BACKGROUND Auto-CPAP machines have been developed to adapt automatically the positive pressure level to the required needs. The auto-CPAP response to transient nasal obstruction was tested during sleep in patients with sleep apnoea/hypopnoea syndrome (SAHS).

RESULTS Nasal resistance increased initially by a mean (SD) of 166 (66)% during the trials with the “Morphée plus” and by 118 (44)% with the “Horizon” machine. The increase in nasal resistance was always accompanied by flow limitation with one exception. Mask pressure initially decreased to the minimal permitted value as nasal resistance increased with the “Morphée plus” device. A late increase of the positive pressure level occurred sometimes. Mask pressure did not change with increasing nasal resistance when the “Horizon” device was used.

CONCLUSION Neither the “Morphée plus” nor the “Horizon” device abolished flow limited breaths and resulting sleep fragmentation. We conclude that, despite the different algorithm of pressure changes, these auto-CPAP machines do not adequately respond to an acute increase in nasal resistance.