COMPARISON OF STATIC AND DYNAMIC MEASUREMENTS OF INTRINSIC PEEP IN MECHANICALLY VENTILATED PATIENTS

Intrinsic positive end-expiratory pressure (PEEPi) is routinely determ ined 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 al so be evaluated dynamically (PEEPi,dyn) by recording the change in pre ssure required to initiate lung inflation. It has been suggested that PEEPi,dyn reflects the lowest regional PEEPi, and therefore underestim ates PEEPi,stat in the presence of heterogenous mechanical properties. The purposes of this study were (1) to compare PEEPi obtained with th ese two methods in mechanically ventilated patients with significant a irway obstruction (AWO) and those without (non-AWO), and (2) to relate any discrepancies observed with other indices of respiratory mechanic s. PEEPi,stat, PEEPi,dyn, and respiratory mechanics were measured duri ng 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 < 0.000 1). In contrast, these values were more comparable in non-AWO, averagi ng 4.6 +/- 0.8 and 5.4 +/- 1.0 cm H2O (p > 0.05). As a result, the rat io of PEEPi,dyn to PEEPi,stat amounted to 0.36 +/- 0.06 for AWO compar ed with 0.87 +/- 0.05 in nonAWO (p < 0.005). Maximal (Rmax) and minima l (Rmin) respiratory resistance were greater in AWO whereas respirator y compliance (Crs) was no different between groups. PEEPi,dyn/PEEPi,st at was inversely related to Delta P, 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 viscoel astic pressure losses.