Philips Respironics patent US7168429B2 describes "variable breathing" as a breathing control layer in the auto CPAP, that monitors the flow signal to determine whether the patient is experiencing erratic breathing, and causes the pressure generating system to adjust the pressure of the flow of breathing gas responsive to detection of erratic breathing.
The variable breathing algorithm seeks to develop a peak inspiratory flow trend on a 4 minute moving average, and measure the deviation above or below that trend. The system is designed to identify variable breathing and to turn over control of the pressure support system, from the auto CPAP controller to the Variable Breathing controller. Without figures, we don't know what that controller response is, but it seems if VB is detected while the pressure is steady, the VB controller will maintain that pressure, but if pressure was increasing before VB controller took control, the pressure is reduced up to 2-cm. Similarly if pressure was decreasing ahead of VB detection, the VB controller will in increase pressure up to 2-cm. The duration of VB controller action is 5 minutes unless VB is still detected.
Here is an example of how variable breathing caused by flow limitation over-rides the auto-pressure algorithm resulting is an unresponsive Auto CPAP Variable Breathing Example