Sharon,
Theophylline and other methylxanthines work by incapacitating the machinery
that "shuts off" cell energy metabolism (ATP cycle, adenyl cyclase is the
enzyme incapacitated if I remember correctly, I might not).  Therefore, at the
cellular level, the body never gets a rest.  For an infant, who clears
methylxanthines slowly, this effect would be especially acute. When it is a
choice between breathing or growing, most peds will opt for the baby
breathing!
Catherine Watson Genna, IBCLC