Thread by @angryhacademic

Head-down bed rest impairs vagal baroreflex responses and provokes orthostatic hypotension
journals.physiology.org/doi/abs/10.115…

30 days head down tilt (HDT) caused 4 of 10 healthy participants to faint after standing up for 5 minutes. some interesting stuff going on...

Heart rate:
Those who did faint, had a much smaller increase in heart rate on standing, just 23 ± 5 bpm, compared to non-fainters 49 ± 10 bpm

This suggests high heart rate is protective of fainting (in these very peculiar conditions)

Blood pressure:
Diastolic blood pressure had a similar response for fainters & not fainters, but systolic blood pressure reduced by about 15 mmHg for fainters, with no such change for non-fainters

there was similar plasma volume reduction in both groups

study also looked at baroreflex responses:
plasma volume (PV) fell before baroreflex response changed during HDT

PV then stayed low but stable; baroreflex slopes declined progressively

PV restored within 1 day of ending HDT, but baroreflex remained depressed for at least 5 days

"although our data do not rule out a contribution from reduced blood volume (BV), they point away from reductions of BV as the sole cause of the baroreflex abnormalities that developed during bed rest"

laying down and sitting norepinephrine and epinephrine were similar pre- and post-HDT

all this suggests to me that plasma/blood volume is less of a contributor to orthostatic intolerance/hypotension than other factors. whether these findings translate to #POTS = ❓❓❓