Linear and nonlinear analysis of pressure and flow during mechanical ventilation

Area of Science:

• Critical Care Medicine
• Respiratory Physiology
• Biomedical Engineering

Background:

• Mechanical ventilation requires accurate assessment of respiratory mechanics.
• Flow dependence of resistance is a complex phenomenon in critically ill patients.
• Existing linear models may not fully capture respiratory system behavior in conditions like COPD and ARDS.

Purpose of the Study:

• To compare linear and nonlinear models for respiratory mechanics during mechanical ventilation.
• To evaluate the flow dependence of resistance in patients with normal respiratory function (NRF), COPD, and ARDS.
• To assess the impact of positive end-expiratory pressure (PEEP) on respiratory system modeling.

Main Methods:

• Recorded airway opening pressure, flow, and volume signals in 32 mechanically ventilated ICU patients (NRF, COPD, ARDS).
• Analyzed data using linear (Pao = PE + Ers V + Rrs V') and nonlinear (Pao = PE + Ers V + k1V' + k2|V'|V') models.
• Evaluated model goodness of fit using root mean square difference and varied PEEP levels (0, 5, 10 hPa).

Main Results:

• Both models fit normal respiratory function data well.
• The nonlinear model provided a better fit for COPD and ARDS patient data.
• Negative nonlinear resistance coefficients (k2) were observed in COPD and ARDS, increasing with PEEP.
• Increased PEEP improved the fit of both models for ARDS and COPD data.

Conclusions:

• Nonlinear modeling is more suitable for characterizing respiratory mechanics in ARDS and COPD patients.
• PEEP appears to reduce the flow dependence of respiratory resistance, particularly in COPD patients.
• This effect of PEEP may be due to indirect increases in lung volume.