> Now I'm really confused. Doesn't a Valsalva dramatically increase your
> intrathoracic pressure, therefore reducing venous return? And back to the
> original post, surely leg raise will increase venous return? So how come
> one augments the other; to me it looks like they cancel each other out!
> Unless you're saying it's not the Valsalva pressure that's vagotonic, it's
> the sudden release of Valsalva pressure that's vagotonic? I need to go
> look up why Valsalva is vagotonic...
As I recall the descriptions, it's the release of Valsalva that increases
vagal tone, therefore raising the legs will increase the venous return.
If I remember my physiology correctly, cardiac output is varied to maintain
a constant systolic BP, with the various organs and tissues autoregulating
blood flow by varying arteriolar resistance. A sudden increase in venous
return would increase the stroke volume as noted, leading to an increased CO
without any systemic need for it. The baroreceptors should respond to that
by acting to decrease CO - presumably by increasing vagal tone.