Comparison of static and dynamic measurements of intrinsic PEEP in mechanically ventilated patients

Intrinsic positive end-expiratory pressure (PEEPi) is routinely determined under static conditions by occluding the airway at end-expiration (PEEPi,stat), the resulting plateau pressure representing the average PEEPi present within a nonhomogeneous lung. In contrast, PEEPi can also be evaluated dynamically (PEEPi,dyn) by recording the change in pressure required to initiate lung inflation. It has been suggested that PEEPi,dyn reflects the lowest regional PEEPi, and therefore underestimates PEEPi,stat in the presence of heterogenous mechanical properties. The purposes of this study were (1) to compare PEEPi obtained with these two methods in mechanically ventilated patients with significant airway obstruction (AWO) and those without (non-AWO), and (2) to relate any discrepancies observed with other indices of respiratory mechanics. PEEPi,stat, PEEPi,dyn, and respiratory mechanics were measured during controlled mechanical ventilation in 22 sedated, paralyzed patients. PEEPi,dyn was significantly less than PEEPi,stat in AWO, averaging 3.0 ± 0.5 (SEM) and 9.3 ± 1.1 (SEM) cm H₂O, respectively (p < 0.0001). In contrast, these values were more comparable in non-AWO, averaging 4.6 ± 0.8 and 5.4 ± 1.0 cm H₂O (p > 0.05). As a result, the ratio of PEEPi,dyn to PEEPi,stat amounted to 0.36 ± 0.06 for AWO compared with 0.87 ± 0.05 in non-AWO (p < 0.005). Maximal (Rmax) and minimal (Rmin) respiratory resistance were greater in AWO whereas respiratory compliance (Crs) was no different between groups. PEEPi,dyn/PEEPi,stat was inversely related to ΔP, the pressure losses attributable to time constant inequalities and viscoelastic tissue properties (r = 0.64, p < 0.005). No correlation was found between this ratio and Rmax, Rmin, or Crs. In conclusion, PEEPi,dyn significantly underestimates PEEPi,stat in patients with AWO, in contrast to those without AWO. The data suggest that this difference is related to regional inequalities in mechanical time constants within the lung and/or increased viscoelastic pressure losses.