Unidirectional breathing (UB), nose-in mouth-out (NMB) or vice versa, is thought
to create PEEP, stabilize small airways, and increase expiratory flow and exhaled tidal volume (VT)
in patients with expiratory obstructive disorders. However, the exact mechanism providing the
benefits of UB remains unknown. Our hypothesis was that the benefits of UB are achieved mainly
through reduction of upper airway dead space. METHODS: Sixteen stable COPD patients requiring
oxygen use at home were enrolled in this prospective study at a tertiary health care center. A
nasal mask and a mouthpiece were used, each having a removable one-way valve to direct the
breathing pattern. Four experimentally defined patterns of spontaneous breathing, NMB, mouth-in
nose-out (MNB), nose-in nose-out (NNB), and mouth-in mouth-out (MMB), were compared. Each
breathing pattern lasted 5 min followed by a 5-min rest period. A NICO device continuously
monitored respiratory parameters. The functional anatomical dead space volume (VD) and expired
Vx values were determined. Breathing efficiency (BE) was calculated as alveolar Vx divided by
expired volume. RESUUTS: Functional anatomical VD was higher in bidirectional breathing (BB)