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 (Zrs), expressed as respiratory resistance (Rrs) and elastance (Ers), in 11 anesthetized dogs from 0.078 to 8.9 Hz using broadband 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 (Pao) of 5, 10, 15, and 20 cm H2O. The Zrs spectra were fit by various models of the respiratory system incorporating different distributions of parallel viscoelastic tissue properties.
RESULTS: Under baseline conditions, both Rrs and Ers exhibited dependence on oscillation frequency, reflecting viscoelastic behavior. The Ers demonstrated significant dependence on Pao. After ALI, both the level and frequency dependence of Rrs and Ers increased, as well as the apparent heterogeneity of tissue properties. Both Rrs and Ers as well as heterogeneity decreased with increasing Pao, approaching baseline levels at the highest levels of Pao.
CONCLUSIONS: These data demonstrate that Zrs can provide specific information regarding the mechanical heterogeneity of injured lungs at different levels of Pao. Moderate increases in Pao 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.