Oxygen ratio control in critical care ventilation using compressed oxygen and blower gas sources

The coupling issue between ratio (gas mix) and pressure controllers is addressed for blower based critical care ventilators. Complications can arise by how oxygen gas, supplied by a proportional flow valve, is introduced relative to the blower. A simple blower model explains why coupling effects are more sensitive when oxygen is introduced downstream of the air blower. A progressive design synthesis is described that eventually led to the solution of complementary flow actuation between blower and valve flow feedback loops. Complementary dynamic flow coupling filters connect with the pressure stage output of the feedback control cascade. The complementary coupling filters frequency response changes according to the set gas ratio helping to equalize differences in the dynamic response between blower and valve at different gas ratio settings. This design solution achieves a more uniform total flow and pressure response across all ratio settings throughout the range of expected patient load.