EFFECT OF PEEP ON THE MECHANICS OF THE RESPIRATORY SYSTEM IN ARDS PATIENTS

In patients with adult respiratory distress syndrome (ARDS) we studied the effect of positive end-expiratory pressure (PEEP) on respiratory mechanics. We used the technique of rapid airway occlusion during constant flow (V) inflation to partition the total respiratory system resistance (Rrs) into the interrupter resistance (Rint,rs) and the additional resistance (DELTARrs) due to viscoelastic pressure dissipations and time constant inequalities. We also measured static (Est,rs) and dynamic (Edyn,rs) elastance of the respiratory system. The procedure was carried out in nine ARDS patients at different inspiratory V and inflation volumes (DELTAV) at PEEP of 0, 5, 10, and 15 cmH2O. We found that during baseline ventilation (DELTAV = 0.7 liter and V = 1 l/s), Est,rs, Edyn,rs, and Rint,rs did not change significantly with PEEP, whereas DELTARrs and Rrs increased significantly only with PEEP of 15 cmH2O. The increase of DELTARrs and Rrs with PEEP was positively correlated with the concomitant changes in end-expiratory lung volume (P < 0.001). At all levels of PEEP, under iso-DELTAV conditions, DELTARrs decreased with increasing V, whereas at a fixed V, DELTARrs increased with increasing DELTAV. A four-parameter model of the respiratory system failed to fully describe respiratory dynamics in the ARDS patients, probably due to nonlinearities.