Changes in levels of sodium or potassium may be relevant to feeling ill but we are discussing orthostatic intolerance and I do not see their relevance to that. I think some people think that a low serum sodium goes with a low blood volume or in simple terms 'not enough salty fluid' but that is not the case. In most cases low sodium is associated with too much fluid.
As indicated above the general assumption behind the term OI is that the heart is unable to pump enough blood to the brain. The commonest reason for that is a low blood volume relative to the context. That gets complicated to explain so there is no single definition of low blood volume relevant here. In haemorrhagic shock you cannot stand up because there is literally an abnormally low amount of blood. But in most faints the problem is that there is not enough blood to fill a dilated vascular tree with the result that not enough blood is being returned to the heart for it to pump out enough to keep the brain full.
As I indicated, I am sceptical that OI in ME has anything to do with either situation, but I am pretty sure that all the physicians who deal with OI think it does - which is why they think heart rate matters. I suspect heart rate may be irrelevant.
I have labwork dating back to when I was healthy, and can see the sharp contrast of healthy vs ME for calcium, sodium, and potassium.
So I was curious if @ahimsa had low values in her labs too.