Question about plateau pressures

I understand teh danger of lung injury in volume ventilation with ARDS (specifically barotrauma and volutrauma). I also understand that plateau pressures are more important than peak airway pressures, adn the plateau pressures are more indicative of alveolar pressure and peak pressures are more indicative of the larger airway pressures and airway resistance. I understand that plateau pressures are static and done on indpiratory hold with no gas flow. I understand that peak pressures are dynamic. I understand that plateau pressures should not be greater than 30 cm H2O. What I do not understand is why my RT department subtracts the peep when determining teh plateau pressures. I may be wrong, but I don't think this is accurrate. If you are on a peep of 12 and do an inspiratory hold and get a plateau pressure of 39, well then isn't your plateau pressure 39? RT was subtracting 12 (peep) from that number and tellimg me that was the plateau pressure. I think that subtracting peep from plateau pressure gives you thoracic compliance, but that you should not be subtracting peep to determine plateau pressure.

This was important because they were telling me the plateau pressures were ok, because 39-12 was only 27 and thus the plateau pressures were < 30, but I don't agree and I think the plateau pressures were actually 39 which is dangerous (patient was maxed out on sedation).

Now, I know the strategies for elevated pressures (ie ... low VT's, pressure control ventilation, pressure regulated volume control ventilation, increased sedation, and even paralytics). I'm not asking that. I just want to know about what the true plateau pressure is (do you or do you not subtract the peep). I honestly don't think you do, because peep increased plateau pressure.