The investigators propose to carry out a prospective randomized cross-over prospective physiological study comparing, Non-Invasive Ventilation and High Flow Oxygen Therapy in patients with chronic uncompensated heart failure. This stable population would make it possible to easily study variations in a physiological parameter without any added risk for the patient, but with all the pathophysiological parameters transposable to the situation of decompensated heart failure.~Indeed, it is accepted that the existence of heart failure with decreased left ventricular ejection fraction (LVEF) (< 40%) exposes patients to a high risk of congestive cardiac decompensation due to the associated presence of diastolic dysfunction1.~The study will be conducted using an alternating design, where a patient receives the two treatments one after the other (cross-over), because the alternating design, compared to the two parallel arm design, reduces the number of patients needed to show a given difference between High Flow Oxygen Therapy and Non-Invasive Ventilation at equal power, if there is neither an order-of-prescription effect (period) nor a residual effect of the first treatment on the second treatment (carry-over). In order to avoid a residual effect on changes in pressure or oxygenation, a wash-out period on conventional oxygen therapy (O2C) should be set between the two treatments (High Flow Oxygen Therapy and Non-Invasive Ventilation) administered to the same patient.
The investigators hypothesize that the airway pressures generated by High Flow Oxygen Therapy could have a beneficial impact in patients with heart failure, particularly by lowering capillary pulmonary arterial pressures. However, results compared to Non-Invasive Ventilation are difficult to predict due to the lack of data in the literature.