Heart Rate Variability During Serial Exercise Testing In Patients With Myalgic Encephalomyelitis, 2023, VanNess Mark et al

Autonomic dysfunction contributes to post-exertional malaise (PEM) in Myalgic Encephalomyelitis (ME). Heart rate variability (HRV) may be useful for examining measures of sympathetic and parasympathetic tonus.

PURPOSE: To continuously monitor HRV from rest to maximal exertion during serial exercise tests.

METHODS: Two patients with ME performed two CPETs 24 hours apart. Raw interbeat interval data collected through ECG were analyzed with Kubios software. HRV values for high and low frequency power (HFP and LFP) during a 5-minute rest period prior to the CPET and during the 5-min epoch leading up to the subject’s maximum HR during the CPET were extracted for both days. Frequency-domain HRV measures were then log transformed (lnHFP and lnLFP), and values were compared from day 1 to day 2 during the rest period and the maximum HR period. Data are reported as (resting period, max exercise period) for each variable. HFP and LFP were then compared to the maximum HR for each day.

RESULTS: In subject 1, both lnHFP (4.84, 6.38) and lnLFP (6.42, 7.33) were higher at rest on day 2, as compared to day 1. In the maximum HR period, lnHFP decreased on day 2 (2.34, 1.66) and lnLFP increased (1.42, 1.53). These values were coupled with a decrease in 13 bpm in the max HR on day 2 (172, 159 bpm). In contrast, for subject 2; both lnHFP (6.30, 6.13) and lnLFP (7.48, 6.71) were lower at rest on day 2. In the maximum HR period, lnHFP increased on day 2 (1.17, 3.42) but lnLFP decreased (2.78, 2.47). This was coupled with an increase of 8 bpm for this subject in the max HR on day 2.

DISCUSSION: While HFP is a measure of parasympathetic tone, LFP captures both sympathetic (SNS) and parasympathetic (PNS) activity. Increased HFP would suggest increased PNS tone and a blunting of max HR, with diminished HFP suggesting vagal withdrawal and greater SNS contribution. However, in subject 1, decreased HFP was associated with a lower max HR; and in subject 2, higher HFP was associated with a higher max HR. The direction of change for both HFP and LFP, along with the change in max HR, from day 1 to day 2 in these two subjects was opposite despite engaging in the same series of CPETs.

CONCLUSIONS: The results of this case series suggest that the autonomic response to exercise in those with ME differs from person to person. Future research may explore additional factors interacting during PEM.
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I wonder if they differ depending on other factors in the same person.

I’ve had wildly different responses to overexertion depending on how much I push it. It can be anywhere from inducing massive “adrenaline rush” that has me overexerting for weeks until I crash to immediately feeling like I hit a hard wall and getting crushing PEM.

 

I'm not even sure those factors would even have anything to do with ME/CFS. I wouldn't be suprised if taking 2 healthy controls couldn't also give you similar results.

 

Max Nelson in South Australia also investigated HRV differences at rest before and after the 2 CPETs and found null results too.

https://www.frontiersin.org/journals/physiology/articles/10.3389/fphys.2021.771899/full

In any event this means that differences in HRV cannot be assumed to be a cause of PEM.