EXPIRATORY SYNCHRONY DURING PRESSURE SUPPORT VENTILATION: A MATHEMATICAL APPROACH
Hong-Lin Du, MD and Yamada Yoshitsugu, MD. Clinical Research Department, Newport Medical Instruments Inc., Newport Beach, California; Surgical Center, The Institute of Medical Science, University of Tokyo, Japan.
A mathematical model was developed in order to provide qualitative information for clinicians to select expiratory trigger sensitivity and optimize expiratory patient-ventilator synchrony during pressure support ventilation (PSV). The model used a first order equation for the respiratory system and a second-order polynomial function for the inspiratory muscle pressure. Solving the model yielded an equation of the inspiratory flow during PSV as follows:
Computing the inspiratory flow rates based on this fundamental equation revealed the following
Results: 1. The ratio of the flow at the end of the patient neural inspiration to peak inspiratory flow (VTI/Vpeak) during PSV is determined by the set support pressure level (Pps), maximal patient inspiratory muscle pressure (Pmus max), time constant (t) of the respiratory system, and patient neural inspiratory time (TI); 2. VTI/Vpeak is affected more by t/TI than by Pps/Pmus max. At the same neural TI, VTI/Vpeak is increased by t/TI in an exponential pattern within the full data ranges. Within the specific mechanic ranges of adult conditions, VTI/Vpeak has an excellent linear correlation with t at a given TI. Increase in Pps/Pmus max slightly shifts the VTI/Vpeak - t/TI curve to right; 3. Single fixed levels of the flow termination criterion used in mechanical ventilators will always have chances of both synchronized termination and asynchronized (premature and delayed) termination, depending on the patient mechanics. When TI remains unchanged, the expiratory synchrony is primarily dependent on t of the respiratory system. Increase in t causes more opportunity of delayed termination and less chance of premature termination. Increase in the set Pps or decrease in Pmus max narrows the synchronized zone, leaving the inspiratory termination predisposed to be in asynchrony. Increasing the expiratory trigger sensitivity shifts the synchronized zone to right, causing less delayed and more premature terminations. A faster Pmax decay after inspiration or expiratory muscle activity broadens the synchronous zone and narrows the delayed termination zone. In conclusion, our mathematical model provides useful qualitative information for clinicians to adjust expiratory trigger sensitivity in order to optimize expiratory synchrony during PSV.