Closing volume tests—that is, single breath O2 test14—were
performed in both groups at rest and after 20 minutes of
recovery (Vmax29; SensorMedics). The pneumotachograph
was filled with 100% O2 and equipped with a N2 meter.
During these tests, the subjects performed a slow, full vital
capacity manoeuvre at inspiratory and expiratory flow rates
of about 0.5 l/s. The slope of the N2 alveolar plateau (slope of
phase III) and the closing volume were calculated. Slow vital
capacity manoeuvres were repeated at least twice—that is,
until the slow vital capacity volumes were within 10% of each
other—and the mean was calculated.