Transcutaneous capnography (PtcCo2) recently showed good agreement with blood gases to assess hypercapnia. There is no study that has evaluated the benefit to realise systematically a nocturnal PtcCo2 in the follow-up of home-ventilated patient with obesity hypoventilation syndrome (OHS). The investigators will evaluate the intake bring by nocturnal transcutaneous capnography, compared to nocturnal oximetry and blood gases, to diagnose nocturnal alveolar hypoventilation in a population of patient presenting an OHS, treated by non invasive ventilation (NIV) at home .

Patients will be evaluated once a year in consultation with a blood gases, and then a nocturnal recording at home including a transcutaneous capnography and an oximetry.

Some data will be registered too : cardiovascular and neurological comorbidities, diabetes mellitus, treatment, parameters of the non-invasive ventilation, duration of compliance and apnea-hypopnea index (if available).

The investigators will perform a sequential analysis of nocturnal hypoventilation with first blood gases, then nocturnal oxymetry and to finish nocturnal PtcCo2. It will allow us to determine if PtcCo2 catch up some nocturnal hypoventilation, undetected by the first two exams.

The investigators plan that the study will last 18 months. The analysis will be descriptive. Quantitative variables will be described with mean, standard deviation, minimum, maximum and median. Quantitative variables are presented with number and percentage.

Comparison of blood gas / oximetry association with blood gas / oximetry / transcutaneous capnography using a McNemar test

Inclusion Criteria:

• Obesity Hypoventilation Syndrom defined by a diurnal PaCo2> 45 mmHg, in stable conditions, in a patient presenting an obesity (BMI > 30 kg/m²)
• treated by non invasive ventilation since at least 3 weeks, and seen in follow-up consultation

Exclusion Criteria:

• minors
• refusal to participate
• under guardianship or trusteeship
• pregnant woman
• other etiology of hypoventilation : copd, kyphoscoliosis, neuromuscular disorders, diaphragmatic pathology

The pneumology department at Nantes University Hospital follows approximately 400 patients treated by non-invasive ventilation, 250 of whom have SOH The investigators plan to include 150 patients in the active patient line followed at Nantes University Hospital, which will make it possible to obtain the number of patients necessary to obtain satisfactory power while taking into account the fact that a certain number of transcutaneous capnographies will not be conclusive.