ABSTRACT OF THE THESIS

Clinical Predictors of Metabolic Syndrome

By

Samir T. Mukherjee

Master of Science in Environmental Health Sciences

University of California, Irvine, 2017

Professor Bongkyoo Choi, Chair

Metabolic syndrome refers to the co-occurrence of several cardiovascular risk factors, including insulin resistance, dyslipidemia, obesity, and hypertension. It is an escalating public health challenge with an estimated worldwide prevalence of 25%. Metabolic syndrome is also a powerful prognostic indicator for atherosclerotic cardiovascular disease, which remains the leading cause of morbidity and mortality within the United States.

Two simple clinical measures, resting heart rate (RHR) and heart rate reserve (HRR), have been shown to be associated with metabolic syndrome in the general population. Currently, however, there is a paucity of evidence in the scientific literature describing this association within the adult working population. Given the significant economic impact of cardiovascular disease on the occupational work force, the purpose of this study was to evaluate RHR and HRR as cost-effective screening metrics for the detection of metabolic syndrome.

Data from the biomarker project of the Midlife in the United States (MIDUS II) cross-sectional survey were analyzed using a combination of binary logistic regression, independent samples t-testing, prevalence ratio analysis, and generation of receiver operating characteristics (ROC) in order to elucidate the strength of the proposed relationships. All statistical analysis was performed using JMP v13.0.0. The results suggested that both RHR and HRR are reliable predictors of metabolic syndrome; however, RHR generally had a stronger association within the younger study population regardless of gender. HRR was a stronger predictor among the older subset of participants, and demonstrated a marginally better discriminative ability throughout the sample population. The results for both predictor variables were significant at the p < 0.05 level. The utility of RHR and HRR as screening tools for metabolic syndrome, however, was lower than expected, as the area under the ROC curve was less than the desired cut-off level of 0.7.

On-the-job deaths due to cardiovascular events are the most common cause of death among firefighters across the U.S. consistently every year since 1977. Work stress is prevalent in the firefighting occupation most often secondary to the strenuous nature of their job duties that can include emergency situations that are sometimes life-threatening and extremely long shift hours. Work stress can contribute to strain or the negative manifestations of stress reactions, which can present mentally and/ or physically. There is extensive literature on the association of psychological strain and metabolic syndrome (MS), which is an indicator of cardiovascular disease risk. Consequently, based on the current literature and known pathophysiology, a causal association between work stress, MS, and cardiovascular disease is biologically plausible. Thus, screening for psychological occupational strain may potentially reveal modifiable cardiovascular risk factors to target in disease prevention efforts.

However, the association between work stress, MS, and cardiovascular disease is understudied in firefighters. This study is the first to examine this relationship in this unique occupational group. In this study, an extensive literature review and cross-sectional analysis of data from firefighters in Southern California was performed. Psychological strain was investigated through measures of mental health, emotional exhaustion, and post-traumatic stress disorder (PTSD) risk indicators through the General Health Questionnaire-12 (GHQ-12), Exhaustion Scale of the Maslach Burnout Inventory (ES-MBI), and the primary care PTSD (PC-PTSD) screening questionnaires. MS was assessed with data on each component of MS, which included waist circumference (WC), fasting glucose, triglycerides (TG), high density lipoprotein (HDL), and blood pressure (BP). Data analysis revealed statistically significant relationships between GHQ-12 and WC, ES-MBI and TG, and PC-PTSD and WC even after controlling for covariates of age and gender. The analysis revealed negative associations between psychological screening tests and MS (and its components), which are unexpected considering the pathophysiology of cardiovascular strain resulting from stress. Due to potential effects of medications on associations with well-controlled components of MS, such as reverse causality, a sensitivity analysis was performed excluding those on medications. This analysis revealed a stronger statistically significant association with GHQ-12 and WC and more consistent positive associations between psychological screening tests and MS that were apparently negative in the initial analysis. Otherwise no other statistically suggestive associations were found. Furthermore, no statistically significant relationships were found with the number of MS criteria and MS itself.

This study reveals possible associations with indicators of psychological occupational strain and metabolic syndrome (and its components). Due to individual variability of reactions to stressors, indicators of psychological strain were used instead of measures of work stress. Despite the multiple strengths of this study, there are also several limitations making further research on this topic necessary in order to make more definitive conclusions about causation. Further investigation is important in informing prevention programs and achieving the overarching goal of decreasing firefighter morbidity and mortality while increasing quality of life through early intervention and prevention of adverse cardiovascular outcome.

Numerous studies performed in various countries consistently find urban bus driving among the unhealthiest of modern occupations. Urban bus drivers have higher rates of mortality and morbidity compared to other occupational groups, particularly with respect to cardiovascular disease. Psychological occupational strain is also prevalent among bus operators due to the nature of their job conditions, which include high job demands and low control, an imbalance in effort-rewards, and low social support from supervisors. Although extensive literature shows that job strain arising from the work environment is an etiological agent in the poor cardiovascular health and mental health of bus drivers, the relationship between mental health and cardiovascular disease with its risk factors has not been studied in this occupation.

This is the first study to investigate whether poor mental health indicators are associated with cardiovascular risk factors in bus drivers, specifically those that make up the American Heart Association (AHA) Life’s Simple 7 predictors of cardiovascular health. In this study, a cross-sectional analysis was performed on self-reported data from 280 bus drivers in Los Angeles County. Psychological strain indicators in the form of mental health, emotional exhaustion, and post-traumatic stress disorder were measured through the General Health Questionnaire (GHQ), Exhaustion Scale of the Maslach Burnout Inventory (ES-MBI), and the Primary Care Post-traumatic Stress Disorder (PC-PTSD) screening questionnaires, respectively. Measures of cardiovascular disease risk was based on data relating to the AHA Life’s Simple 7 (LS7) cardiovascular health metrics, which include blood pressure (BP), cholesterol, diabetes, body mass index (BMI), diet, physical activity, and smoking.

The prevalence of bus drivers meeting each LS7 metric criteria was approximately 21% for BP, 11% for cholesterol, 8% for diabetes, 51% for BMI, 21% for physical activity, 9% for diet, 17% for smoking, and 2.5% for inadequate cardiovascular health (CVH) based on overall LS7 score. Statistically significant relationships were found between ES-MBI and BMI, GHQ and BMI, GHQ and BP, GHQ and cholesterol, GHQ and diabetes, and GHQ and inadequate CVH even after controlling for covariates of age, gender, race, education, and socioeconomic status. To investigate whether sex differences exist between associations, stratified analysis by sex was performed. For males, statistically significant relationships between ES-MBI and BMI, GHQ and cholesterol, GHQ and diabetes, and GHQ and inadequate CVH were found even after controlling for covariates combined. For females, statistically significant relationships between GHQ and physical activity and GHQ and BMI were found.

This study reveals possible associations with indicators of psychological occupational strain and cardiovascular disease and its risk factors. Further investigation is important to address the limitations of this study and to inform policy and programs that aim to reduce cardiovascular risk in bus drivers by improving their working conditions and promoting their mental health.

Carpal tunnel syndrome (CTS) is a common peripheral neuropathy with known personal, medical and biomechanical risk factors including specific work-related physical exertion. CTS accounts for approximately 10% of work-related musculoskeletal disorders (WMSDs) which make up almost 31% of all work-related injuries in the United States (US), 2015. Among the US working adults CTS injury results in a median of 28 days away from work, resulting in loss of work time, loss of earning power, low rate of returning to pre-injury status, and high cost for treatment. It is important to determine whether any work-related factors other than the known biomechanical risk factors could be potential contributing factors to CTS among working individuals in the U.S. Recent research has found that work characteristics (e.g., long work hours or non-standard employment arrangements) and occupational psychosocial factors (e.g., high job demand or work-life imbalance) could impact on a range of health outcomes either as direct risk factors or by interacting with physical or other personal or work-related risk factors. However, there has been a paucity of evidence on whether work characteristics or occupational psychosocial factors are associated with CTS in working populations with no evaluation of these factors in a general U.S. working population.

Data from the 2015 National Health Interview Survey (NHIS) and its supplementary Occupational Health Supplementary (OHS) Survey were analyzed using Chi Square for bivariate comparisons and multiple logistic regression models to examine associations between occupational psychosocial risk factors and prevalence of CTS, while controlling for demographic characteristics, relevant health comorbidities, and work-related physical exertion. The prevalence of self-reported ever CTS diagnosed by a healthcare provider was 6.3% while the prevalence of those reporting CTS symptoms in the last 12 months was 3.0%, translating to 3.4 and 1.7 million U.S. adult workers. The psychosocial factors found to be associated with ever having a diagnosis of CTS were hostile work environment, worrying about losing one’s job, and work-life interference. High job demand and worrying about losing job were found to be associated with those with a past diagnosis of CTS and experiencing CTS symptoms within the last 12 months. The role of these occupational psychosocial risk factors for CTS along with physical job demands of frequent lifting and pushing at work should be evaluated further in longitudinal studies.

Psychosocial and environmental work factors and their effects on obesity and cardiovascular disease risk among firefighters

By

Javier Garcia Rivas

Doctor of Philosophy in Public Health

University of California, Irvine, 2019

Professor BongKyoo Choi, Chair

During the last two decades, US firefighters have gradually gotten heavier to become the third heaviest occupational group in the country and workplace has been identified as a salient environmental factor which contributes to the development of obesity among this occupational group. Nevertheless, few studies have studied the impact of the work environment and individual characteristics integrated in a socio-ecological model design.

The long term goal of this investigation is to make a contribution to the research of individual, occupational and environmental factors that play a role in the development of obesity in firefighters. The specific aims of this study were to 1) determine the association between worksite food environmental factors and obesity among male firefighters who work 24-hour shifts. 2) investigate the relationship between health-related (nutrition and physical exercise) self-efficacy and weight outcomes i.e. body mass index, waist circumference and body fat percent. Concurrently, analyze important occupational preceding variables (e.g. institutional, supervisor and coworker support) that facilitate/hamper self-efficacy; and 3) compare four sedentary behavior measures (number of steps, <100 and <150 counts per minute threshold in actigraphy, sitting time by posture sensor inclinometer and self-reported sedentary work) with regards to obesity and CVD risk factors.

The present study found an indirect association between environmental fast-food density and obesity indicators in male firefighters. The association was mediated by station- and individual-level eating behaviors. Secondly, organizational support was the most important occupational predictor of exercise and nutrition self-efficacy. However, only exercise self-efficacy was directly associated negatively with all three obesity indicators (Waist circumference, BMI and Body Fat Percent). Finally, self-reported sedentary work was identified as the best predictor of CVD risk factors when compared to other physical activity and sedentary behavior measures. Sitting time assessed by inclinometry was also an important predictor of systolic blood pressure and HDL cholesterol. Other measures of occupational inactivity (step count and sedentary behavior assessed by actigraphy) did not perform well as predictive tools.

The present dissertation offers an integral theoretical approach to a public health issue based on an occupational health perspective in order to prevent weight-related diseases and improve the health of firefighters.

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