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.

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.

Inclusion Criteria:

1. Major patient
2. Patient who has undergone cardiac surgery with extracorporeal circulation for more than 24 hours and has been extubated for at least 6 hours.
3. Pre-operative heart failure with left ventricular ejection fraction < 40%.
4. Invasive monitoring by pulmonary arterial catheter (Swan-Ganz) with transpulmonary thermodilution and by radial arterial catheter intended for use in the intra- and post-operative monitoring of the patient.
5. Treatment with conventional oxygen therapy (O2C) without an increase in O2 flow for more than 30 minuts, without signs of hypoxemia (SpO292%)
6. Stable hemodynamic parameters: no changes in hemodynamic parameters (arterial pressures and pulmonary and cardiac index) by more than 10% over the 30 minutes prior to inclusion and no increase in cardiotropic medication for at least 30 minutes.
7. Patient with an oxygenation rate ≤ 6L
8. For women of child-bearing age: negative pregnancy test at inclusion
9. Person who has read and understood the information letter and signed the consent form
10. Affiliation to a social security scheme

Exclusion Criteria:

1. Contraindication to the use of NIV or HFO

2. Patient with orotracheal intubation or tracheotomy

3. Renal failure with hemodialysis or hemofiltration

4. Uncontrolled state of shock (PAS<90 mmHg and/or cardiac index<1.8 L/min/m² and/or norepinephrine>0.2 μg/kg/min and/or dobutamine>10 μg/kg/min)

5. Acute respiratory failure as defined by :

   • clinical signs: respiratory rate >35/min, signs of struggle, SpO2<92% on O2C
   • indication for treatment with NIV or HFO at the discretion of the clinician
   • oxygen dependency requiring O2C>6L/min O2C flow at the oxygen mask

6. Chronic advanced respiratory disease

7. Chronic treatment with NIV or CPAP at home

8. Non-operated aortic or mitral insufficiency ≥ II/IV

9. Administration of loop diuretics within 3 hours prior to inclusion for intravenous forms and within 6 hours for oral forms.

10. Presence of altered consciousness defined by a Glasgow score < 15/15 or cognitive dysfunction defined by a CAM-ICU score > 0/4

11. Pregnant or parturient or nursing woman or proven lack of effective contraception

12. Person deprived of liberty by an administrative or judicial decision or person placed under judicial protection / subtutorship or guardianship

13. Person participating in another trial / having participated in another trial that may interfere with the procedures that are the subject of the research within a 4-week time frame

No applicable