Physiological Effects of Continuous Negative External Pressure for Acute Hypoxemic Respiratory Failure

This study is a prospective randomized cross-over trial. Eligible, consenting participants will undergo 4 strategies of respiratory support: high-flow nasal cannula (HFNC) only, HFNC + continuous negative external pressure (CNEP) of 10 cmH2O, HFNC + CNEP of 20 cmH2O, HFNC + CNEP of 30 cmH2O. Throughout the study period, HFNC will be managed at a constant flow rate with fraction of inspired oxygen (FiO2) titrated to achieve goal oxygen saturation (SpO2) of 90-97%, measured via continuous pulse-oximetry. Each of the 4 strategies will be performed for 45 minutes per strategy, interspersed with a 15-minute washout period of HFNC only. Participants will be randomized to the sequence of strategies for respiratory support. A summary of the sequence of trial procedures is as follows: 1. Perform baseline measures. 2. Wait 5-10 minutes for recovery. 3. Initiate first random treatment assignment for up to 45 minutes. 4. HFNC-only washout for 15 minutes 5. Initiate second random treatment assignment for up to 45 minutes. 6. HFNC-only washout for 15 minutes 7. Initiate third random treatment assignment for up to 45 minutes. 8. HFNC-only washout for 15 minutes 9. Initiate fourth random treatment assignment for up to 45 minutes. 10. Return to usual care (HFNC only) If a participant does not tolerate a given level of CNEP, the patient will be returned to HFNC for 5 minutes and then given the option to attempt that CNEP level again. If the patient declines reattempt or does not tolerate that CNEP level on reattempt, the HFNC-only washout will be instituted for 15 minutes before proceeding to the next protocol step.