Non-invasive ventilation (NIV) has been widely used in heart failure patients with supporting evidence. However, the drawbacks and contraindications associated with NIV limit its applicability in certain patients. Recently, high-flow oxygen therapy (HFOT) has gained popularity, particularly in the context of the COVID-19 pandemic, due to its documented benefits, improved patient comfort and fewer contraindications. Studies have suggested that HFOT can generate positive end-expiratory pressure (PEEP) similar to NIV, thereby increasing end-expiratory lung volume. However, the specific effects of PEEP remain unknown, as previous research only monitored the upper airway pressure. Therefore, this study aims to explore the flow-pressure relationship between HFOT and NIV in heart failure patients using electrical impedance tomography (EIT). This prospective randomized crossover clinical trial will be conducted at a single medical center with multiple intensive care units. Participants will be randomly assigned to Groups A and B using a computerized randomization process. Each group will undergo specific protocols for 5-10 minutes per phase, during which parameters including respiratory rate, heart rate, blood pressure, peripheral oxygen saturation, and oxygen concentration will be recorded. NIV will be administered in continuous positive airway pressure (CPAP) mode. Additional parameters such as tidal volume, respiratory rate, minute ventilation, leak flow, and peak inspiratory pressure will be recorded for NIV. The study protocols for Group A will follow the sequence of oxygen mask, HFOT 40L, HFOT 50L, HFOT 60L, oxygen mask, CPAP 4cmH2O, CPAP 5cmH2O, and CPAP 6cmH2O. Group B will follow the sequence of oxygen mask, CPAP 4cmH2O, CPAP 5cmH2O, CPAP 6cmH2O, oxygen mask, HFOT 40L, HFOT 50L, and HFOT 60L. This means that each intervention will be performed in the order listed, with one intervention completed before moving on to the next. The participants will be positioned in a semi-recumbent position at 45 degrees, and the EIT belt will be placed around the fifth (or sixth) intercostal space for monitoring. The EIT signals will be filtered with a cut-off frequency set at 10 beats below the current heart rate. The entire procedure is estimated to take approximately 1-1.5 hours, and recalibration will only be performed in case of significant signal abnormalities. All data will be stored for offline analysis.