Controlled randomized crossover trial in patients with chronic obstructive respiratory disease (COPD), who are treated with Nasal High Flow (NHF) therapy during wakefulness with a small amount of CO2 added to the inhaled air to keep the patients' PaCO2 stable despite the washout effect. Aim of the study is to examine respiratory rate, tidal volume and work of breathing under NHF without the CO2 washout effect.

In this controlled randomized crossover trial, 10 stable patients with chronic obstructive respiratory disease (COPD) are treated with Nasal High Flow (NHF) therapy during wakefulness. NHF treatment will inevitably result in increased upper airway washout and thus in a slight reduction of PaCO2 of approximately 2 mmHg. For compensation, a small amount of CO2 is added to the inhaled air during this experimental situation, which keeps the patients' PaCO2 stable. This will allow to examine the effects of NHF therapy on respiratory rate, tidal volume and work of breathing, that are not attributable to the CO2 washout effect.

Inclusion Criteria:

• Hospitalized COPD patients GOLD (3 and 4) in stable phase after an acute exacerbation.
• Written informed consent is required for participation.

Exclusion Criteria:

• Acute respiratory failure with clinical instability or respiratory acidosis defined by pH<7.35
• Severe acute physical illness that does not allow the subject to participate in a clinical trial
• Unable to give consent
• Language, cognitive, or other impairments that may prevent independent completion of questionnaires