Spousal bereavement is associated with more pronounced ex vivo cytokine production and lower heart rate variability: Mechanisms underlying cardiovascular risk?
Introduction
The loss of a spouse is a highly stressful event. Indeed, the death of a spouse ranks first on the social readjustment rating-scale (Moon et al., 2013). The period surrounding spousal bereavement puts people at considerable increased risk for morbidity and mortality (Moon et al., 2013). Excess mortality among those who are widowed is highest in the first three to six months after bereavement and decreases over time (Stahl et al., 2016). Heart disease accounts for the largest proportion of these deaths attributed to spousal bereavement (Stahl et al., 2016). Depression, anxiety, and somatic complaints such as fatigue and sleep disturbances are hallmark characteristics of bereavement (Assareh et al., 2015).
Autonomic nervous system functioning is likely dysregulated when one is bereaved. Higher parasympathetic activity facilitates energy conservation (Thayer and Sternberg, 2006). Parasympathetic underactivity has been linked with a number of adverse mental and physical health outcomes. The variability in heart rate in the high frequency range is directly mediated by the vagus nerve and serves as a marker for vagally regulated heart rate variability (described as HRV for the remainder of this manuscript). Lower tonic HRV is a marker of all-cause mortality (Thayer and Sternberg, 2006); importantly, lower HRV is a well-established risk factor for cardiovascular disease, as well as diabetes, even after controlling for other cardiovascular risk factors (Haensel et al., 2008). Stress and depression can lower HRV.
A few studies have examined the association between HRV and bereavement with conflicting results. One study which consisted of 10 bereaved individuals and two other comparison groups (one depressed and one healthy comparison group), did not show differences between spousal bereavement and HRV (O’Connor et al., 2002). However, in this study, bereaved individuals who exhibited more depressive symptoms had lower HRV than those bereaved who reported less depressive symptoms (O’Connor et al., 2002). Yet in this study, widowers participated up to 24 months after the death of their spouse (O’Connor et al., 2002). Given cardiovascular risk is highest in the initial months after the loss (Moon et al., 2013), those who lost their spouse more recently may be most at risk for lower HRV. Another study showed that bereavement was associated with reduced HRV (Buckley et al., 2012); however, participants in this study consisted of bereaved spouses, bereaved significant others (i.e. romantic partner, unmarried), and bereaved parents. Given the psychological processes one experiences when losing a child are different from losing a spouse (i.e., more intense grief) (Sanders, 1980), it is possible that the data of those who lost a child disproportionally contributed to this finding (Stanley et al., 2004).
Psychological stress can enhance inflammation. Inflammation of the vessel wall promotes both the initiation and progression of atherosclerosis (Ellenbogen et al., 2002; Lai and Linden, 1992; Ridker et al., 2000; Roy et al., 2001). Given that the stress-response system boosts inflammation, which is a major factor underlying cardiovascular disease, researchers have begun investigating links between bereavement and inflammation. In a study of 64 older adults (36 widows), bereaved participants had higher circulating plasma levels of IL-1RA and IL-6 if they also carried the IL-6 174 SNP, a single-nucleotide polymorphism (SNP) in the promoter region of the IL-6 gene (Schultze-Florey et al., 2012). Secondary data analysis from Midlife in the United States (MIDUS) II biomarkers project revealed that those who experienced the death of a person close to them 5–63 months prior to assessment had higher levels of inflammatory circulating biomarkers IL-6 and E-selectin, but not of ICAM-1 or C-reactive protein (CPR) (Cohen et al., 2015). Although partially impacted by third variable influences, these studies provide some evidence that bereavement is associated with increased levels of circulating pro-inflammatory cytokines.
A major limitation of the use of circulating (serum or plasma) cytokines as biomarkers of inflammation is that their levels are often close to, and more often below, the limit of detection of the assay and exhibit extreme variability as result of a number of factors including diurnal variation, changes in plasma volume, and enlargement of the cell pool (Steptoe et al., 2007). A more complete cytokine signature can be obtained by analyzing the capacity of immune cells to produce inflammatory mediators after ex vivo stimulation. This method more likely represents the in vivo situation where cytokines are produced by the immune system in response to stress or infection (Korenromp et al., 2011; Mommersteeg et al., 2008). Moreover, the ex vivo stimulation method allows to discriminate between production of cytokines by monocytes/macrophages and by T cells, depending on the stimulus applied in the cultures (Korenromp et al., 2011; Mommersteeg et al., 2008).
In the current study, we examined individuals shortly after the loss of a spouse who had been married for at least three years (most of whom were married for much longer). We examined whether or not (a) bereaved individuals had lower HRV than age-matched non-bereaved individuals (b) bereaved individuals showed more pro-inflammatory cytokine production by peripheral blood leukocytes stimulated with lipopolysaccharide than age-matched non-bereaved individuals. We hypothesized that bereaved individuals would exhibit lower HRV than non-bereaved individuals (Hypothesis 1). We also hypothesized that bereaved individuals would have higher levels of ex vivo cytokine production than age-matched non-bereaved comparisons (Hypothesis 2). We also explored whether those who were bereaved reported more depressive symptoms than non-bereaved individuals. We expected that, on average, bereaved individuals would meet the clinical cut-off score for major depressive disorder (MDD) based on the extant literature suggesting that symptoms in the early stages of bereavement mirror those of patients with MDD (Fried et al., 2015).
Section snippets
Study sample
Thirty-two recently bereaved individuals (Mean = 89.68 days since death, SD = 17.09) and 33 age-matched comparisons completed a blood draw, EKG, and self-report questionnaires. The primary aim of this study was to determine the mechanisms that underlie the increased CVD risk among bereaved adults. Individuals who recently experienced the loss of their spouse were contacted and recruited from obituaries, support groups, flyer distribution, online postings, and community events. Comparison
Results
Important characteristics of the study sample are presented in Table 1. There was no significant difference between bereaved and age-matched comparisons in regard to age, smoking status, sleep disturbance, ethnicity, education level, alcoholic drinks per week, and comorbidities. Sex was distributed equally among both groups. Interestingly, BMI was lower among bereaved individuals than aged-matched comparisons. The pro-inflammatory cytokine composite index (IL1β, IL-6, TNFα, CCL2, CCL4, IL-6R,
Discussion
Spousal bereavement was associated with lower HRV and a more pronounced ex vivo cytokine production in both unadjusted and adjusted models. Indeed, even after adjusting for confounding factors including age, sex, BMI, physical activity, comorbidities, sleep disturbance, alcohol use, and depressive symptoms–spousally bereaved individuals demonstrated an enhanced pro-inflammatory response and lower HRV. Our findings add to the growing literature regarding the mechanisms that may underlie
Conclusions
In sum, this study adds to our growing understanding of the mechanisms that underlie bereavement-related cardiovascular risk. Future longitudinal studies are needed to determine the temporal relation between the risks. Understanding the biological mechanisms that underlie bereavement could allow researchers to create therapeutic targets for interventions to reduce or prevent the toll of a “broken heart.”
Conflicts of interest
The authors have no conflicts of interest.
Acknowledgements
This work was supported by the National Heart, Lung, and Blood Institute (1R01HL127260-01). The technical assistance of Mr. Jia Liu is gratefully acknowledged. We are very grateful to Patricia Morales and Kristi Parker for project coordination, and Levi Saucedo for data management. Finally, we appreciate DeWayne Williams and Derek Spangler for cleaning and scoring the HRV and Lani DuFresne for editing.
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