Background: Nasal High Flow (NHF) therapy had shown to increase airway pressure, reduce dead space, and improve mucociliary clearance and gas exchange in patients suffering from chronic obstructive pulmonary disease (COPD). The long-term and short-term physiological aspects of NHF are subject of ongoing research. Different flow rates and cannula sizes have been studied and found to yield different therapeutic effects. Larger sized cannulas tend to seal the nares and produce increased positive airway pressure, while smaller cannulas increase the washout effect. Currently, a new cannula is available, with one smaller and one larger opening (Optiflow+ Duet, Fisher\&Paykel Healthcare, NZ).
Procedure: Following a baseline period without high flow therapy, NHF (35L/min) will be applied via the nasal cannula (Duet or Standard in randomized order). After a wash out phase, the NHF interface will be replaced for another measurement with the respective other cannula (Standard or Duet).
Respiratory rate, tidal volume, minute ventilation, inhalation to exhalation ratio (Ti/Te), transcutaneous pCO2 and intercostal EMG will be recorded during each partial measurement.
