The effects of positive end-expiratory pressure on respiratory resistance in patients with the adult respiratory distress syndrome and in normal anesthetized subjects

We investigated the effects of positive end-expiratory pressure (PEEP) upon respiratory resistance during mechanical ventilation in 21 subjects anesthetized for surgery (normal subjects) and in 11 patients with the adult respiratory distress syndrome (ARDS). We measured tracheal pressure (Ptr) near the end of the endotracheal tube through a 1.5-mm ID catheter and airflow (V) at 0, 5, and 10 cm H2O PEEP (normal subjects) and at 0, 5, 10, 15, and 20 cm H2O PEEP (patients with ARDS). We computed respiratory system static elastance (Est(rs)), maximal (Rrs(max)) and minimal (Rrs(min)) inspiratory resistance by the end-inspiratory occlusion method during constant-flow inflation. Rrs(min) represents the ohmic respiratory resistance, whereas Rrs(max) plus the additional respiratory impedance caused by the stress adaptation phenomena of the respiratory system tissues and to time constant inhomogeneities between lung units (pendelluft). The difference (Rrs(max) - Rrs(min)) has been termed DRrs. We also computed expiratory resistance (Rrs(exp)) at preselected volume (50% of expiration; Rrs(exp50)) and flow (0.3 L/s; Rrs(exp0.3)) using the equation: Rrs(exp) = (Pel(rs)(t) - Ptr(t))/Flow(t), where elastic recoll pressure (Pel(rs)) at time double-ended dagger was computed as: Est(rs) * V(t) + PEEP, in which V(t) is the volume above end-expiratory volume at time t. We found that (1) at PEEP 0, expiratory resistance (Rrs(exp50): 7.38 +/- 1.92 versus 5.35 +/- 1.97 cm H2O*L-1*s) and DRrs (3.08 +/- 1.9 versus 1.66 +/- 0.77 cm H2O*L-1*s) were significantly higher in the ARDS group than in the normal group. Rrs(min) values were not different between the two groups. (2) DRrs and expiratory resistance increased significantly with PEEP, particularly in patients with ARDS. In conclusion, contrary to a commonly accepted notion, PEEP did increase rather than decrease respiratory system resistance in patients with ARDS, particularly at PEEP equal or higher than 10 cm H2O.