Noninvasive determination of respiratory system mechanics during mechanical ventilation for acute respiratory failure.

In 10 acutely ill patients mechanically ventilated for management of acute respiratory failure, respiratory system mechanics were determined with the interrupter technique as described recently (J Appl Physiol 1984; 56:681-690). Flow, volume, and tracheal pressure were measured throughout a series of brief expiratory interruptions. A plateau in tracheal pressure during interruption was observed in all patients, indicating respiratory muscle relaxation as well as equilibration between alveolar and tracheal pressure. Measurement of the plateau in postinterruption tracheal pressure, corresponding volume, and preceding flow enabled determination of the passive elastic and flow-resistive properties of the total respiratory system. In general, the volume-pressure relationship was linear over the expired volume examined and did not necessarily pass through the origin, indicating deviation of the end-expiratory lung volume during mechanical ventilation from the equilibrium position of the respiratory system. Elastance, or inverse slope of this relationship, averaged 16.88 +/- 1.90 (SE) cmH2O X 1(-1). The pressure-flow relationship of the respiratory system was curvilinear; resistance averaged 19.74 +/- 2.08 (SE) cmH2O X 1(-1) X s at a flow rate of 1.0 1 X s-1. In 6 patients the pressure-flow relationship was concave upward. An upward convexity, particularly towards end expiration, was present in the remaining patients. This was associated with characteristic "supramaximal" flow transients after expiratory interruptions, indicating the presence of dynamic airway compression and expiratory flow limitation. In conclusion, the interrupter technique enabled detailed examination of the passive elastic and flow-resistive properties of the total respiratory system in mechanically ventilated patients using simple, noninvasive equipment.(ABSTRACT TRUNCATED AT 250 WORDS)