Reversibility of Bronchial Obstruction in Children Born Preterm

1. Outpatient follow-up - Parents are asked to sign the study consent form if the child meets the indication criteria. If the trial subject is not taking inhaled therapy, he/she may immediately continue to Check-up A. If the subject is taking short- or long-acting bronchodilators, these medications must be discontinued before each check-up and at the following intervals: * Short-acting β2 agonists (SABA) - 12 hours before the examination * Short-acting inhaled cholinergic antagonists (e.g. ipratropium bromide) - 12 hours before the examination * Long-acting inhaled betamimetics (long-acting β2 agonists, LABAs) - 48 hours before the examination * Long-acting muscarinic antagonists (LAMA) - one week before the examination 2. Check-up A - Baseline pre-interventional spirometry may be performed if the subject has no evidence of acute respiratory infection or has not had an acute respiratory infection two weeks previously. Otherwise, the examination should be rescheduled. Before baseline spirometry, the child is randomized to a dilatation test with inhaled salbutamol, ipratropium, or a combination of fenoterol/ipratropium. After a defined time interval (15 - 30 minutes), post-interventional spirometry follows. The subject is invited to attend Check-up B (which preferably follows 1 to 30 days after Check-up A, but no later than 3 months). 3. Check-up B - Baseline pre-interventional spirometry may be performed if the subject has no evidence of acute respiratory infection or has not had an acute respiratory infection two weeks previously. Otherwise, Check-up B should be rescheduled. Before baseline spirometry, another inhaled bronchodilator according to the preceding randomisation is applied. After a defined time interval (15 - 30 minutes), post-interventional spirometry follows. Resting spirometry with a dilatation test with another drug, the choice of which is guided by randomization, is performed. 4. Check-up C - Baseline pre-interventional spirometry may be performed if the subject has no evidence of acute respiratory infection or has not had an acute respiratory infection two weeks previously. Otherwise, Check-up C should be rescheduled. Baseline spirometry with the last bronchodilator not yet used is performed.