Very well, reposted from a few hundred pages ago, a list of things Shepard needs to survive, slightly updated:
When viewing the following keep in mind several things: The Citadel is massive. Shepard is at the base of the ring, essentially under the Presidium Tower in the center of the Citadel where the Crucible is docked. Shepard has hardly any armor and no kinetic barriers as proven by the blood spatter from Marauder Shields attack.

This explosion more than envelops the area where Shepard was in the decision chamber, the sheer size of this explosion should let us assume that it is LARGE and FATAL to anything near it.
A list of things Shepard would need to survive in the Destroy ending in order to take the famous breath:
1. Potentially fatal injuries already sustained.
2. The initial explosion of the Destroy tube (the strength and force of this explosion alone could outright kill Shepard at point blank range).
3. Being ground zero when the Red Wave is sent out (it's enough force to move space debris and Reapers, it would most definitely hurt Shepard being that close).
4. The Red Wave itself. "You can wipe out all Synthetic life if you want --- Even YOU are partly Synthetic," it's entirely possible ALL of Shepard's implants would be deactivated on contact with the Red energy.
5. The Citadel ring explosion, image above (or, The Crucible Explosion, or The Tower explosion, it's source does not limit it's destructive force).
6. The vastness of space without a helmet. Or air for that matter.
7. Re-entry into Earths atmosphere without kinetic barriers or whole armor or a helmet (the sheer heat of freefall would vaporize Shepard if other things hadn't already.)
8. Impact with the ground, assuming anything is left of Shepard it would have to be scooped up with a shovel and bucket.
The infamous "Shepard Breath" scene I'm 99% positive it takes place on Earth, see my sig for London Rubble/Breath scene Rubble comparison. Shepard is in one piece, bloodied and battered, and takes a breath on Earth after the list above, the only logical explanation is that Shepard never left Earth. Whether or not things occurred in your head or in real life while you were unconscious is up in the air at this point.