Valsalva maneuver

[Just_Kidney, ADN, BSN, RN]

I posted this in the nursing student forum but I'm not getting a lot of response so I'm going to try it here:

Okay, so maybe I'm just dense, but I'm really confused by this. The Valsalva maneuver promotes a vasovagal response (parasympathetic response): lowering B/P, pulse and increasing vasodilation, right? But my NCLEX review book says to have patients perform the Valsalva maneuver (unless contraindicated) when doing central line tubing changes to increase pressure in the central veins thus helping to prevent an air embolism? But how would the Valsalva maneuver increase central venous pressure? Also, I know it's contraindicated in patients with increased intercranial pressure, recent eye surgery, etc because it increases ICP, which also does not make sense to me physiologically.

Can someone please explain this, I'm just not getting it!