Great question! Although this is not my area of expertise, I believe your first interpretation is correct, it is the standard sequence for inhaled therapies before airway clearance. Bronchodilators are given first to open up the airway, reduce bronchospasm risk, and improve delivery of subsequent inhaled meds. Osmotic agents then draw water into the airway to hydrate and loosen mucus. rhDNase breaks down DNA in thick secretions, reduces mucus viscosity, and are best given before clearance so the loosened mucus can actually be moved out. Then airway clearance techniques are used (PEP, autogenic drainage, manual techniques, etc.). So, you open with bronchodilators, thin with osmotic agents and rhDNase, and then clear with airway clearance.
Jessica