Pressure Support and Rise Time
Rise time is a valuable tool for improving patient synchrony during mechanical ventilation. However, it’s important to remember that on many ventilators, the rise time setting affects both control and pressure support breaths. This becomes especially relevant in modes that combine SIMV and pressure support.
For example, if a patient shows signs of flow starvation on their loops, you might consider decreasing the rise time to increase flow delivery. However, in SIMV with pressure support, this adjustment also decreases the rise time of pressure support breaths. Unlike control breaths, which are time- or volume-cycled, pressure support breaths are flow-cycled. On the Servo-U ventilator, this parameter is called End Inspiration and is expressed as a percentage of peak inspiratory flow.
When rise time is decreased, the flow cycle threshold is often reached more quickly, leading to a reduced tidal volume for pressure support breaths. To mitigate this, you should reassess and optimize the flow cycle (End Inspiration) parameter to minimize tidal volume loss.
The first slide illustrates the impact of not optimizing flow cycle settings, while the second slide demonstrates a reduction in End Inspiration% from 30% to 10%. This increases the area under the curve, effectively increasing tidal volume.
TL;DR: Just use assist control, and you won’t have to worry about any of this 😂