Determinants of dynamic hyperinflation during metronome-paced tachypnea in COPD and normal subjects

• MPT is a simple, reproducible test that elicits dynamic hyperinflation (DH) in COPD.
• Expiratory time is the main determinant of DH during MPT in subjects with COPD.
• DH can also be induced during MPT in normal subjects but to a lesser degree.
• DH in normal subjects correlates with end-tidal CO2, not expiratory time.

In COPD, dynamic hyperinflation (DH) occurs during exercise and during metronome-paced tachypnea (MPT). We investigated the relationship of DH with breathing pattern and ventilation (V˙E) in COPD and normal subjects (NS).In 35 subjects with moderate COPD and 17 younger healthy volunteers we measured inspiratory capacity (IC), breathing frequency (fR), expiratory time (TE), ventilation (V˙E) and end-tidal carbon dioxide tension (PETCO2) at baseline and after 30 s of MPT at 40 breaths/min with metronome-defined I:E ratios of 1:1 and 1:2. A reduction in IC (ΔIC) was taken to indicate DH. In COPD subjects, DH correlated with TE but not with V˙E or PETCO2, and was best predicted by total lung capacity. NS also showed DH (although less than in COPD), which correlated with PETCO2 but not with fR, TE or V˙E.We conclude that MPT evokes DH in both NS and patients with COPD. TE is the most important determinant of DH during MPT in patients with COPD.