Abstract : Precapillary pulmonary hypertension (PH) is defined by a mean pulmonary pressure > 25 mmHg associated with a normal wedge pressure (≤ 15 mmHg). Increase in vascular resistances in PH is due to vasoconstriction, vascular wall remodelling and thromboses of small pulmonary arterioles. Hypoxia is known to cause similar changes in pulmonary vasculature. Although some cohorts studies have shown that nocturnal hypoxemia (NH) is frequent in PH, accounting for up to 77% of cases, the mechanisms of this NH were not well established, being associated to periodic respiration like the Cheyne Stokes respiration (CS) by some authors, and to obstructive apneas-hypopneas (OAH) by others.The aim of our study was to search for sleep-related breathing disorders in PH, to determine their prevalence and their mechanisms. Fourty six patients hospitalized in the pulmonary department of Antoine Béclère Hospital, which is the national referral center for PH in France, underwent a one night polysomnography (CIDELEC) with a transcutaneous capnography (TOSCA). Our population was homogeneous with 29 patients having idiopathic pulmonary arterial hypertension (IPAH) and 17 patients having chronic thrombo-embolic PH (CTEPH). Patients had no severe functional limitation (FEV1 and TLC ≥ 60 % of predicted), nor severe obesity (BMI < 35 Kg/m²), and they were in a steady state with optimal PH treatment for at least three months. The majority of patients were in NYHA functional class II, had a 6 minutes walking distance > 400m and a mean cardiac index measured on right heart catheterization within normal ranges (3,2 ± 0,6 L/min/m²).Thirty eight out of the 46 patients (82,6%) had a NH as defined by a sleeping time spent with a SpO2 < 90%, > 60min and/or an oxygen desaturation index ≥ 20/h. These patients spent 48.9 ± 35.9% of their sleeping time with a SpO2 < 90%. The most frequent mechanism underlying theses abnormalities (76% of desaturators patients) was due to ventilation/perfusion (VA/Q) heterogeneity, isolated or associated to OAH. Apneas-hypopneas (AH) were frequent in PH patients with an AH index (AHI) ≥ 5/h in 89% of cases, and a mean AHI of 24.9 ± 22.1/ h. Most of these respiratory events were obstructive, only four patients had central AH with a CS pattern in 3 out them. A single case of alveolar hypoventilation associated with a moderate AHI was identified and was caracterized by a sustained nocturnal desaturation associated with a significant increase in transcutaneous CO2 pressure ( > 30 mmHg). No clinical nor hemodynamic factor was found to be predicting for NH, although minor correlations were found between nocturnal desaturation parameters and PaO2, and nocturnal desaturation parameters and small airways obstruction.We conclude that NH is frequent in IPAH and CTEPH, due to VA/Q mismatch and/or OAH. Future studies are needed to determine the impact of the correction of this NH on PH patients regarding their NYHA functionnal class, their hemodynamic parameters and their responsiveness to PH specific treatment.