The goal of this study randomized single-center crossover study is to evaluate the impact of asymmetrical versus symmetrical nasal high flow (NHF) therapy on work of breathing (WOB). The main question it aims to answer are:

1. Does the asymmetrical NHF interface improve the work of breathing compared to the symmetrical NHF interface? Researchers will compare the symmetrical NHF interface to the symmetrical NHF interface to see if the the asymmetrical interface has a greater impact on the work of breathing.

Participants will :

1. receive NHF via both interfaces (FiO₂ 0.21, 50 L/min) for 15 minutes, in random order, with esophageal pressure monitoring and bioelectrical impedance analysis performed.
2. baseline characteristics will me monitored throughout the interventions
3. Esophageal pressure and minute ventilation will be monitored.

Each participant will undergo all phases of the study while seated comfortably, breathing room air. Vital signs, including blood pressure, heart rate, and peripheral oxygen saturation (%SpO₂), will be continuously monitored and recorded throughout.

Respiratory mechanics will be assessed using esophageal pressure (Pes) monitoring. A thin latex balloon catheter (10 cm long, 3-5 cm circumference; Cooper surgical, USA), filled with 0.5 mL of air, will be placed in the mid-esophagus (~45 cm from the nares) and will be connected to a pressure transducer (RSS-100HR, Hans Rudolph Inc., USA). Correct positioning will be confirmed by observing equal fluctuations in Pes and airway pressure (Paw) during occluded breaths. Balloon integrity will be verified before, after, and as needed during the procedure.

Participants will be instructed to breathe quietly on room air (FiO₂ 0.21) to allow for baseline assessment. Volume will be recorded using a heated pneumotachometer (Series 3813; Hans Rudolph Inc., USA) and the Research Pneumotach System (RSS100-HR), sampling at 50 Hz, a sampling rate exceeding the Nyquist criterion for respiratory signals. A bioelectrical impedance technology device (ExSpiron, Respiratory Motion Inc., Waltham, MA) will then be attached around the chest to assess lung volume changes.

Participants will then be randomized in a 1:1 ratio to one of two intervention sequences using a computer-generated randomization schedule.

• Sequence 1: Symmetrical NHF Interface - 3-minute washout period - Asymmetrical NHF Interface
• Sequence 2: Asymmetrical NHF Interface - 3-minute washout period - Symmetrical NHF Interface In the seated position, participants will both 15-minute phases, separated by a 3-minute washout period to prevent residual effects. During the "Symmetrical Interface" phase, participants will breathe using the conventional symmetrical NHF cannula, and during the asymmetrical phase, the asymmetrical interface (DUET, Fisher and Paykel Healthcare, Auckland, New Zealand) will be used. High-flow oxygen will be delivered using a commercial system (AIRVO 3; Fisher and Paykel Healthcare, Auckland, New Zealand) set at a flow of 50 L/min, temperature of 37°C, and FiO₂ of 0.21.

Inclusion Criteria:

1. age ≥18 years old
2. absence of symptoms and signs of respiratory failure (SpO2>95% at FiΟ2 0.21)

Exclusion Criteria:

1. pregnancy
2. SpO2<94% at FiΟ2 0.21
3. neuromuscular disease
4. contraindications to esophageal pressure monitoring (e.g., uncontrolled coagulopathy, esophageal disease, nasal trauma, allergy to local lidocaine)