The goal of this observational study is to assess the potential non-invasive tools (e.g. regional ventilation, respiratory muscle response, lung mechanic's parameters) to identify the risk of failure when using high flow nasal cannula (HFNC) or non-invasive ventilation (NIV).

The main question, it aims to answer is:

Does abnormal regional ventilation could predict HFNC or NIV failure?

This is a single-center, prospective, physiological study.

The study will enroll patient who has used either HFNC or NIV for the indications of 1. to prevent worsening acute hypoxemic respiratory failure or 2. to prevent reintubation. Once being confirmed to meet the inclusion criteria, the research team will apply the electrical impedance tomography (EIT) on the patient and start recording for 5-10 minutes as well as perform diaphragm and parasternal intercostal muscle ultrasound when the patient is using HFNC or NIV. The current HFNC or NIV setting, some lung mechanic's parameters (e.g. rapid shallow breathing trial (RSBI), negative airway pressure generated during the first 100 milisecond (P0.1), end-expiratory occlusion pressure(ΔPocc) derived from NIV machine), patient's characteristic, dyspnea scores (intensive care respiratory distress observation scale, IC-RDOS; work of breathing scale; self-report of dyspnea visual analog scale, D-VAS), ICU and hospital length of stay will be also collected.

The investigator will use EIT to visualize where air goes in the patient's lung and inspiratory muscle ultrasonography (diaphragm and parasternal intercostal muscle) to visualize the muscle activities when using HFNC or NIV. EIT is a measurement often used in the ICU since it allows easy visualization and is a non-invasive technique. The sensors measure electrical current changes during inspiration and expiration and will not cause any pain or radiation concerns. The EIT belt (3-cm diameter) will be placed around the chest without causing adverse event/discomfort/pain.

The duration of study will be approximately 30 minutes. The study will be conducted one time for one enrolled patient.

HFNC or NIV (device) failure is considered as either intubation/reintubation or death, whatever has come first. It will be followed up to 7 days.

The investigators will proceed with the measurements only with agreement from the clinical team. The investigators will analyze the data collected offline with a dedicated software afterwards.

After these study procedures are completed, data will also be collected from the participant's chart and combined with relevant information from the HFNC or NIV settings and vital functions. Confidentiality will be maintained throughout the entire study.

Inclusion Criteria:

• Age of ≥ 18 years
• Using NIV or HFNC anywhere in the hospital

Exclusion Criteria:

• Chronic CO2 retention
• Neuromuscular disorder
• Requiring home NIV (CPAP or BIPAP)
• Tracheostomy
• Contraindication to EIT placement: pacemaker/defibrillator implantation, burns at the area of EIT placement

Patient who is using high flow nasal cannula (HFNC) or non-invasive ventilation (NIV) to either prevent 1. intubation in the context of acute hypoxemic respiratory failure or 2. reintubation in the context of high risk group of extubation.