Intro Airway clearance techniques (ACTs) are the most burdensome therapies for people with cystic fibrosis (CF). Effects of different techniques on clinical outcomes are unknown.

Aim Identify combinations of ACT breath pressure and length that are associated with improved FEV₁%predicted in children with CF.

Methods Expired breaths from 145 children with CF using positive expiratory pressure (PEP) or Oscillating PEP (OPEP) devices during habitual ACTs were captured over 16months. Treatment data were summarised into mean mid-expiratory breath pressure and breath length. ACT efficacy was investigated by iteratively comparing the effect on FEV₁ of all possible combinations of pressure and length against weeks with no ACTs using linear mixed effects regression models, where weekly FEV₁ was extrapolated for each participant using flexible polynomials fitted to observed FEV₁ data (n=1116).

Results ~45K treatments (137 people,~4M breaths) and ~21K ?no ACT? days (141 people) were analysed. Compared to no treatment, a significant positive association with FEV₁ was observed for 3 breath profiles (32% of treatments) (Table). Breaths outside these 3 profiles had no effect.

Conclusions Pressure-length breath profiles other than those normally prescribed may be facilitated by different ACT devices and may result in benefit.

Table. Breath profiles associated with improved FEV₁%predicted