Volume-cycled decelerating flow. An alternative form of mechanical ventilation

The linearly decelerating flow waveform for volume-cycled mechanical ventilation is an option on many modern ventilators. We have developed mathematical models for two available forms of volume-cycled decelerating-flow ventilation (VCDF). These equations use clinician-chosen ventilator settings as inputs (frequency, tidal volume, peak inspiratory flow or inspiratory time fraction, and end-inspiratory pause), and patient-determined inputs which describe the patient's ventilatory impedance (inspiratory [RI] and expiratory [RE] resistance and respiratory system compliance [C]. The equations predict key outcome variables: mean airway pressure; and peak, mean, and end-expiratory alveolar pressures. The mathematical expressions were validated in a mechanical lung analog. Values observed in the test lung were compared to values predicted by the mathematical models for a wide range of ventilator settings and impedance combinations (RI and RE, 5 to 40 cm H2O.s/L; C, 0.02 to 0.10 L/cm H2O). The correspondence between observed and predicted values was generally excellent across the broad range of inputs tested (r greater than or equal to 0.98). Outcome variables were quite sensitive to clinician-chosen inputs over certain critical ranges. Carefully applied, VCDF offers several theoretic advantages for the clinical setting; however, appropriate caution must be exercised to avoid the application of tissue-injuring pressure.