Study objective: To determine whether tidal expiratory
airflow patterns change with increasing airways obstruction in patients
with cystic fibrosis.
Design: An observational
Setting: Lung function laboratory.
Patients: Sixty-four children and young adults with cystic
Measurements: After measuring
FEV1 and airways resistance using body plethysmography,
each subject was seated and asked to mouth breathe through a
pneumotachograph for 2 min. The collected data were analyzed, and three
expiratory airflow pattern-sensitive indexes were computed. The first
index was derived from the ratio of the time to reach peak expiratory
flow to the total expiratory time (tptef/te).
The second index, Trs, was an estimate of the time constant of the
passive portion of expiration. The third index, S̄, describes the
slope of the whole post-peak expiratory flow pattern after
Results: Compared with FEV1, the
index tptef/te was a poor indicator of airways
obstruction (r2 = 0.15, p = 0.002). Trs showed a
strong relationship with the severity of airways obstruction
(r2 = 0.46, p < 0.001). Using S̄, the
postexpiratory profile could be categorized into three shapes, and
provided a good indicator of airways obstruction when linear and
concave-shaped profiles occurred (r2 = 0.42,
p < 0.001). Convex-shaped flow profiles had to be treated separately
and were indicative of normal lung function.
Conclusions: In a cross-sectional study of patients with
cystic fibrosis, increase in airways resistance above normal is
reflected by quantifiable changes in the expiratory airflow