Preprint Wearable heart rate variability monitoring identifies autonomic dysfunction and thresholds for post-exertional malaise in Long COVID, 2025, Ruijgt+

[Hutan]

Typically, higher HRV indicates better overall health, although significant inter-individual variability exists [15], partly related to sex and age-related reductions [16,17].

Inclusion criteria were 1) minimal one SaRS-CoV-2 infection, as diagnosed by a general practitioner, and 2) being treated by a general practitioner for long COVID-related symptoms, including severe fatigue, brain fog, sleep disturbances, and particularly post-exertional malaise. A control group consisted of participants matched for age and sex to the long COVID group. Control participants could have had a SaRS-CoV-2 infection, but without long COVID symptoms. Exclusion criteria for the control group were chronic illnesses and related medication use or being an elite athlete.

Long COVID participants had on average a higher BMI compared to the age- and sex healthy controls. Patients with long COVID had a lower power output and oxygen uptake consumption (V̇ O2) at VT1 compared to controls (p<0.001).

The paper notes that HRV varies with sex and age. It says that the control group was matched on sex and age, which is great, but there was a higher percentage of males in the control group (47.6% versus 31.0% - surprisingly, this was reported with a p value indicating it was not a statistically significant difference, but it seems like an important difference).

There was a significant difference in BMI (23.1 for the controls versus 25.7).

I think the differences in sex ratio and BMI (and presumably baseline fitness) are a problem. I think we would need to see this study replicated with carefully matched controls. As @Yann04 said, it would be great to see the study done with chronic illness controls.

 

[Hutan]

A recent literature overview confirmed a lower HRV in patients with long COVID, although methodological quality was poor and measurements were very short and not related to specific daily life activities or sleep [22]. These findings suggest autonomic dysfunction, characterised by diminished parasympathetic activity, which may be a key feature in long COVID patients [8,19], but how HRV is related to the pathophysiology of ppost-exertional malaise in long COVID is currently unknown. Indeed, HRV in patients with myalgic encephalomyelitis/chronic fatigue syndrome (ME/CFS) remained lower shortly after exercise, while HRV increased in healthy controls [23].

The authors suggest that lower HRV has been repeatedly reported in long Covid and the lack of a rebound after exercise has been reported in ME/CFS.

Current optimal treatment to avoid PEM in patients with long COVID includes avoiding overexertion, and as such, long COVID participants were instructed to avoid exercise above the heart rate at the first ventilatory threshold. This was determined in a submaximal exercise test on a bike, three to six weeks prior to the HRV assessment. Furter details can be found in the supplemental file. VT1 was determined using the V-slope and ventilatory equivalent methods and was assessed by two experts. Heart rate at the first ventilatory threshold was defined as the patient-specific PEM threshold. Heart rate pacing was not advised in the control group.

I agree with @poetinsf, I don't think this idea of keeping below the heart rate associated with the ventilatory threshold in order to avoid triggering PEM has been proven.

It's worth noting that the patients were advised to restrict physical activity for three to six weeks, as in, not exceeding the heart rate associated with the ventilatory threshold. The control group was not given that advice. Three to six weeks could have been long enough to change someone's fitness, regardless of the level of fitness before that.

Post-hoc analyses allowed us to classify long COVID patients based on daily-life-impairments into mild and moderate patients. We reasoned that ∼15 ml O2.kg-1.min-1 is needed for daily life activities [26]. Patients with VT1 values >15 ml O2.kg-1.min-1 were classified as mildly impaired, and patients with VT1 values <15 ml O2.kg-1.min-1 were classified as moderately impaired.

 

[rvallee]

I haven't looked at the paper yet - it does look interesting. But, I wonder if what we are looking at is just the difference between fit people and people who are deconditioned? That AI result I posted above suggests that it could be.

There's probably some of that in general, but 2 years ago when I developed POTS and I started monitoring my vitals, I saw my HRV go from about low 30s on average to my current average of 60s over the course of about 6 months, and I wasn't doing any exercises of any kind. So HRV may be affected by conditioning, but it isn't a direct relationship.

Also related, but in the last year or so, especially the last 3 months, I have significantly improved, function a lot more, and although I'm still not fit by stretch of the imagination, I am definitely much more conditioned than I was a year ago, with no change to my HRV.

It's also very noticeable how my HRV changes from immune challenges, whether an acute illness (strep A, then later COVID), but also from vaccination.

HRV seems related to conditioning, but it's definitely not a simple relationship of more fit = higher or more stable HRV.

 

[Grigor]

Sports physician Kasper Janssen, who's the co-author of the paper, gave a talk recently about how he uses the HRV. You can turn on the auto-translator in the captions section.

 

[Hutan]

That's an interesting talk. The pace is fast and the translation is a bit off, so it makes it hard to get all the points

Janssen explains what causes HRV at 12 minutes. When you breath out, the rib cage contracts, leaving the heart less space to pump. So the heart rate slows down. When you breathe in, the rib cage expands, making the operation of the heart easier, so the heart rate speeds up.

So, breathing rate is relevant. So too is the function of the nerves that sense what is going on and adjusts the heart's operation.

Janssen also talks about various wearables and their apps.

He strongly suggests avoiding alcohol. Also, short episodes of exercise that does not lift the heart rate over the heart rate at the anaerobic threshold.

 

[Utsikt]

Also, short episodes of exercise that does not lift the heart rate over the heart rate at the anaerobic threshold.

That sounds like a very dangerous suggestion.