Background: Although involvement of autonomic dysfunction in ME/CFS has been proposed, conflicting evidence has made it difficult to draw firm conclusion regarding autonomic nervous system activity at rest in ME/CFS patients. Furthermore, little attempts have been made to study autonomic nervous activation in response to physical exercise, which is remarkable as severe exercise intolerance is one of the core features of ME/CFS.
Purpose: To examine whether autonomic nervous activation at rest, during an exercise stressor, and during recovery from exercise is impaired in patients with ME/CFS.
Methods: Twenty ME/CFS patients and 20 healthy, sedentary controls participated in a case–control study. Different autonomous variables, including cardiac (blood pressure (BP), heart rate (HR), and heart rate variability measures in time (RMSSD) and frequency (LF/HF) domains), respiratory and electrodermal responses were assessed using electrophysiological measures. All assessments were performed during 10 minute periods preceding (=at rest) and following (=recovery) an acute bout of exercise (=submaximal bicycle exercise test). In addition, cardiorespiratory assessments were performed continuously during exercise testing.
Results: CFS/ME patients showed similar resting HR, BP, RMSSD, electrodermal function and respiratory rate as controls. Although LF and HF in the ME/CFS patients were lower (p = .038, p = .024 resp) than in controls, the LF/HR ratio was similar (p = .314), indicating decreased sympathetic and parasympathetic activation in ME/CFS at rest while maintaining the sympatho/vagal balance.
Exercise capacity and performance parameters were similar between the ME/CFS and controls (p > .05), as were HR and BP responses during exercise. Although the LF/HF ratio increased in both groups, reflecting sympathetic dominance and parasympathetic inhibition during exercise, in ME/CFS the increase was not large enough to reach significance (p = .059) as was the case in the controls (p = .001) demonstrating that although similar sympathetic and parasympathetic modulation takes place during exercise in ME/CFS as in healthy people, the magnitude of this modulation is impaired in ME/CFS.
After the exercise mean HR declined (p < .001) in both groups, but a differential response was seen regarding full recovery. HR during recovery did not significant differ from HR at rest (p = .578) in controls, indicating HR quickly recovered to the original baseline levels following exercise. However in the ME/CFS group a significantly higher HR was observed during recovery compared to rest (p = .031), and the 10 min recovery was insufficient for the HR to return to baseline levels (p = .037).
Conclusion(s): At rest, parameters in the time-domain represented normal autonomic function in ME/CFS, while frequency-domain parameters indicated the possible presence of diminished (para)sympathetic activation. Although a similar (para)sympathetic modulation took place during exercise in ME/CFS as in healthy people, the magnitude of this modulation was impaired in those with ME/CFS. Reduced parasympathetic reactivation during recovery from exercise was observed in ME/CFS for the first time.
Implications: Delayed HR recovery and/or a reduced HR recovery as seen in ME/CFS have been associated with poor disease prognosis, high risk for adverse cardiac events, morbidity and sudden death in other pathologies, which implies that future studies should examine whether this is also the case in ME/CFS and how to safely improve HR recovery in this population.
Keywords: Autonomic nervous system; Heart rate; Aerobic exercise
Funding acknowledgements: This study was funded by the Ramsay Research Fund of the ME Association (United Kingdom).
