Background: Breast cancer survivors often experience fatigue. Previous studies suggest that vagally mediated heart rate variability (HRV) is related to fatigue among breast cancer survivors. However, the mechanism behind this relationship is unclear. Research in non-cancer populations indicates that people with lower HRV experience more intrusive thoughts than individuals with higher HRV. Breast cancer survivors commonly experience cancer-related intrusive thoughts and survivors who experience intrusive thoughts at diagnosis have increased fatigue. Thus, cancer-related intrusive thoughts may mediate the relationship between HRV and fatigue. Aims: This study investigated whether cancer-related intrusive thoughts mediate the relationship between HRV and fatigue among breast cancer survivors. This study also investigated social support as a potential moderator in the mediational model, as the relationship between intrusive thoughts and fatigue appears to be attenuated by high social support.
Methods: This study utilized multiple samples of survivors. The first sample was comprised of breast cancer survivors who had completed surgery but not begun adjuvant treatment. Survivors' HRV was recorded and they completed measures of fatigue (MFSI), intrusive thoughts (IES/IES-R), and social support (ISEL). The second sample consisted of women who had completed their breast cancer treatment. Similarly, survivors HRV was recorded and they completed measures of fatigue (MFSI), intrusive thoughts (IES), and social support (ISEL). The third sample consisted of longitudinal data from second sample participants, collected 3 months and 6 months after the initial visit.
Results: In the first sample, HRV was not associated with fatigue. Similarly, HRV was not associated with cancer-related intrusive thoughts. Cancer-related thoughts were associated with fatigue, although they did not mediate the hypothesized relationship between HRV and fatigue. Further, there was no evidence that social support moderated the proposed mediation. The same pattern of results was observed in the second sample. In the third sample, cancer-related intrusive thoughts and fatigue demonstrated effectively zero variance over time and thus the results must be interpreted with caution. HRV did not predict the slope of fatigue, nor did it predict the slope of cancer-related intrusive. The slope of cancer-related intrusive thoughts did not mediate the relationship between HRV and the slope of fatigue.
Conclusions: Across two samples of breast cancer survivors there was no evidence to support cancer-related intrusive thoughts as a mediator of the relationships between HRV and fatigue and social support did not appear to moderate the mediation. The longitudinal analysis was unable to yield reliable data as the slopes of cancer-related intrusive thoughts and fatigue had virtually no variance. These findings suggest that the relationship between HRV and fatigue may not be as strong as previously suggested. It is possible that the association between HRV and fatigue is larger among particular groups of survivors, such as women with lower socio-economic status and comorbid health conditions. Thus, future work should consider the role of moderators. Further exploration of fatigue, intrusive thoughts, and other sources of stress and distress faced by breast cancer survivors is crucial to improving their quality of life.