Rapid breathing

My son has had intermittant rapid breathing since he was about 9 months old.  His pulsocs has always been good - 98 or 99 and there has never been a wheeze present.  A couple of his many er visits showed hyperinflated lungs and a CT scan of his lungs had evidence of interstitial marking and a broken glass appearance (both non specific findings we were told). Heart tests, brain mri and ph probe were normal.  These tests were done when he was admitted to the hospital for a week when he was one for a fast RR (ranged between 60 to 80 breaths a minute)normal.  He is now being treated for asthma with 220 flovent 2 puffs 2x a day, singulair and xopenex as needed.  His RR has been alot better with only a few episodes of fast breathing since treatment begun.  I am concerned with the high dose of steroids and there affect on his growth. Have you ever seen a case were a child has had fast breathing with nothing else and they grew out of it?  We are currently ruling out any forms of CHILD.  They found one irregular ABCA3 mutation on one allele so he is scheduled for a repeat CT scan on the 11th.  We are under the care of an excellent pulmonologist and we are hoping the CT scan is clean and our son is only a carrier of the ABCA3 mutation but given his history they seem to think there is a chance that he may have it on the other allele.  If the CT scan is clean - are there any other things that may cause his fast breathing when he is not on the Flovent? He is now 18 months old.

[lawteddy]

Thank you.