Analysis of the mechanisms of expiratory asynchrony in pressure support ventilation: a mathematical approach

A mathematical model was developed to analyze the mechanisms of expiratory asynchrony during pressure support ventilation (PSV). Solving the model rev ealed several results. 1) Ratio of the flow at the end of patient neural in spiration to peak inspiratory flow (VTI/V-peak) during PSV is determined by the ratio of time constant of the respiratory system (tau) to patient neur al inspiratory time (TI) and the ratio of the set pressure support (Pps) le vel to maximal inspiratory muscle pressure (Pmusmax). 2) VTI/V-peak is affe cted more by tau/TI than by Pps/Pmus max. VTI/Vpeak pancreases in a sigmoid al relationship to tau/TI. An increase in Pps/ Pmus max slightly shifts the VTI/V-peak-tau/TI curve to the right, i.e., VTI/V-peak becomes lower as Pp s/Pmus max increases at the same tau/TI. 3) Under the selected adult respir atory mechanics, VTi/V-peak ranges from 1 to 85% and has an excellent linea r correlation with tau/TI. 4) In mechanical ventilators, single fixed level s of the flow termination criterion will always have chances of both synchr onized termination and asynchronized termination, depending on patient mech anics. An increase in tau/TI causes more delayed and less premature termina tion opportunities. An increase in Pps/Pmus max narrows the synchronized zo ne, making inspiratory termination predisposed to be in asynchrony. Increas ing the expiratory trigger sensitivity of a ventilator shifts the synchroni zed zone to the right, causing less delayed and more premature termination. Automation of expiratory trigger sensitivity in future mechanical ventilat ors may also be possible. In conclusion, our model provides a useful tool t o analyze the mechanisms of expiratory asynchrony in PSV.