Thread by @ZdenekVrozina

In people with long COVID, arterial stiffness in the large vessels looked no different from people who’d recovered cleanly.
The deficit sits one level down - in the smallest vessels, and specifically in how fast they can react.🧵

A new paper from Tübingen measured microvascular reactivity - how quickly a muscle re-oxygenates after its blood supply is cut off for a few minutes and then released. That re-flooding step is called reperfusion. A near-infrared sensor on a forearm muscle tracks how oxygenated the tissue is throughout.

29 patients with symptoms lasting more than 12 months after infection and a real impact on daily life, against 33 people who had the same infection and recovered without trouble. Everyone was infected in 2020–2021.

Reperfusion was slower in the patients (1.67 vs 2.23 %/s). Meanwhile large-artery stiffness, the ankle-brachial index, and resting values didn’t differ. So the deficit isn’t in the large arteries and isn’t about the resting state - it’s in the small vessels’ ability to dilate on demand.

The patients are less fit, so they perfuse worse. The authors expected that - the patients did have lower aerobic fitness (VO₂max). So they adjusted the reperfusion difference to subtract the effect of fitness. It survived. And it didn’t track with how much physical activity people reported.

Fitness is subtracted statistically, not physically, and activity was captured by a questionnaire with three-month recall. So the honest version - fitness doesn’t fully explain the deficit - not that it’s been proven unrelated to conditioning.

The patient group carried more cardiovascular risk and medication - a quarter were on anticoagulants, versus 3%. The authors admit they can’t tell whether that’s a consequence of COVID or something that predated it. Part of the measured gap could ride on that imbalance.

What might blunt that on-demand dilation?
Lower availability of nitric oxide (vasodilator released by the vessel lining), autoantibodies against vascular receptors, microclots, smoldering inflammation. None of these mechanisms were measured here

It slots into a growing pile of data on endothelial and microvascular dysfunction after COVID. And it fills in the exercise-intolerance picture. Other work has shown reduced oxygen extraction at the periphery and damaged mitochondria in muscle. This adds a middle link - the delivery of oxygen through the smallest vessels.

It’s a case-control study at a single time point. The sample is small and mildly underpowered.

One detail actually works in the finding’s favor. The controls had also been infected and could carry hidden vascular changes. That would tend to wash the difference out - so the real effect may be larger, not smaller.

In post-COVID exercise intolerance there’s an objective peripheral vascular limit you can measure right at the muscle. For rehab, the brake may be in how oxygen reaches the muscle in the first place.

Thiel at al., Impaired microvascular reactivity in post-COVID-19 syndrome is independent of cardiorespiratory fitness. journals.physiology.org/doi/full/10.11…