The ResMed S9 VPAP S BiLevel machine helps noncompliant OSA (obstructive sleep apnea) patients receive quiet and comfortable therapy for improved sleep. The S9 VPAP S machines provides two pressure levels, one for inhalation and another for exhalation, for an easier time breathing and smoother transitions between pressure changes. Select a model with or without a Heated Humidifier (H5i). Or choose a model with the heated humidifier, plus the ClimateLine tubing to maintain temperature and humidity.