The two types of pressure support, pressure support proportional to volume (Volume Assist) and pressure support proportional to flow (Flow Assist) can be combined. Flow Assist and Volume Assist are set by the user, the ventilator’s microprocessor calculates continuously at each point of the breathing cycle the proportional pressure support for the patient. If set correctly, only the illness-related higher work of breathing is continuously adapted and compensated – the physiological work of breathing continues to be provided by the patient alone.
Flow Assist helps overcome the resistance and the pressure support is proportional to the flow. In contrast, Volume Assist compensates the elastic resistance (C) caused by reduced elasticity of the lungs. The pressure support is proportional to the tidal volume. The relationship between the inspiratory effort and the pressure support remains constant for the same settings, while the pressure support varies for every breath. If no spontaneous breathing is detected, the mechanical support also ceases. Adequate apnoea and minute volume monitoring must therefore be in place.