Firefighting is known to be a physically demanding and dangerous occupation. Nonetheless, it is surprising that the leading cause of death on-duty among firefighters is heart disease rather than burns or smoke inhalation (8,18,33). Explanations for high proportionate cardiovascular morbidity and mortality are the combined effects of smoke and chemical exposure, irregular physical exertion, heavy equipment and materials handling, heat stress, shift work, and psychological stressors on firefighters with underlying coronary heart disease and left ventricular hypertrophy (14,30,32).
In other words, research has documented that most on-duty cardiovascular disease (CVD) events (fatal and nonfatal) are likely to be work precipitated by strenuous duties (16,18,19). However, at the same time, parallel studies have shown definitively that the risk of such on-duty CVD events is concentrated among the highest risk individuals: firefighters with underlying disease (known or previously subclinical) (12,13). Thus, on-duty CVD events result from the interaction of excess stressors on a susceptible, preexisting biologic substrate.
Although strenuous duty may precipitously trigger a CVD event, the underlying disease develops as a result of multiple, interacting factors gradually over many years. The Metabolic Syndrome (MetSyn), as an expression of comorbid clustering of multiple risk factors, is a major predictor of diabetes mellitus, CVD, and all-cause mortality. Accordingly, the MetSyn is a likely precursor of many on-duty CVD events in firefighters. A recent review has described the estimates of the population- attributable fraction for the MetSyn as 6–7% for all-cause mortality, 12–17% for CVD, and 30–52% for diabetes (9). Moreover, Gami et al. found current evidence from a large number of longitudinal studies that people with MetSyn had a significantly increased risk of cardiovascular events and death (11).
On the other hand, there is robust scientific evidence that enhanced cardiorespiratory fitness (CRF) is an important protective factor against developing CVD and CVD triggering by exertion, and improving CRF can ameliorate established CVD risk factors such as hypertension, obesity, hyperinsulinemia, and hyperlipidemia that underlie the MetSyn (4,22,23,29).
Accordingly, the interrelationship between CRF and the risk of MetSyn is highly relevant, especially in occupational populations whose jobs involve periods of high physiologic demands on the CV system and significant CVD risks. To date, only 1 small study limited to a single fire department has examined this association in firefighters using older MetSyn criteria (6). Therefore, we investigated the prevalence of MetSyn based on updated criteria in a large cohort of male career firefighters from 3 states and 11 fire departments and examined the association of MetSyn with CRF, with and without adjustment for age. We hypothesize that lower CRF is associated with a higher prevalence of MetSyn in career firefighters, and CRF in general has the stronger relationship with MetSyn compared with age.
Experimental Approach to the Problem
The present cross-sectional study was nested within a federally funded multicenter, prospective cohort investigation of CRF, health, and employment outcomes among career firefighters. Fire departments with established medical programs including maximal exercise testing were sought. Eleven career departments in 3 Midwestern States were included. The approach of studying fire departments performing maximal exercise treadmill testing allowed us to use the objectively measure CRF as the primary independent variable, whereas the comprehensive cardiovascular risk assessments done through the remainder of the fire departments medical evaluations permitted us to characterize MetSyn prevalence as the dependent variable.
Male career firefighters, ≥18 years of age, were recruited from eligible fire departments in 3 Midwestern states as described above. Inclusion criteria were informed consent to participate, having completed a maximal exercise test during the course of a fire department-sponsored medical examination, and having no restrictions on duty at examination. Excluded subjects failed to meet one or more of the above criteria or had undergone the index exercise tests for the evaluation of symptoms, retirement pensions, disability, or exit examinations. The study was approved by the Institutional Review Board (IRB) of Harvard School of Public Health and local IRBs as appropriate.
The CRF was determined from symptom-limited maximal treadmill exercise testing with electrocardiogram monitoring and estimation of oxygen consumption (METS) following the Bruce protocol. The participants were encouraged to continue exercise until volitional exhaustion, even after exceeding 85% of their maximum predicted heart rate defined as (220 − age). The cohort achieved an average of 97.9% (SD 6.6) of maximal age-predicted heart rate on these tests. During the exercise test, the maximum METS achieved (maxMETS) were recorded. Firefighters were then categorized according to their baseline exercise tolerance or CRF into 4 groups, representing very low, low, intermediate, and high fitness levels defined as ≤10 METS, >10–12 METS, >12–14 METS, and >14 METS, respectively.
Assessment of Cardiovascular Risk Factors
Height was measured in the standing position with a clinic stadiometer. Body weight was measured with light clothes on a calibrated scale. Body mass index (BMI) was calculated as the weight in kilograms divided by the square of height in meters. Blood pressure was measured using an appropriately sized cuff with the subject in the seated position. Pulse and blood pressure assessments were obtained in a resting state from the physical examination (and were not taken from preexercise test values). Fasting venous blood samples were analyzed for HDL-cholesterol (HDL-chol), triglycerides, and glucose using standardized methods.
Definition of MetSyn
The prevalence of MetSyn among the study population was determined using modified criteria from the Joint Scientific Statement (1). If ≥3 of the following 5 risk factors were present, then the participant was categorized as having MetSyn: abdominal obesity (elevated waist circumference), modified here to BMI ≥ 30; hypertriglyceridemia (≥150 mg·dl−1); reduced HDL-chol <40 mg·dl−1; elevated blood pressure (Systolic ≥ 130 and/or diastolic ≥85 mm Hg) or antihypertensive drug treatment; or hyperglycemia (blood glucose ≥ 100 mg·dl−1) (1).
Baseline characteristics were described using the mean (SD) in the case of quantitative variables and the frequency for categorical variables. Group comparisons were performed using the χ2-test for categorical variables and generalized linear models for continuous variables. The effect of METS (as a continuous variable) on different metabolic abnormalities and MetSyn was assessed with the use of logistic regression models. Unadjusted models and models adjusting for age and CRF, respectively, are presented. Analyses were performed using SPSS 18.0 and SAS 9.2. All the tests presented are 2-sided, and a p value ≤0.05 is considered significant.
The average age and BMI of our cohort were 39.6 (SD 8.5) and 29.3 (SD 4.3), respectively. More than one-quarter of the study population met the criteria for MetSyn (28.3%); 21.7% had none of the defining criteria, 28.2% met a single criterion, and 21.7% met 2. The most frequent metabolic abnormality in our cohort was HDL-chol <40 mg·dl−1 (40.8%) closely followed by elevated blood pressure or antihypertensive medication (39.8%). The prevalence of individual MetSyn components and metabolic abnormalities according to clinical guidelines are presented in Table 1. As the number of metabolic abnormalities increased, the mean CRF in METS decreased in a dose-response fashion, which was highly significant even after adjustment for age (Table 1).
The prevalence of MetSyn increased with age. For example, the youngest group of firefighters (18–29 years of age) showed a prevalence of MetSyn of 15.1%, whereas in the oldest age group (50–62 years of age), the prevalence was 34.9% (p = 0.012). The relationship between age, CRF in METS categories, and the proportion of MetSyn is visualized in Figure 1. The prevalence of MetSyn increased with decreasing fitness, which had a much stronger effect than age did (Figure 1).
In categories of low fitness and young age, however, the numbers were very small. Therefore, some caution is warranted in examining the subgroup prevalence estimates.
The mean CRF level of the entire study population was 12.0 (1.9) (Table 1). Firefighters with MetSyn had a mean maximal METS of 11.1 (SD 1.8) compared with 12.8 (SD 1.7) in study participants who met none of the criteria (p < 0.0001, adjusted for age). The prevalence of metabolic abnormalities is presented in Table 2 according to the mean CRF levels.
In the lowest fitness group, 66 participants had the MetSyn (51.2%), whereas in the highest fitness group, only 5 participants fulfilled ≥3 criteria (5.2%) (p < 0.0001, adjusted for age). The crude analysis showed statistically significant associations between all numbers of metabolic abnormalities and different CRF fitness levels, and these results remained statistically significant even after adjustment for age for 0, 1, and ≥3 criteria (Table 2). In the group with METS, ≤12, 37.9 % of the participants had MetSyn compared with 15.9% among those achieving >12 METS (p < 0.0001, adjusted for age). For fitness levels ≤12 METS compared with those >12 METS, the odds of having MetSyn is 3.24 (95% confidence interval [CI] 2.4–4.4; unadjusted) and 2.93 (95% CI 2.1–4.1; adjusted for age).
The logistic regression analysis of CRF as a continuous parameter (METS) demonstrated that with every one unit increase in METS, the odds of having the MetSyn is reduced by 31% (odds ratio 0.69 [95% CI 0.63–0.76]) after adjusting for age (Table 3). In contrast, age was significant only in the unadjusted model but was no longer significant after adjustment for CRF (Table 3).
The present cohort study in US firefighters, from 3 different states, showed strong inverse cross-sectional associations between MetSyn and CRF. The highly significant and independent dose-response relationship persisted even after adjustment for age. Therefore, our study confirms our hypotheses that the prevalence of MetSyn is inversely related to CRF among male career firefighters. This clearly suggests improving CRF as a means to decrease MetSyn and hence, CVD risk in this occupation. Moreover, in this population, CRF, a modifiable risk parameter, was more important than age, a nonmodifiable risk factor. After adjustment for CRF, age was no longer significant in predicting the risk of MetSyn. However, CRF was significant in predicting the odds of MetSyn, suggesting that every one additional MET of CRF decreases the odds of MetSyn by 31% even after adjusting for age.
Furthermore, we found an alarmingly high prevalence (28.3%) of MetSyn among career emergency responders expected to be younger, more active and healthy than the general U.S. adult population, who has a prevalence of MetSyn reported to be approximately 24% (dependent on the criteria used) (3,10). The high prevalence of MetSyn and failure to find a healthy worker effect in this study occurred despite the rather conservative approach to use BMI ≥ 30 instead of waist circumference. A recent study in firefighters has shown that the false-positive rate for BMI-derived obesity was relatively low when compared with both body fat percentage and waist circumference (28). Furthermore, the same investigation found that the BMI was more likely to underestimate obesity than overestimate it by using other measures of body composition such as body fat (28). Moreover, other studies have validated BMI ≥ 30 as an accurate proxy for the detection of waist circumference ≥ 102 cm with only about 2% of the sample being misclassified (24), whereas the suggested cutoff for abdominal obesity for defining the MetSyn is only >94 cm. Accordingly, other studies have used a BMI criterion instead of waist circumference with even lower BMI cutoff values (BMI > 28.8) (31). Thus, our MetSyn prevalence estimates are quite conservative. Finally, BMI is a simpler and less operator-dependent measure than weight circumference is, which minimized the potential for misclassifying subjects in our study.
A previous study published by our group on a smaller subset of the currently enrolled firefighters found beneficial independent effects of increasing the frequency, duration, and intensity of physical exercise activities on CVD risk profiles (7). Our present study provides stronger support for the original findings. Despite the increasing prevalence of MetSyn with older age, the inverse association between MetSyn and CRF remained highly statistically significant even after adjusting for age. Furthermore, CRF seems to be the stronger factor in this relationship (Figure 1). As expected, the prevalence of MetSyn increased with the age of the participants. However, the MetSyn prevalence gradient from the youngest (18–29 years) to the oldest (50–62 years) firefighters was much less marked (15.1% compared with 34.9%) than that seen across CRF: 5% in the most fit group (>14 METS) compared with 51% in the least fit group (<10 METS). Thus, our results suggest that prevalent MetSyn is for the most part reversible in theory by modifying one's fitness level, rather than the inevitable result of aging.
Our results are also in general agreement with those of a smaller, previous investigation in firefighters. Donovan et al. described a comparable trend in age-dependent increase in the prevalence of MetSyn and a significant association with CRF and similar findings of the individual MetSyn-defining criteria (6). However, the absolute numbers are not directly comparable because of Donovan's smaller sample size and the more recently revised guidelines for diagnosing MetSyn used in this study. Additionally, given our large sample size, we were able to investigate the relative, independent effects of CRF and age and demonstrate that CRF was more of an important predictor of MetSyn than age, and it appears that age-associated declines in CRF explain most of the age-related increase in MetSyn observed before adjustment for CRF.
Interestingly, in our study, the prevalence of decreased HDL-chol was the most common metabolic abnormality (40.8%). This result is in agreement with data from a smaller sample of male police officers in Buffalo, NY, where low HDL-chol was also the most common MetSyn component (48%) (15). This could be explained by the more sedentary behavior of firefighters and their high prevalence of obesity and overweight. Accordingly, over 50% of the study participants failed to reach >12 METS, which has been suggested as the minimum CRF required to safely perform firefighting duties (27).
Recent cross-sectional studies in the general population have demonstrated that fitness seems to have a stronger association with MetSyn than diet has (21). These findings emphasize the importance of incentives to increase CRF in firefighters. Previous data have demonstrated that the long-term effects of a worksite health promotion program for firefighters were generally positive for all groups, consistent with lasting effects and diffusion of program benefits across experimental groups within the worksites (25). Our findings strongly support the need to extend and improve these programs so that they include guidelines for increased CRF in firefighters. The goals of the fitness program should be to optimize work performance, improve safety, promote and maintain higher levels of CRF, and exert beneficial effects on modifiable CVD risk factors (32).
Our study has some modest limitations, including the cross-sectional design. Also, because of the very small number of participating women firefighters in our study, only male participants were included in the analyses.
On the other hand, our investigation has a number of important strengths. First, objectively measured CRF is a stronger predictor of over-all fitness than self-reported physical activity (2,26), and we can be confident that we captured maximal or near-maximal exertion in most participants as evidenced by their achievement of maximal age-predicted peak heart rates. Second, the sample size was large and represented urban and suburban departments from 3 different states. Third, we had information concerning lipid-lowering and antihypertensive medication, which allowed us to use the newest criteria for the definition of MetSyn (1). Finally, our sample had demographic and anthropometric characteristics similar to those found in other epidemiologic studies of firefighters and other emergency responders (5,17,20). Therefore, the results can be generalized to most male career public safety professionals in the U.S.A.
In conclusion, we found a high prevalence of MetSyn, which is strongly inversely correlated with CRF. These results remained statistically significant even after adjustment for age. Moreover, low CRF was found to be a much stronger independent predictor of MetSyn, whereas increasing age was not significantly associated with MetSyn after adjusting for CRF. Because of these findings, fire departments should increase their emphasis on regular assessments of maximal aerobic capacity and the promotion of physical fitness. This would decrease the prevalence of MetSyn and most likely also decrease the future risk of CVD in career firefighters.
There is a highly significant inverse dose-response association between CRF and MetSyn. Moreover, in this population, CRF, a modifiable risk parameter, was more important than age, a nonmodifiable risk factor. Firefighters should be given strong incentives to improve their fitness, which would decrease prevalent MetSyn, a likely precursor of CVD. Fire departments should increase their emphasis on regular assessments of maximal aerobic capacity and the promotion of physical fitness. The results are likely generalizable to other similar professions such as law enforcement, emergency medical services, and the military.
The authors would like to thank all of the participating firefighters and Fire Departments; the staff and clinical leadership of the clinics who examined the firefighters; Dr. Sara Jahnke, Ms. Brianne Tuley, Dr. Antonios Tsismenakis, Dr. Lilly Ramphal, and the late Dr. William Patterson for their contributions to the underlying longitudinal study. The authors also thank Dr. Charles A. Czeisler, Dr. Steven W. Lockley, Mr. Jason Sullivan, and ‘the Harvard Work Hours Health and Safety Group’ for providing data on one of the fire departments. This investigation was supported by the Federal Emergency Management Agency (FEMA) Assistance to Firefighters Grant (AFG) program's awards EMW-2006-FP-01493 (PI: Dr. S.N. Kales), EMW-2009-FP-00835 (PI: Dr. S.N. Kales), and EMW-2007-FP-02197 (PI: Dr. C.A. Czeisler). S.N. Kales has served as expert witness in medicolegal cases involving firefighters. The other authors report no conflict of interest.
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