Inferior Vena Cava Collapsibility for Fluid Responsiveness

Ultrasound of the IVC is the people's volume responsiveness test. The idea is that you can non-invasively tell if a patient will benefit from getting more IV fluids. Everybody wants this to work so we can have a simple test to help us in critical situations. It is more reliable in mechanically ventilated patients with a fixed tidal volume but how about the non-intubated patient? Can we trust it?