Quantifying mechanical heterogeneity in canine acute lung injury - Impact of mean airway pressure

Background: The heterogeneous pattern of acute lung injury (ALI) predisposes patients to ventilator-associated lung injury. Currently, there is no simple technique that can reliably quantify lung heterogeneity during the dynamic conditions of mechanical ventilation. Such a technique may be of use in optimizing mechanical ventilatory parameters such as rate, tidal volume, or positive end-expiratory pressure. Methods: To determine the impact of heterogeneity on respiratory mechanics, the authors measured respiratory impedance (Z(rs)), expressed as respiratory resistance (R-rs) and elastance (E-rs), in 11 anesthetized dogs from 0.078 to 8.9 Hz using broad-band pressure and flow excitations under baseline conditions and after ALI produced by infusion of 0.08 ml/kg oleic acid into the right atrium. Data were obtained at mean airway pressures ((P) over bar (ao)) of 5, 10, 15, and 20 cm H2O. The Z(rs) spectra were fit by various models of the respiratory system incorporating different distributions of parallel viscoelastic tissue properties. Results: Under baseline conditions, both R-rs and E-rs exhibited dependence on oscillation frequency, reflecting viscoclastic behavior. The E-rs demonstrated significant dependence on (P) over bar (ao). After ALI, both the level and frequency dependence of R-rs and E-rs increased, as well as the apparent heterogeneity of tissue properties. Both R-rs and E-rs as well as heterogeneity decreased with increasing (P) over bar (ao), approaching baseline levels at the highest levels of (P) over bar (ao). Conclusions: These data demonstrate that Z(rs) can provide specific information regarding the mechanical heterogeneity of injured lungs at different levels of (P) over bar (ao). Moderate increases in P.. seem to be beneficial in ALI by reducing heterogeneity and recruiting lung units. These noninvasive measurements of lung heterogeneity may ultimately allow for the development of better ventilation protocols that optimize regional lung mechanics in patients with ALI.