Abstract
Introduction
Firefighting is a highly demanding technical skill often done in difficult circumstance, requires bearing heavy personal protective apparel, working above or below the ground, and exposes firefighters to smoke inhalation and high temperatures. Different aspects of Firefighters’ health have been studied in the field of occupational medicine [1–3]. Studies on firefighters have demonstrated that, their heart rate immediately increases and remains high for a long time in emergency situations [4, 5]. After a long period of stillness, these emergency situations can impose severe physical and psychological stress on firefighters. Under these circumstances, adrenergic hormones are released in large amounts and high cardiovascular demand further adds to the physical and chemical exposures, causing problems in the cardiovascular system [6]. This is exacerbated by evidence suggesting that up to 75% of firefighters are overweight and 40% of them are obese; this is also true for one-third of the recently employed personnel [7–11]. In fact, in the United States, firefighters rank third in terms of prevalence of obesity (30% had BMI>30 kg/m2) among 41 male work groups according to the statistics reported by the National Health Interview Surveys [12].
Unfortunately, not only are there significant risk factors for the cardiovascular system due to the job, but the prevalence of cardiovascular diseases (CVDs) is high in firefighters. It is estimated that (CVDs), such as coronary artery disease [13], are responsible for 45% and 25% of fatal work accidents among firefighters and medical emergency personnel, respectively. Annually in the United States, one out of every 1000 firefighters suffers fatal or non-fatal cardiovascular events at work [6]. The rate of morbidity and mortality of CVDs is relatively high among firefighters due to occupational exposure to chemical agents, carrying heavy loads, irregular physical activity, thermal stress and psychological work-related stressors in conjunction with underlying coronary artery disease or hypertrophy of the left ventricle [14–17]. Moreover, obese firefighters are at even higher risk for occupational mortality due to CVDs [18, 19].
Increased physical activity can generally decrease the mortality and prevalence of CVDs and also reduce CVD risk factors. Epidemiological studies on the general population have shown that increased physical activity and aerobic fitness (AF) significantly decrease the risk of coronary artery disease [20, 21]. Increasing the AF and decreasing the CVD risk factors have also been found to significantly lower the morbidity and mortality associated with coronary artery disease [22–24]. In a study by Blair et al, an increase of 1 metabolic equivalent (MET) in AF in the treadmill test resulted in an 8% reduction in the mortality rate of the understudy participants [25]. The results of another study also confirmed that one MET increase in AF resulted in 13% and 15% reduction in general mortality and morbidity rates and mortality and morbidity due to CVDs, respectively [20].
While firefighting is a demanding occupation both physically and psychologically and performing the tasks safely is of utmost importance in this job, the cardiovascular health status of firefighters has not significantly improved in the past decades despite great advances in cardiovascular medicine. Moreover, in Iran, medical surveillance has not been provided for firefighters regularly during their work life [18]. The only reliable study related to this issue was done by Mehrdad et al. [18] that sought to assess the relationship of AF with CVD risk factors in firefighters. Given the prevalence of CVD within the profession, assessment of CVD risk factors in firefighters is especially important and finding strategies to decrease CVD risk factors can help improve the health status of firefighters. Therefore, we conducted the following study to assess the relationship of aerobic fitness with cardiovascular risk factors in firefighters.
Materials and methods
Study design and population
This study was completed in 2012 in a large industrial facility in Tehran. All male firefighters in this facility were enrolled in the study (n: 157). Demographic data and medical and work history of the participants were retrieved by direct interviewing and recorded in a questionnaire specifically designed for this purpose. The questionnaire included age, level of education, work history, cigarette smoking status, drug intake, shift work, and history of cardiac disease in subjects or their first-degree relatives. All workers participated voluntarily in this study and signed informed consent form. This study was approved by the Ethics Committee of Iran University of Medical Sciences.
Assessment of CVD risk factors
Subjects’ height was measured in a standing position and they were weighed while wearing light-weighted clothes, such as T-shirts and cotton pants. Body mass index (BMI) was calculated in Kg/m2. Resting systolic blood pressure (RSBP), resting diastolic blood pressure (RDBP) and resting heart rate (RHR) of participants were measured in a seated position. BMI was classified based on WHO 2000 statement [26] to underweight or normal weight (<25 kg/m2) and overweight and fat (≥25 kg/m2). Blood pressure was classified to normal systolic and diastolic blood pressure (<120 and <80 respectively) and higher than normal values (≥120 or ≥80 respectively) [27]. Fasting venous blood samples were analyzed for total cholesterol (total chol.), HDL-chol., LDL-chol., triglycerides (TGL) and glucose using standardized methods.
Assessment of AF
AF assessment and submaximal AF testing of participants were performed according to the guidelines of the ACSM [28] The Young Men’s Christian Association protocol was adopted for this test using two to four three-minute stages of continuous exercise [28]. AF was assessed using the bicycle ergometer (Bike Excite Forma, version 700E, Technogym, Barcelona, Spain). Cycle ergometer testing used a stationary bike to estimate VO2 max. Assuming that a linear correlation exists between the heart rate and oxygen consumption during physical activity, VO2 max of each subject was estimated based on their heart rate in response to the respective activity. This protocol aimed to achieve 85% of the maximum heart rate predicted for age at the end of the test.
For each test, the height of the chair was adjusted for each individual in such way that
with the pedal at the 6 o’clock position, the subject’s knee angle was 5 to 10°.
Participants pedaled at a constant speed of 60 cycles/min. The speed of pedaling increased
when the heart rate reached a predetermined percentage of maximum heart rate. This process
was continued until 75% of the maximum heart rate predicted was achieved. The test was
predicted to increase the heart rate in a stable fashion between 110 beats per minute and
85% of the maximum heart rate predicted for age for a minimum of two consecutive stages.
In this protocol, each stage took at least 3 minutes and heart rate was recorded at 15 to
30 seconds after the second and third minutes. If the two heart rates recorded had more
than 5 beats/min difference, the participant continued his activity for one more minute in
the respective stage. This test comprised of the following stages: Warming up: The test was commenced by
initiating the physical activity. This stage lasted for one-minute and the
candidates accelerated the speed to 60 cycles/min. The power output was 50 watts in
this stage. First stage: This stage
took three minutes. The power output was increased to accelerate the heart rate to
60% of the maximum safe heart rate for the respective
individual. Transition stage: In this
stage, the power was exponentially increased until reaching the heart rate required
for the second stage (75% of the maximum heart rate). Second stage: After reaching the heart rate required (75% of the
maximum safe heart rate), this stage was started and lasted for three minutes. The
power of the ergometer in this stage was changed based on the alterations of the
heart rate to maintain the heart rate within the desired
range. Cooling down: After completion
of the test, recording was finished and the subjects continued to pedal freely for
one minute.
Eventually, the ergometer calculated the AF of the individual in millimeter/minute for each kilogram of weight according to MET. The termination criteria for the test were reaching 85% of the heart rate predicted, complaints of chest pain, severe headache, dizziness, tremor, nausea, dyspnea, paleness, cold skin, disorientation, inappropriate affect or participant’s willingness to end the test [29].
Statistical analysis
The mean, standard deviation (SD) and range of quantitative variables were calculated. For analysis, groups will be created using the 50th and 75th percentiles of AF MET functioning across the group. The use of a percentile as a cutoff for defining groups has been applied in other studies [30, 31]. The t-test or ANOVA was used for comparing variables among the groups. The chi-square test was used for comparing the qualitative variables. The effect of MET on different CVD risk factors was assessed by applying logistic regression models. The results of statistical analysis were expressed as odds ratio (OR) with 95% confidence intervals (95% CI). All tests presented were two-sided and a P < 0.05 was considered significant. Analyses were performed using SPSS11.0.
Results
In this study, 157 male firefighters in an industrial facility were evaluated. The mean age of subjects was 34.18 years (range 21–60 years) with a mean work experience of 8.30 years (range 1–37 years). The mean BMI of subjects was 25.61 (range 17.26–40.18 kg/m2).
The mean RSBP was 116.93 mmHg (range 100–180 mmHg). The mean RDBP was 76.03 mmHg (range 60–130 mmHg). Of participants, 34 (21.7%) were smokers, 106 (67.50%) were married and 56 (35.7%) had regular physical activity. The mean AFof participants based on VO2 max was 33.76 mL.kg.–1.min–1 (range 10.10–50.30). The mean AF predicted for participants was 33.76 mL.kg.–1.min–1 (range 26–47). Also, the mean AF based on MET was 9.64 MET (range 2.89–14.37).
Two cutoff points for AF which represent the population statistically were used to categorize cardiovascular fitness: AF = 9 represented the closest integer to the AF mean value which was 9.64 and AF = 11 closely related to 75th percentile (11.32). Cardiovascular fitness was divided to three groups based on their AF value: low fit aerobic capacity with AF<9, moderate fit aerobic capacity with AF ranging from 9to11, and high fit aerobic capacity with AF>11. Of participants, 66 (42.0%), 52 (33.1%) and 39 (24.9%) had AF<9 MET, between 9–11 MET and >11 MET.
Table 1 presents the mean values for CVD risk factors in the three groups of AF<9, between 9–11 and AF>11 MET. Significant differences were noted among the three groups for all variables with exception of fasting blood sugar, triglycerides, and total cholesterol. (P < 0.05). Post-hoc analysis of between group differences indicates significantly differences between individuals with >11 MET versus individuals with <9 MET for all factors with exception of total cholesterol, fasting blood sugar, and RSBP. In this analysis, the high AF group was significantly younger with lower BMI, triglycerides, LDL, RHR, RDBP, and higher HDL. No significant differences were noted in the post-hoc analysis between the mid and high AF groups and the only differences between the 9–11 MET and <9 MET group was that the mid AF group was significantly young with lower BMI.
Because individuals in the 9–11 and >11 MET groups were not different, data were collapsed into two groups, of <9 MET (low AF) and >9 MET (high AF) for determination of the relative effects of CVD risk factors within each group. Table 2 compares the frequency of subjects with CVD risk factors in the two groups of AF<9 MET and AF ≥9 MET. The frequency of subjects with CVD risk factors in the group with AF<9 MET was significantly higher than that in the group with AF ≥9 MET (P < 0.05) for all factors except triglycerides. For more accurate assessment of the correlation of AF with CVD risk factors, logistic regression analysis was used to determine the relationship of all factors to low AF (Table 3). Individuals with low AF were more than 5 times as likely to smoke, not participate in physical activity and have higher LDL levels than their peers with high AF. Additionally, this group was 4-5 times as likely to be over the age of 30, have BMI higher than 25 and have lower Hb levels, as well as nearly 4 times as likely to have lower HDL levels.
Discussion
Presence of CVD risk factors in firefighters can be very concerning considering their stressful job. Thus, it is important to pay attention to CVD risk factors and adopt strategies to decrease them in firefighters. Our study results show significant association between CVD risk factors and AF, and indicates that firefighters with higher AF have a more favorable status in terms of these CVD risk factors. Specifically, AF status (i.e.,<9 METs versus >9 METs) is significantly associated with age, BMI, cigarette smoking, physical activity, lipid profile, RSBP and RDBP (P < 0.05).
Our findings in this Iranian population are similar to previous studies of CVD and AF in firefighters. In a cross sectional study by Baur et al., on 968 male firefighters, it was revealed that higher AF was significantly associated with lower diastolic blood pressure, less body fat, and lower lipid profile (P≤0.0272). Increased AF independently decreased CVD risk factors in firefighters [32]. Comparison of specific findings in our study to previous work evaluating BMI, lipid and glucose profiles, blood pressure and heart rate, smoking and physical activity, and hemoglobin.
BMI
Mentally, some firefighters consider themselves as active individuals [31]. In our study, the mean BMI in firefighters with AF <9 MET was significantly higher than that in subjects with greater AF and individuals with BMI greater than 25 were 5 times more likely to be in this low MET group. In previous studies, the reverse correlation of AF and BMI has less commonly been reported [9–11]. One study showed that in obese firefighters, increasing the AF to more than 12 MET was associated with a reduction in BMI by 1.6 units. In firefighters with low AF, the man body fat was 24.9 versus 16.4 in firefighters with high AF [32]. In a study by Cl ark et al, by increasing the BMI in firefighters, their AF significantly decreased [7].
Blood profile
In our study, the mean level of LDL-chol, HDL-chol and triglycerides in AF>11 MET was significantly lower than AF<9 MET, but fasting blood sugar and total cholesterol had no significant difference in all groups. In a previous study [32], a significant association was reported between total cholesterol, HDL-chol, LDL-chol, and blood glucose with AF in firefighters. In obese firefighters (BMI>30) AF higher than 12 MET was associated with 5 mg/dl increase in the mean HDL-chol, 30 mg/dl reduction in the mean triglycerides and 9 mg/dl reduction in the mean blood glucose level. In firefighters with AF more than 14 MET, the mean HDL-chol was 15.2 mg/dl higher than that in firefighters with AF<10 MET and this difference was statistically significant (P < 0.001). In addition to cholesterol and blood sugar our study also investigated a unique measure of blood hemoglobin. In our study, the mean hemoglobin level of firefighters with AF<9 MET was significantly lower than in firefighters with higher AF and individuals below the cutoff value for CVD were nearly 5 times more likely to be in the low MET group.
Blood pressure and heart rate
In our study, post hoc analysis of the mean values of RDBP and heart rate in firefighters with AF<9 MET were significantly higher than those in subjects with AF>11. Individuals with AF>11 MET had a 5-6 mmHg lower RSBP and RDBP. Results of another study revealed that each one MET increase in AF was associated with a mean reduction of 0.71 mmHg in RDBP [32]. Epidemiological evidence suggests that even small amounts of reduction (change) in blood pressure can reduce the risk of coronary artery disease and stroke by 6% and 16%, respectively [33, 34].
Cigarette smoking and physical activity
In this study, the frequency of smoker firefighters and those without physical activity was significantly higher in the group with AF<9 MET. In fact, individuals who did not participate in regular physical activity and those who smoked were 5.5 to 6.5 times more likely to have an AF<9 METs, respectively, than their peers. These results association smoking and physical activity with AF are consistent with the findings of Punakallio, et al. [35].
Study limitations
The cross-sectional design, relatively small sample size and inclusion of only males were among the limitations of this study. Given the design, these results are only able to evaluate basic association between CVD risk factors and AF levels, and are not meant to indicate causation. However, the significant results in this small sample indicate the existence of associations that require awareness and futher evaluation. Moreover, the use of objective assessment of AF in firefighters instead of using a questionnaire to evaluate the association with major CVD risk factors strengthens this small-sample, cross-sectional study.
Conclusions
In general, firefighting is a high risk job and improving the AF of firefighters must be considered as a suitable strategy to decrease the potential risk of cardiovascular events. Our results suggested that, generally, decrement of AF has significant association with increase of CVD risk factors. However, longitudinal studies are recommended to further confirm these results. By designing strategies and encouraging the firefighters to increase their AF, we may be able to decrease the risk of CVD risk factors in them.
Conflict of interest
The authors have no conflict of interest to report.