Comparison of static and dynamic measurements of intrinsic PEEP in anesthetized cats

Dynamic measurements of intrinsic positive end-expiratory pressure (PEEPi,dyn) considerably underestimate values obtained under static conditions (PEEPi,stat) in patients with severe airway obstruction. This may be related to regional differences in respiratory system mechanical properties and/or viscoelastic behavior. To evaluate this concept, PEEPi,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 between IRV and MCh, averaging 2.70 ± 0.33 (SE) and 2.70 ± 0.25 cmH₂O, respectively. PEEPi,dyn was significantly less with MCh (0.25 ± 0.05 cmH₂O) than IRV (2.05 ± 0.28 cmH₂O) (P < 0.0001), resulting in a lower PEEPi,dyn/PEEPi,stat ratio for MCh (0.10 ± 0.02) than for IRV (0.76 ± 0.03) (P < 0.0001). Compared with control values (33.5 ± 3.7 cmH₂O · l^-1 · s), maximum resistance (Rmax) was unchanged during IRV (29.1 ± 2.1 cmH₂O · l^-1 · s) but increased considerably with MCh (288.8 ± 18.4 cmH₂O · l^-1 · s) (P < 0.0001). Similar changes in minimum resistance (Rmin) and ΔR (Rmax - Rmin) were noted. There was a strong inverse relationship between ΔP, an index of time constant inequalities and viscoelastic pressure losses and PEEPi,dyn/PEEPi,stat ratio. No correlation was found between this ratio and Rmax, Rmin, ΔR, or compliance. In conclusion, PEEPi,dyn considerably underestimates PEEPi,stat in acute nonhomogeneous airway obstruction with MCh in contrast to IRV, where the magnitude and distribution of mechanical properties remain unaltered. These findings support the concept that the difference between PEEPi,dyn and PEEPi,stat is related to regional time constant inequalities and/or increased viscoelastic pressure losses.