COMPARISON OF STATIC AND DYNAMIC MEASUREMENTS OF INTRINSIC PEEP IN ANESTHETIZED CATS

Dynamic measurements of intrinsic positive end-expiratory pressure (PE EPi,dyn) considerably underestimate values obtained under static condi tions (PEEPi,stat) in patients with severe airway obstruction. This ma y be related to regional differences in respiratory system mechanical properties and/or viscoelastic behavior. To evaluate this concept, PEE Pi,stat and PEEPi,dyn were compared in six anesthetized paralyzed cats during dynamic hyperinflation produced by inverse ratio ventilation ( IRV) and aerosolized methacholine (MCh). PEEPi,stat did not differ bet ween IRV and MCh, averaging 2.70 +/- 0.33 (SE) and 2.70 +/- 0.25 cmH(2 )O, respectively. PEEPi,dyn was significantly less with MCh (0.25 +/- 0.05 cmH(2)O) than IRV (2.05 +/- 0.28 cmH(2)O) (P < 0.0001), resulting in a lower PEEPi,dyn/ PEEPi,stat ratio for MCh (0.10 +/- 0.02) than f or IRV (0.76 +/- 0.03) (P < 0.0001). Compared with control values (33. 5 +/- 3.7 cmH(2)O.1(-1).s), maximum resistance (Rmax) was unchanged du ring IRV (29.1 +/- 2.1 cmH(2)O.1(-1).s) but increased considerably wit h MCh (288.8 +/- 18.4 cmH(2)O.1(-1).s) (P < 0.0001). Similar changes i n minimum resistance (Rmin) and Delta R (Rmax - Rmin) were noted. Ther e was a strong inverse relationship between Delta P, an index of time constant inequalities and viscoelastic pressure losses and PEEPi,dyn/P EEPi,stat ratio. No correlation was found between this ratio and Rmax, Rmin, Delta R, or compliance. In conclusion PEEPi,dyn considerably un derestimates PEEPi,stat in acute nonhomogeneous airway obstruction wit h MCh in contrast to IRV, where the magnitude and distribution of mech anical properties remain unaltered. These findings support the concept that the difference between PEEPi,dyn and PEEPi,stat is related to re gional time constant inequalities and/or increased viscoelastic pressu re losses.