Objective: The aims of the present study were 1) to evaluate a method for identification of "slowly" distensible compartments of the respiratory system (rs), which are characterized by long mechanical time constants (RC) and 2) to identify "slowly" distensible rs-compartments in mechanically ventilated patients.
Design: Prospective study on a physical lung model.
Setting: Intensive Care Unit, University Hospital, Tübingen.
Patients and participants: 19 patients with severe lung injury (acute respiratory distress syndrome, ARDS) and on 10 patients with mild lung injury.
Measurements and results: Positive end-expiratory pressure (PEEP)-increasing and -decreasing steps of about 5 cmH2O were applied and the breath-by-breath differences of inspiratory and expiratory volumes (delta V) were measured. The sequence of delta Vs were analyzed in terms of volume change in the "fast" compartment (Vfast), the "slow" compartment (Vslow), total change in lung volume (delta VL) and mechanical time constant of the slow compartment (RCslow). Thirty-eight measurements in a lung model revealed a good correlation between the preset Vslow/delta VL and Vslow/delta VL measured: r2 = 0.91. The Vslow/delta VL measured amounted to 0.94 +/- 0.15 of Vslow/delta VL in the lung model. RCslow measured was 0.92 +/- 0.43 of the RCslow reference. Starting from a PEEP level of 11 cmH2O PEEP-increasing and PEEP-decreasing steps were applied to the mechanically ventilated patients. Three out of ten patients with mild lung injury (30%) and 7/19 patients with ARDS (36.8%) revealed "slowly" distensible rs-compartments in a PEEP-increasing step, whereas 15/19 ARDS patients and 1/10 patients with mild lung injury showed "slowly" distensible rs-compartments in a PEEP-decreasing step (78.9% vs 10%, P < 0.002, chi-square test).
Conclusions: The gas distribution properties of the respiratory system can be easily studied by a PEEP-step maneuver. The relative contribution of the "slow" units to the total increase of lung volume following a PEEP step could be adequately assessed. "Slowly" distensible rs-compartments could be detected in patients with severe and mild lung injury, however significantly more ARDS patients revealed "slow" rs-compartments in PEEP-decreasing steps. The influence of "slowly" distensible rs-compartments on pulmonary gas exchange is unknown and has yet to be studied.