If you read Prehospital Emergency Care, 9th edition by Joseph Mistovich, in Chapter 6 on Lifting and Moving Patients, it does mention that Trendelenburg is no longer a treatment for shock due the fact that it causes abdomen organs to compress the diaphram and increase the pt's difficulty breathing. In addition for pt's with head bleeds it makes the situation worse for them.
I was not referring to the Mistovich book.
Compression of the abdominal organs is one of the points of using it. A patient in shock is sufferering from a fluid perfusion problem, not primarily from a breathing problem. I couldn't find a study, but from my experience I would be willing to bet there is considerably more breathing restirction from immobilization than from trendelenburg. In the event that the position has a significant effect or the breathing effort, the patient is likely to have issues that are bigger than can be managed without invasive techniques anyway.
Just as a thought, if you were concerned about an intraabdominal bleed, compression of the compartment would be a good thing.
You are right,Trendelenburg has been a contraindication in CHI for sometime, but we are speaking about valcular volume and movement for the prupose of central perfusion as far as I can tell.