Design: Prospective, observational, cross-over, clinical physiologic study of HFNC and NIV in patients with COPD and hypercapnic respiratory failure Intervention: Participants will be consecutively ventilated with the two devices, i.e., HFNC and NIV. HFNC 30 and 50 L/min will be applied in a random order (sealed, opaque envelope). HFNC will be set with a temperature at 37°C or 34°C if perceived as too warm. NIV settings will be adjusted based on the clinical assessment of the Respiratory Therapist as per standard practice and will not be modified during the test. In both groups, FiO2 will be adjusted to achieve a SpO2 of at least 92%.
Setting: The intervention will be applied in the Medical-Surgical ICU of St. Michael's Hospital.
Duration: Each device will be studied for up to 30 minutes. The entire study will take place over up to 3 hours. There is no additional follow-up thereafter.
Procedures: The study will start first by recording patients under spontaneous breathing considered as a baseline (sequence #1). Under spontaneous breathing, patients will be receiving oxygen therapy as it is usually done between 2 NIV sessions. Then, the patient will have up to 30 minutes to acclimatize to HFNC and select his/her preferred flow rate (sequence #2). NIV will be applied (sequence #3). The NIV settings will be adjusted based on the clinical assessment of the Respiratory Therapist and will not be modified throughout the study. Then, according to randomization order (sealed envelopes), patients will receive a flow corresponding to the preferred flow selected by the patient and a second flow, either HFNC 30 L/min or HFNC 50 L/min depending on the initial choice (sequences #4 and #5) (see flow diagram). FiO2 will be adjusted to achieve a SpO2 of 90 to 94%. Each condition will be studied fup to 30 minutes and will be separated by a 5-minute washout period during which patients will be receiving oxygen therapy. The measurements will be collected at baseline (under spontaneous breathing) and during the last five minutes of each condition. In case of clinical intolerance as considered by the attending physician, the study will stop. In case the patient is considered dependent of NIV by clinicians, we will still enroll the patient without doing baseline period after discussion with clinicians.
The pulse oximeter (SpO2) and transcutaneous CO2 monitor (SenTec Digital Monitoring System (SDMS)) will be continuously monitored throughout the study period. Exspiron or Electrical Impedance Tomography (EIT) device will be calibrated to measure minute ventilation. Four surface electrodes will be placed bilaterally to record each hemidiaphragm activity using a specific recording system (Acqknowledge software, Biopac Systems) A bedside ultrasound examination will be performed, using a SonoSite system (Fujifilm) equipped with a 10-15 MHz ultrasound linear probe. Diaphragm thickness will be measured both at end-inspiration and end-expiration. This technique will be applied at the end of each sequence (before ventilatory assist under conventional O2 therapy). In patients becoming rapidly intolerant to NIV disconnection, we will limit this "baseline" period under O2 to a minimum (5 minutes). The thickness and contraction of the intercostal muscles and abdominal muscles will also be assessed.
Interpretation of the ultrasound results to determine diaphragm and other muscle thickness will be read at a later time by an individual blinded to the intervention.
