To maximize PaCO2 removal p-Low must be set at zero cmH2O. This allows for a rapid peak expiratory flow, which creates a venturi effect that help drafts paCO2 removal.
Additionally, the high peak expiratory flow rate promptly ends the t-Low phase and allows for the p-High phase to be resumed earlier to optimize alveolar recruitment and exposure time.
If p-Low is set greater than zero, the peak expiratory flow rate will be decreased and delayed.
Furthermore, a p-Low setting higher than zero creates additional expiratory resistance during the release phase resulting in a more turbulent expiratory flow pattern and decreases the venturi effect.
note- even though p-Low is set at zero lung derecruitment is not a issue, for t-Low is titrated to maintain expiratory lung volume.
Notice the above image showing that during p-Low there is still pressure in the airways above zero, even when the ventilator is set with a p-Low at 0 cmH2O.