Of relevance to the amyloid question, the peer review file contained —
Authors replied —
Authors replied —
In this article, we tested the notion that accumulation of amyloid-containing deposits inside capillaries causes local hypoxia by local stoppage of blood flow. This has been hypothesized to be the cause of the diverse symptoms of long COVID (Kell et al. 2022, Aschman et al, 2023), and this was what we intended to address here.
We agree with the reviewer that a halted RBC flux is a possible reason for a potentially reduced blood flow. Unfortunately, we cannot directly measure microvascular blood flow, and do not know if local blood flow is reduced. The measurements suggested by the reviewer are presently intractable in humans and unfortunately, thus not possible for us to verify in this cohort. No animal model of Long COVID currently exists, to verify this hypothesis.
As opposed to the hypothesis, our NIRS-derived muscle deoxygenation data does not support the theory that a reduced microvascular blood flow and lower local O2 delivery causes hypo-circulation in skeletal muscle during exercise, although we realize these data are not conclusive.
There are various other potential mechanisms that can explain a potentially reduced O2 transport in patients. Since we do not find proof of an impaired (or unimpaired) blood flow, we prefer not to speculate about possible ways by which (local) blood flow can (or cannot) be impaired in long COVID.
In light of the reviewer suggestions, we have now revised the results section to include the following revised text:
Results, page 6, line 205-210: “Neither did we observe any signs of skeletal muscle tissue hypoxia, as the skeletal muscle capillary-to-fiber ratio, capillary density (Fig.2B) and intracellular and circulating lactate concentrations (ED Fig.3G+6A) were not different between long COVID patients and controls. Therefore, we conclude that post-exertional malaise cannot be explained by the hypothesis that these deposits block vessel perfusion, causing local tissue hypoxia (Kell et al. 2022, Aschman et al, 2023).”
And also within the limitations section of the discussion, to highlight that functional impairments may still persist:
Discussion, page 9, line 323-326: “We only evaluated the presence of amyloid, but not the function of the endothelium, or local blood flow. As such, the possible contribution of chronic endotheliitis or reduced blood flow to the development of post-exertional malaise and/or exercise intolerance in long COVID remains open (Charfeddine et al, 2021, Turner et al, 2023)”
Reviewer #2 said:
Authors replied —
We are aware of the work that other scientists have done on endothelial dysfunction in patients with long COVID. There is however not a single marker for endothelial abnormalities and endotheliitis, and as such, drawing conclusions from the results of one particular marker is, in our opinion, inappropriate. Such a study requires a more elaborative approach from different angles, and as such, we reason that studying endothelial dysfunction in long COVID and its potential link with exercise intolerance and the development of PEM requires multiple new experiments. This would, in our view, slow down the current publication specifically targeting skeletal muscle aberrations and post-exertional malaise. We have added this avenue for future research to the discussion, as the reviewer suggests.
Discussion, page 9, line 323-326: “We only evaluated the presence of amyloid, but not the function of the endothelium, or local blood flow. As such, the possible contribution of chronic endotheliitis or reduced blood flow to the development of post-exertional malaise and/or exercise intolerance in long COVID remains open (Charfeddine et al, 2021, Turner et al, 2023)”
Discussion, page 9, line 323-326: “We only evaluated the presence of amyloid, but not the function of the endothelium, or local blood flow. As such, the possible contribution of chronic endotheliitis or reduced blood flow to the development of post-exertional malaise and/or exercise intolerance in long COVID remains open (Charfeddine et al, 2021, Turner et al, 2023)”
Reviewer #3 said:
Authors replied —
In this article, we tested the notion that accumulation of amyloid-containing deposits inside capillaries causes local hypoxia by local stoppage of blood flow. This has been hypothesized to be the cause of the diverse symptoms of long COVID (Kell et al. 2022, Aschman et al, 2023), and this was what we intended to address here.
We agree with the reviewer that a halted RBC flux is a possible reason for a potentially reduced blood flow. Unfortunately, we cannot directly measure microvascular blood flow, and do not know if local blood flow is reduced. The measurements suggested by the reviewer are presently intractable in humans and unfortunately, thus not possible for us to verify in this cohort. No animal model of Long COVID currently exists, to verify this hypothesis.
As opposed to the hypothesis, our NIRS-derived muscle deoxygenation data does not support the theory that a reduced microvascular blood flow and lower local O2 delivery causes hypo-circulation in skeletal muscle during exercise, although we realize these data are not conclusive.
There are various other potential mechanisms that can explain a potentially reduced O2 transport in patients. Since we do not find proof of an impaired (or unimpaired) blood flow, we prefer not to speculate about possible ways by which (local) blood flow can (or cannot) be impaired in long COVID.
In light of the reviewer suggestions, we have now revised the results section to include the following revised text:
Results, page 6, line 205-210: “Neither did we observe any signs of skeletal muscle tissue hypoxia, as the skeletal muscle capillary-to-fiber ratio, capillary density (Fig.2B) and intracellular and circulating lactate concentrations (ED Fig.3G+6A) were not different between long COVID patients and controls. Therefore, we conclude that post-exertional malaise cannot be explained by the hypothesis that these deposits block vessel perfusion, causing local tissue hypoxia (Kell et al. 2022, Aschman et al, 2023).”
And also within the limitations section of the discussion, to highlight that functional impairments may still persist:
Discussion, page 9, line 323-326: “We only evaluated the presence of amyloid, but not the function of the endothelium, or local blood flow. As such, the possible contribution of chronic endotheliitis or reduced blood flow to the development of post-exertional malaise and/or exercise intolerance in long COVID remains open (Charfeddine et al, 2021, Turner et al, 2023)”
