Although some studies have brought some evidences about the efficacy of positive expiratory pressure (PEP)-mask therapy as an airway clearance technique, yet it is not clearly understood what is the contribution of this technique in modifying peripheral ventilation inhomogeneity, a typical feature of patients with Cystic Fibrosis (CF).

The aim of this study is to investigate how PEP-MASK affects ventilation inhomogeneity in children and adolescents, with moderate to normal CF lung disease by the change in acinar airways (Sacin), lung clearance index (LCI) and conductive airways (Scond) indexes derived from nitrogen multiple-breath washout test (N2MBW).

Investigational device: PEP-MASK=Positive Expiratory Pressure delivered through a face mask.

Standard Operating Procedures used for evaluating CF lung disease and ventilation inhomogeneity.

Sample size: Based on preliminary data collected on seven subjects who performed nitrogen multiple-breath washout test before and after PEP-MASK, assuming a variation of 20% of Sacin, the study requires a sample size of 18 subjects.

Inclusion Criteria:

• CF diagnosis
• Patients hospitalized for a scheduled intravenous antibiotics cycle and regularly followed-up by the CF Centre
• ≥ 15 kilograms
• FEV1 ≥ 40%predicted
• Ability to perform NMBW test
• Ability to perform spirometry
• Willing to adhere to protocol procedures
• Patients in treatment with PEP-MASK

Exclusion Criteria:

• Pulmonary Exacerbation within two last week
• Burkholderia cepacia infection
• Patients in lung transplantation waiting list
• Patients undergoing non invasive mechanical ventilation or oxygen therapy

Technicians performing multiple-breath washout tests are blinded to treatment allocation; physiotherapists performing the treatment are blinded to multiple breath washout results; patients are blinded to the purpose of the study; the statistician will be blind to treatment allocation together with outcome assessors.