(Inspiratory to expiratory flow at 50% of expiratory volume) in tidal breathing effect in obstructive airways

 
I am investigating the effect of bronchodilator on asthmatic children the parameter  i have calculated during tidal breathing.
please i want a paper with a good reference talking about this parameter  during tidal breathing in obstructive airways(Asthma), and the differnces in this parameter between normal and asthmatic children. and the changes of this parameter after receiving bronchodilator.