Determination of volume-dependent respiratory system mechanics in mechanically ventilated patients using the new...

In patients mechanically ventilated for severe respiratory failure, respiratory system mechanics are non-linear, i.e., volume-dependent. We present a new computer-based multipoint method for simultaneously determining volume-dependent dynamic compliance and resistance. Our method is based on continuously determined tracheal pressure (Ptrach). Tidal volume is subdivided into six volume slices of equal size. One compliance value (intrinsic PEEP considered) and one resistance value are determined for each volume slice by applying of the least-squares-fit (LSF) analysis based on the linear RC-model; we therefore call this the SLICE method. The method gives the course of dynamic compliance and resistance within the tidal volume. The method was evaluated using physical models of the respiratory system with linear and non-linear passive mechanical properties. The relative error of the method is smaller than ±5%. The method needs no special ventilatory pattern. Using data from 14 patients mechanically ventilated for adult respiratory distress syndrome (ARDS) we found a very good correspondence between the measured end-inspiratory airway pressure (Paw,Ie) and the end-inspiratory alveolar pressure (Palv,Ie) calculated from the dynamic compliance values determined with the SLICE method (Palv,Ie = 1.02 * Paw,Ie + 0.097; r2 = 0.977). The SLICE method allows continuous monitoring of non-linear pulmonary mechanics on a breath-by-breath basis at the bedside.