PTSD symptoms in firefighters and to evaluate the interactive mechanisms underlying the development of these symptoms following trauma exposure. The current findings offer a better understanding of those who are at high risk for PTSD and of the factors that should be targeted by efforts to prevent the development of PTSD symptoms in firefighters. Based on these findings, clinical interventions, especially those targeting the cognitive reappraisal of traumatic stressors and those that bolster individual resilience, may be effective for firefighters with a low level of resilience who have experienced many traumatic events. Finally, as Bonanno (2004) indicated, finding multiple pathways or factors to facilitate resilience might shed light on the development of effective intervention against PTSD. Role of funding source This work was supported by Grant No KFI-2009-Next generation-001 from Korea National Emergency Management Agency.
Conflict of interest The corresponding author confirms that all authors have disclosed any potential competing interests regarding the submitted article and the nature of those interests.
Acknowledgments We acknowledge Grant support of the Korea National Emergency Management (No. KFI-2009-Next generation-001). We also thank all the participating firefighters in the present study.
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Cohen, S., Kamarck, T., Mermelstein, R., 1983b. A global measure of perceived stress. J. Health Soc. Behav. 24, 385–396. Connor, K.M., 2006. Assessment of resilience in the aftermath of trauma. J. Clin. Psychiatry 67, 46–49. Connor, K.M., Davidson, J.R., 2003. Development of a new resilience scale: the Connor–Davidson Resilience Scale (CD-RISC). Depress. Anxiety 18, 76–82. Fergus, S., Zimmerman, M.A., 2005. Adolescent resilience: a framework for understanding healthy development in the face of risk. Annu. Rev. Public Health 26, 399–419. Gilbar, O., Plivazky, N., Gil, S., 2010. Counterfactual thinking, coping strategies, and coping resources as predictors of PTSD diagnosed in physically injured victims of terror attacks. J. Loss Trauma 15, 304–324. Gray, M.J., Litz, B.T., Hsu, J.L., Lombardo, T.W., 2004. Psychometric properties of the life events checklist. Assessment 11, 330–341. Hammack, S.E., Cooper, M.A., Lezak, K.R., 2012. Overlapping neurobiology of learned helplessness and conditioned defeat: implications for PTSD and mood disorders. Neuropharmacology 62, 565–575. Hauffa, R., Rief, W., Brahler, E., Martin, A., Mewes, R., Glaesmer, H., 2011. Lifetime traumatic experiences and posttraumatic stress disorder in the german population results of a representative population survey. J. Nerv. Ment. Dis. 199, 934–939. Hayes, A., 2013. An Introduction to Mediation, Moderation, and Conditional Process Analysis: a Regression-Based Approach. Guilford, New York. Hoge, E.A., Austin, E.D., Pollack, M.H., 2007. Resilience: research evidence and conceptual considerations for posttraumatic stress disorder. Depress. Anxiety 24, 139–152. Kessler, R.C., Sonnega, A., Bromet, E., Hughes, M., Nelson, C.B., 1995. Posttraumaticstress-disorder in the national comorbidity survey. Arch. Gen. Psychiatry 52, 1048–1060. Lazarus, R.S., Folkman, S., 1984. Stress, Appraisal, and Coping. Springer Publishing Company, New York. Lee, J.S., Joo, E.J., Choi, K.S., 2013. Perceived stress and self-esteem mediate the effects of work-related stress on depression. Stress Health 29, 75–81. Lim, H.K., Woo, J.M., Kim, T.S., Kim, T.H., Choi, K.S., Chung, S.K., Chee, I.S., Lee, K.U., Paik, K.C., Seo, H.J., Kim, W., Jin, B., Chae, J.H., 2009. Disaster Psychiatry Committee in Korean Academy of Anxiety, Disorders, 2009. Reliability and validity of the Korean version of the Impact of Event Scale-Revised. Compr. Psychiatry 50, 385–390. Luthar, S.S., Cicchetti, D., Becker, B., 2000. The construct of resilience: a critical evaluation and guidelines for future work. Child Dev. 71, 543–562. Maier, S.F., 2001. Exposure to the stressor environment prevents the temporal dissipation of behavioral depression/learned helplessness. Biol. Psychiatry 49, 763–773. Park, J., Seo, Y., 2010. Validation of the Perceived Stress Scale (PSS) on samples of Korean University Students. Korean J. Psychol.: Gen. 29, 611–629. Pedrelli, P., Feldman, G.C., Vorono, S., Fava, M., Petersen, T., 2008. Dysfunctional attitudes and perceived stress predict depressive symptoms severity following antidepressant treatment in patients with chronic depression. Psychiatry Res. 161, 302–308. Pietrzak, R.H., Johnson, D.C., Goldstein, M.B., Malley, J.C., Rivers, A.J., Morgan, C.A., Southwick, S.M., 2010. Psychosocial buffers of traumatic stress, depressive symptoms, and psychosocial difficulties in veterans of Operations Enduring Freedom and Iraqi Freedom: the role of resilience, unit support, and postdeployment social support. J. Affect. Disord. 120, 188–192. Pietrzak, R.H., Johnson, D.C., Goldstein, M.B., Malley, J.C., Southwick, S.M., 2009. Psychological resilience and postdeployment social support protect against traumatic stress and depressive symptoms in soldiers returning from Operations Enduring Freedom and Iraqi Freedom. Depress. Anxiety 26, 745–751. Preacher, K.J., Rucker, D.D., Hayes, A.F., 2007. Addressing moderated mediation hypotheses: theory, methods, and prescriptions. Multivar. Behav. Res. 42, 185–227. Roy, A., Carli, V., Sarchiapone, M., 2011. Resilience mitigates the suicide risk associated with childhood trauma. J. Affect. Disord. 133, 591–594. Vieweg, W.V.R., Julius, D.A., Fernandez, A., Beatty-Brooks, M., Hettema, J.M., Pandurangi, A.K., 2006. Posttraumatic stress disorder: clinical features, pathophysiology, and treatment. Am. J. Med. 119, 383–390. Wagner, D., Heinrichs, M., Ehlert, U., 1998. Prevalence of symptoms of posttraumatic stress disorder in German professional firefighters. Am. J. Psychiatry 155, 1727–1732. Weiss, D.S., 2007. The impact of event scale: revised. In: Wilson, J.P., Tang, C.S. (Eds.), Cross-Cultural Assessment of Psychological Trauma and PTSD. Springer, New York, pp. 219–238. Wingo, A.P., Wrenn, G., Pelletier, T., Gutman, A.R., Bradley, B., Ressler, K.J., 2010. Moderating effects of resilience on depression in individuals with a history of childhood abuse or trauma exposure. J. Affect. Disord. 126, 411–414.
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Research report
Resilience buffers the impact of traumatic events on the development of PTSD symptoms in firefighters Jong-Sun Lee a, Yeon-Soon Ahn c, Kyoung-Sook Jeong c, Jeong-Ho Chae d, Kyeong-Sook Choi b,n a
Graduate School of Medical Science and Engineering, KAIST, Daejeon, Republic of Korea Department of Neuropsychiatry, Eulji University School of Medicine, Eulji University Hospital, 95 Dunsanseo-ro, Seo-gu, Daejeon 302-799, Republic of Korea c Department of Occupational and Environmental Medicine, Dongguk University, Ilsan Hospital, Goyang, Republic of Korea d Department of Psychiatry, College of Medicine, The Catholic University of Korea, Seoul, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 10 October 2013 Received in revised form 18 February 2014 Accepted 19 February 2014 Available online 19 March 2014
Background: Resilience is considered to be a powerful protective factor in buffering the detrimental impact of traumatic stress on the development of posttraumatic stress disorder (PTSD). However, there is a striking lack of research concerning the development of a model of resilience, especially one including both risk and protective factors. The aim of this study was to investigate the possible mediators and moderators influencing the relationship between traumatic stress and PTSD using a moderated mediation analysis. Methods: Study participants included 552 Korean firefighters from four large cities. The subjects completed a series of self-report measures including the Life Event Checklist, the Perceived Stress Scale, the Occupational Stress Scale, the Impact of Event Scale-Revised, and the Connor–Davidson Resilience Scale. Results: Traumatic stress had both a direct and an indirect, via perceived stress, impact on the development of PTSD symptoms. Additionally, the mediation of the association between traumatic stress and PTSD symptoms via perceived stress was moderated by individual resilience. In particular, under the same level of traumatic stress, firefighters with high levels of resilience (scores Z 75, upper 25th percentile or Z90, upper 10th percentile) were protected from both the direct and indirect impacts of traumatic stress relative to those with lower levels of individual resilience. Conclusions: The current findings provide a comprehensive picture of individuals who should be considered at high risk for the development of PTSD symptoms following traumatic stress and identify the factors that should be targeted by efforts to prevent PTSD. & 2014 Elsevier B.V. All rights reserved.
Keywords: Traumatic stress PTSD Perceived stress Resilience Firefighters Moderated mediation
1. Introduction According to the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR; APA, 2000), exposure to traumatic events is the first criterion for the development of posttraumatic stress disorder (PTSD). Rescue workers, such as firefighters, constitute a high-risk group relative to the general population because they are repeatedly exposed to traumatic events on a daily basis. Notably, the estimated prevalence of PTSD among firefighters ranges from 17% to 22% (Vieweg et al., 2006; Wagner et al., 1998), whereas that of the general population ranges from 1% to 8% (Hauffa et al., 2011; Kessler et al., 1995). Despite this discrepancy, there is a striking lack of research
n
Corresponding author. Tel.: þ 82 42 611 3443; fax: þ 82 42 611 3445. E-mail address: [email protected] (K.-S. Choi).
http://dx.doi.org/10.1016/j.jad.2014.02.031 0165-0327/& 2014 Elsevier B.V. All rights reserved.
concerning PTSD in firefighters. The identification of risk and protective factors and an understanding of the interactive mechanisms underlying the association between exposure to traumatic events and the development of PTSD symptoms have significant implications for elucidating the factors that should be targeted for the prevention of PTSD and identifying the groups that should be considered at high risk for this disorder. One of the risk factors for the development of PTSD symptoms is a maladaptive cognitive appraisal in response to a traumatic event (Bryant and Guthrie, 2007). Individuals with previous or ongoing exposure to trauma are more likely to perceive stressful events as threatening and uncontrollable due to their susceptibility to feelings of pervasive helplessness (Hammack et al., 2012; Maier, 2001) and their limited coping resources (Gilbar et al., 2010; Pietrzak et al., 2010). Evidence of the mediating effect of perceived stress in the relationship between attachment anxiety and PTSD symptoms has been found in individuals who are exposed to
J.-S. Lee et al. / Journal of Affective Disorders 162 (2014) 128–133
7 years of ongoing traumatic events (Besser et al., 2009). However, there is a relative lack of research investigating whether perceived stress can mediate the association between exposure to traumatic events and PTSD symptoms in firefighters. Similarly, there is a dearth of research regarding protective factors against the development of PTSD. Recent studies have noted that not all individuals exposed to trauma develop PTSD symptoms and that some are able to manage and cope with adversity; such findings indicate the importance of resilience as a protective factor (Agaibi and Wilson, 2005; Hoge et al., 2007). Resilience can be defined as the ability to adapt and successfully cope with acute or chronic adversity (Connor, 2006; Luthar et al., 2000). Resilience reduces susceptibility to depression and suicide in individuals with childhood trauma (Roy et al., 2011; Wingo et al., 2010) and in veterans (Pietrzak et al., 2009). Therefore, it is important to investigate whether the moderating effects of resilience also protect firefighters who have been exposed to traumatic events from developing of PTSD. Recent models of resilience include risk as well as protective factors that may interact to reduce negative consequences and facilitate positive ones (Fergus and Zimmerman, 2005). In this context, the present study aimed to investigate risk and protective factors in the relationship between traumatic stress and PTSD symptoms, using a moderated mediation analysis. This approach allows for the evaluation of mediation and moderation in the same model as well as for the identification of the ranges within which the moderators have a statistically significant conditional indirect effect (Preacher et al., 2007). This method offers a more comprehensive picture by elucidating specific mechanisms by which exposure to traumatic stress leads to the development of PTSD symptoms and by identifying the individuals who may be more susceptible to such symptoms when exposed to similar experiences. The present study hypothesizes that (1) traumatic stress will increase the risk of developing PTSD symptoms via heightened perceived stress and work-related stress, and that (2) firefighters with high levels of individual resilience will be protected from the detrimental impact of traumatic stress on the development of PTSD. Given that the subjects participated in the present study were firefighters, two types of perceived stress measures were included – one is general and one is job-specific. To examine the above hypotheses, this study first investigated whether perceived stress and work-related stress mediate the association between traumatic stress and PTSD symptoms. This study then examined whether the indirect effect of traumatic stress on PTSD via perceived stress and work-related stress is moderated by individual resilience. This is the first study to evaluate a model of resilience by investigating the interaction between risk and protective factors related to PTSD symptoms using a moderated mediation analysis.
2. Methods and materials 2.1. Participants and procedure A total of 679 firefighters from four large cities in Korea participated in the present study. However, 16 firefighters with previous psychiatric histories and 19 firefighters with family histories of psychiatric illness were excluded from the final analyses. An additional 92 firefighters were also removed from the final analyses due to missing data or low reliability (e.g., the same response on all questionnaires). Thus, the final analyses included a total of 552 firefighters, for whom demographic information is presented in Table 1. Following the completion of informed consent forms, participants filled in a series of questionnaires including the Life Event Checklist, the Perceived Stress
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Table 1 Characteristics of the sample (N ¼522). Characteristics of the sample
%
N
Age 20–29 30–39 40–49 50–59 60–69
15.9 44.2 30.6 9.1 0.2
88 244 169 50 1
93.3 6.7
515 37
years years years years years
Gender Male Female Marital status Married Single Divorced Bereaved Education Middle school High school College/university Graduate school
61.1 33.7 0.9 4.3
337 186 5 24
1.3 34.4 57.1 7.2
7 190 315 40
LEC IES-R PSS KOSS-SF CD-RISC
1.22 10.26 12.54 48.33 64.52
M
SD 1.63 12.92 6.20 7.18 17.08
LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale.
Scale, the Occupational Stress Scale, the Impact of Event ScaleRevised, and the Connor–Davidson Resilience Scale. All data were assigned numeric identifiers to ensure the confidentiality of participants and stored accordingly. Total time for completion of the measures ranged from 40 to 50 min. This study was approved by the Institutional Review Board of Eulji University Hospital and the Institutional Review Board (IRB) of Dongguk University Ilsan Hospital.
2.2. Assessments Demographic information such as gender, age, level of education, and marital status was obtained using a self-report questionnaire. Traumatic events were identified using the validated Korean version of the Life Event Checklist (Bae et al., 2008), which was originally developed by Blake et al. (1995) at the National Center for PTSD using the Clinician-Administered PTSD Scale (CAPS). The Life Event Checklist includes 17 domains of potentially traumatic events, such as disasters, accidents, physical and sexual assaults, and combat-related events, which were identified using multiplechoice responses to specify different levels of exposure to these events (“experienced,” “witnessed,” “learned of,” “not sure,” and “doesn't apply”). The number of traumatic events experienced was summed and included in the final analyses. The original and Korean version of the Life Event Checklist has been shown to possess good psychometric properties (Bae et al., 2008; Gray et al., 2004). Perceived stress was assessed by the validated Korean short version of the Perceived Stress Scale (Park and Seo, 2010), a selfreport questionnaire originally developed by Cohen et al. (1983b). This measure is widely used to identify the extent to which respondents feel that their stress is unpredictable, uncontrollable, and overwhelming (Cohen et al., 1983a). It is a 10-item self-report questionnaire using a five-point Likert scale ranging from 0 (never)
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to 4 (very often). Total scores range from 0 to 40, with higher scores indicating greater perceived stress. Job specific perceived stress was measured using the Korean Occupational Stress Scale- Short Form (Chang et al., 2005). This questionnaire was developed and validated using a nationwide epidemiological study to assess the work-related stress of Korean employees with regard to the physical environment, job demands, job-related control, interpersonal conflict, job insecurity, organizational system, rewards, and occupational climate. This measure contains 24 items and uses a four-point Likert scale ranging from 1 (never) to 4 (always). Total scores range from 24 to 96, with higher scores indicating greater workplace stress. PTSD symptoms were measured using the validated Korean version of the Impact of Event Scale-Revised (Lim et al., 2009). This measure was originally developed by Weiss (2007) to assesses the experience of posttraumatic symptoms such as intrusion, avoidance, and hyper-arousal and consists of 22 items rated on a fivepoint Likert scale ranging from 0 (not at all) to 4 (extremely). Total scores range from 0 to 80, with higher scores indicating more posttraumatic symptoms. Individual resilience was measured using the validated Korean version of the Connor–Davidson Resilience Scale (Baek et al., 2010), which was originally developed by Connor and Davidson (2003). This 25-item scale uses a five-point Likert scale ranging from 0 (not true at all) to 5 (true nearly all of the time) to assess the ability of individuals to tolerate challenges, seek social networks, and adapt to changes. Total scores range from 0 to 100, with higher scores indicative of greater resilience. This measure demonstrates high internal consistency, test–retest reliability, and convergent validity in a general clinical population (Connor and Davidson, 2003). 2.3. Statistical analyses Correlational analyses of five variables (traumatic events, perceived stress, job-related stress, posttraumatic symptoms, and individual resilience) were performed using SPSS 21.0 Software. Moderated mediation analyses were performed on SPSS macro utilizing PROCESS, which was provided by Hayes (2013). The PROCESS analysis in this study included one independent variable (traumatic events), two mediators (perceived stress and workrelated stress), one moderator (individual resilience), and one dependent variable (PTSD symptoms). Gender (male/female) was treated as a covariate, and the variables were mean-centered prior to the analysis. The number of bootstrap samples for the biascorrected bootstrap confidence intervals (CIs) was 5000. The normality of the data for each variable was checked using skewness, kurtosis values, and visual inspection of histograms and box plots. To test the moderated mediation analysis (Model 15), the following conditions were considered: (1) if the effect of the mediator on the dependent variable depends on the moderator, then the effect of the independent variable on the mediator should be significant, and (2) that the conditional indirect effect of the independent variable on the dependent variable via the mediator depends on the presence of a certain range of the moderator. To demonstrate moderated mediation, the second condition is essential (Preacher et al., 2007).
3. Results 3.1. Demographic characteristics of the sample Study participants ranged from 24 to 60 years old (M ¼37.79, SD ¼7.90); 93.3% (n ¼515) were male and 6.7% (n ¼ 37) were female. A total of 61.1% (n ¼337) were married, 33.7% (n ¼186)
were single, 0.9% (n ¼ 5) were divorced, and 4.3% (n ¼ 24) were widowed. The educational experience of participants ranged from 9 to 20 years. Further details concerning the characteristics of the sample are presented in Table 1. 3.2. Preliminary correlation analyses Correlations of the five variables are presented in Table 2. Traumatic events were positively associated with PTSD symptoms, perceived stress, and work-related stress. PTSD symptoms were significantly correlated with perceived stress and inversely correlated with individual resilience. Perceived stress was positively related to work-related stress. Finally, work-related stress was significantly associated with individual resilience. An inspection of correlation values revealed that all the variables used in the present study had significant associations with each other. Therefore, there was no unnecessary factor for further analysis. 3.3. Moderated mediation First, the mediator variable model (Table 3) indicated that the number of traumatic events the firefighters experienced was positively correlated with their level of perceived stress (p o0.001). A similar pattern was observed in the association between traumatic events and work-related stress in that the number of traumatic events experienced by the firefighters was positively associated with the amount of work-related stress (p ¼0.046). Second, the dependent variable model indicated that PTSD symptoms were positively influenced by perceived stress (po0.001) and significantly affected by traumatic stress (po0.001). However, workrelated stress did not affect PTSD symptoms. These results suggest that perceived stress, but not work-related stress, partially mediates the relationship between traumatic events and the development of PTSD symptoms. A bootstrapped 95% CI confirmed that the indirect effect of perceived stress was significant (LLCI: 0.5927, ULCI: 1.7883). The interaction between perceived stress and individual resilience (Interaction 1) and the interaction between traumatic events and individual resilience (Interaction 3) were significant, but the interaction between work-related stress and individual resilience (Interaction 2) was not. Importantly, the conditional indirect effect of traumatic events on PTSD symptoms via perceived stress differed depending on the degree to which a subject was resilient (Table 3). More specifically, the conditional indirect effect was weaker in the upper 25th percentile of firefighters (scoresZ75), whereas it was significant up to the 50th percentile of firefighters (a bootstrapped 95% CI did not include zero). These results imply that the greater the number of traumatic events experienced by the firefighters, the greater the level of their perceived stress; the latter, in turn, increases their vulnerability to developing PTSD symptoms. However, firefighters with a high level of individual resilience (upper 25th percentile) Table 2 Mean, Standard deviation, correlation between main variables (N ¼522). Variables
1
1. LEC 2. IES-R 3. PSS 4. KOSS-SF 5. CD-RISC
1 0.28nnn 0.27nnn 0.09n 0.04
2
3
4
5
1 0.26nnn 0.05 0.23nnn
1 0.28nnn 0.003
1 0.16nnn
1
LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale. nnn n
po 0.001. p o 0.05.
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Table 3 Moderated mediation analysis (N ¼ 522). Outcome variable: PSS
Mediator variable model LEC
Outcome variable: KOSS-SF
B (SE)
t
P
B (SE)
t
P
0.98 (0.16)
6.31
o 0.001
0.38 (0.19)
2.00
0.046
Outcome variable: PTSD symptoms B (SE) Dependent variable model PSS KOSS-SF IES-R CD-RISC Interaction 1 Interaction 2 Interaction 3 CD-RISC
0.41 0.02 1.60 0.17 0.02 0.00 0.04
t
(0.09) (0.08) (0.32) (0.03) (0.00) (0.00) (0.02)
Effect (SE)
P
o 0.001 NS o 0.001 o 0.001 o 0.001 NS 0.03
4.70 0.21 5.06 5.86 4.24 0.10 2.16 t
LLCI
Conditional direct effect of traumatic life events on PTSD symptoms at values of the individual resilience 18.52 (10th percentile) 2.3988 (0.4673) 5.13nnn 12.52 (25th percentile) 2.1383 (0.3868) 5.53nnn 0.52 (50th percentile) 1.6174 (0.3146) 5.14nnn 10.48 (75th percentile) 1.1399 (0.3929) 2.90nn 25.48 (90th percentile) 0.4888 (0.6220) 0.79 Mediator
Moderator: CD-RISC
Indirect effect (SE)
ULCI
1.4807 1.3785 0.9994 0.3681 0.7330
LL 95% CI
3.3168 2.8982 2.2354 1.9117 1.7106 UL 95% CI
Conditional indirect effect of perceived stress in the relationship between traumatic life events and PTSD symptoms at specific levels of individual resilience PSS 18.5174 (10th percentile) 0.7671 (0.2036) 0.3814 1.1712 12.5174 (25th percentile) 0.6484 (0.1704) 0.3471 0.9942 0.5174 (50th percentile) 0.4109 (0.1191) 0.1966 0.6737 10.4826 (75th percentile) 0.1933 (0.1103) 0.0479 0.4002 25.4826 (90th percentile) 0.1035 (0.1654) 0.5189 0.1540 Interaction 1: PSS CD-RISK; Interaction 2: KOSS-SF CD-RISK; Interaction 3: LEC CD-RISK. LEC: Life Event Checklist; IES-R: Impact of the Event Scale-Revised; PSS: Perceived Stress Scale; KOSS-SF: Korean-Occupational Stress Scale-Short Form; and CD-RISC: Connor–Davidson Resilience Scale. nnn nn
p o 0.001. p o 0.01.
may be protected from the risk of developing PTSD symptoms (Fig. 1). The conditional direct effect of traumatic events on PTSD symptoms also differed depending on the amount of a subject's individual resilience. The conditional direct effect on the upper 10th percentile of firefighters (scores Z90) was not significant, whereas the direct effect of traumatic stress events on PTSD symptoms on firefighters with scores up to the 75th percentile was significant (Table 3). This result indicates that firefighters with high levels of individual resilience (upper 10th percentile) are better protected from the risk of developing PTSD symptoms compared with those with lower levels of individual resilience (up to the 75th percentile). The final model of moderated mediation in the present study is presented in Fig. 2.
4. Discussion The present study aimed to identify the mechanisms underlying interactions among the risk and protective factors related to PTSD symptoms using a moderated mediation analysis. This study yielded two key findings. First, perceived stress partially mediates the relationship between traumatic stress and PTSD symptoms. This implies that exposure to traumatic events directly and indirectly, via the amplification of perceived stress, influences the development of PTSD symptoms. Second, the mediating pathway of the association between traumatic stress and PTSD symptoms through perceived stress was moderated by individual
resilience. Resilience also buffered the direct pathway of traumatic stress on PTSD symptoms. Although the relationship between exposure to trauma and PTSD is well-known and frequently studied, the specific mechanisms by which PTSD symptoms develop following such experiences have yet to be fully established. A stressor tends to trigger a stress reaction if that stressor is perceived as threatening and overwhelming. In this manner, the transactional model of stress highlights the importance of the perception of stress in the association between stressful events and psychiatric symptoms (Lazarus and Folkman, 1984). The present findings suggest that firefighters with high levels of traumatic stress may be more likely to perceive their stressors as uncontrollable and threatening which, in turn, results in a vulnerability to the development of PTSD symptoms. These findings is in line with prior research showing that high levels of perceived stress are highly predictive of the severity of depressive symptoms (Lee et al., 2013; Pedrelli et al., 2008). The present study expands on previous findings by demonstrating that trauma exposure in firefighters has a significant impact on PTSD symptoms via perceived stress. The fact that trauma exposure directly influences PTSD symptoms apart from indirect links via perceived stress indicates that there may be additional relevant variables mediating the association between trauma exposure and PTSD symptoms. Thus, future research should explore a greater variety of mediators to provide a better understanding of the specific mechanisms operating in the relationship between trauma exposure and the development of PTSD symptoms.
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However, the mediating effect of work-related stress in the relationship between traumatic stress and PTSD symptoms was not supported. The present results demonstrate that the greater the number of traumatic events a firefighter has experienced, the higher the level of work-related stress the firefighter will have. However, this does not necessarily lead to the development of PTSD symptoms. As Cohen et al. (1983b) indicated, the fact that perceived stress, but not work-related stress, mediates the relationship between trauma exposure and PTSD symptoms implies that psychological symptoms can be influenced by an individual's perception of global stress levels rather than specific stress levels. It is important to note that not all firefighters with high levels of trauma exposure develop PTSD symptoms via an increase of perceived stress. Some firefighters with high levels of resilience (upper 25th percentile; Z 75) appeared to overcome adversity and were protected from the risk of developing PTSD symptoms, whereas those with lower levels of individual resilience (up to the 75th percentile; o75) were more susceptible to the development of PTSD symptoms via the escalation of perceived stress (conditional indirect effect). Furthermore, firefighters with high levels of individual resilience (upper 10th percentile; Z90) can escape from the detrimental impact of traumatic stress on PTSD symptoms (conditional direct effect) compared with those with scores up to the 90th percentile who experienced the same level of
A B C D E
trauma stress. These findings illustrate the importance of individual resilience as a protective factor against PTSD symptoms in firefighters with high levels of traumatic stress and/or perceived stress. The finding that resilience is not significantly correlated with traumatic stress events suggests that resilience might be a trait rather than a state. The current results further underscore individual resilience as a key factor when evaluating fitness for duty in firefighters frequently exposed to traumatic events. This study has several limitations that should be noted. The cross-sectional design of this study limits its ability to confirm a causal relationship between the variables that were investigated. Thus, the results of the present study should be interpreted cautiously, and future research using longitudinal and experimental designs is needed to clarify the direction of any causal relationships between the observed variables. Secondly, the present research relied solely on self-report questionnaires. Even though all the scales have been validated, future studies utilizing standardized interviews or observational methods would provide more accurate and detailed information regarding the complex mechanisms underlying the relationship between traumatic stress and the development of PTSD symptoms. Thirdly, although the life events checklist used in the present study included traumatic events that are often experienced by the firefighters, future research focusing on firefighter-specific traumatic events would provide more
A
B
A B C D E
A B
C C D
D
E
E
Fig. 1. The number of traumatic stress events (left) and the level of perceived stress (right) as a function of individual resilience. This interaction predicts PTSD symptoms. The slope indicates that as the number of traumatic events (left) or the level of perceived stress (right) increases PTSD symptoms also increase. However, the slope is increasingly reduced as the level of individual resilience increases.
Individual resilience Global Perceived stress Job specific perceived stress
Traumatic stress events
PTSD symptoms
Fig. 2. The final moderated mediation model. The indirect link associating traumatic stress and PTSD symptoms via perceived stress is moderated by individual resilience. The direct link associating traumatic stress and PTSD symptoms is moderated by individual resilience. The dotted line indicates a non-significant pathway in the association between traumatic stress and PTSD symptoms.
J.-S. Lee et al. / Journal of Affective Disorders 162 (2014) 128–133
relevant information in understanding firefighters' traumatic experiences. Finally, the present study included only a few female participants. For this reason, future research with a large number of female participants is needed to determine whether the results of the present study can be generalized to women. Despite these limitations, the present study is the first to use a moderated mediation analysis to investigate the possible mediators and moderators of PTSD symptoms in firefighters and to evaluate the interactive mechanisms underlying the development of these symptoms following trauma exposure. The current findings offer a better understanding of those who are at high risk for PTSD and of the factors that should be targeted by efforts to prevent the development of PTSD symptoms in firefighters. Based on these findings, clinical interventions, especially those targeting the cognitive reappraisal of traumatic stressors and those that bolster individual resilience, may be effective for firefighters with a low level of resilience who have experienced many traumatic events. Finally, as Bonanno (2004) indicated, finding multiple pathways or factors to facilitate resilience might shed light on the development of effective intervention against PTSD. Role of funding source This work was supported by Grant No KFI-2009-Next generation-001 from Korea National Emergency Management Agency.
Conflict of interest The corresponding author confirms that all authors have disclosed any potential competing interests regarding the submitted article and the nature of those interests.
Acknowledgments We acknowledge Grant support of the Korea National Emergency Management (No. KFI-2009-Next generation-001). We also thank all the participating firefighters in the present study.
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