The flow-time waveform predicts respiratory system resistance and compliance.

Purpose: Knowledge of patients' lung compliance and resistance aids clinical management. We investigated whether these values, readily measured during volume assist-control ventilation (VACV), could also be estimated during pressure assist-control ventilation (PACV).

Methods: Data were collected in 12 mechanically ventilated human subjects. During VACV, peak pressure, plateau pressure, end-expiratory pressure, tidal volume, and inspiratory flow rate were measured. During PACV, inspiratory pressure, end-expiratory pressure, and tidal volume were recorded. The linear component of the pressure-time waveform was extrapolated to time and flow axes. Using the equation of motion for the respiratory system, assuming a nonlinear resistance, we calculated inspiratory resistance and compliance. During VACV, compliance and inspiratory resistance were calculated in the conventional manner.

Results: In ventilated subjects, mean compliance during PACV was 37.06 ± 15.65 mL/cm H(2)O, and during VACV, 36.93 ± 12.18 mL/cm H(2)O. Mean inspiratory resistance during PACV was 15.17 ± 5.14 cm H(2)O/L per second, whereas during VACV, it was 12.50 ± 2.99 cm H(2)O/L per second. A strong correlation is evident between compliance and inspiratory resistance calculated during PACV vs VACV (r(2) of 0.73 and 0.51, respectively).

Conclusions: During PACV, the inspiratory flow waveform is linear, and its slope contains information regarding inspiratory resistance and compliance. Calculated values correlate with those during VACV.