Respiratory system inertance corresponds to extravascular lung water in surfactant-deficient piglets

In various cardio-pulmonary diseases lung mass is considerably increased due to intrapulmonary fluid accumulation, i.e. extravascular lung water (EVLW). Generally, inertance is a physical system parameter that is mass-dependent. We hypothesized that changes in lung mass influence the inertive behavior of the respiratory system. EVLW and intrathoracic blood volume (ITBV) were compared with respiratory system inertance (Irs) in four piglets before and after broncho-alveolar lavage (BAL) that induced surfactant deficiency with interstitial edema. EVLW and ITBV were determined using the double-indicator dilution technique, Irs by multiple linear regression analysis. Measurements were taken before, and 1 and 2 h after BAL. EVLW increased threefold (from 6.2 ± 0.8 mL/kg at baseline to 17.7 ± 0.9 mL/kg (p < 0.001) after BAL). Irs increased by 35% (from 0.17 ± 0.02 to 0.23 ± 0.04 cmH2O s2/L (p = 0.036) after BAL) and was tightly correlated to EVLW (r2 = 0.95, p < 0.023). ITBV did not change significantly after BAL. We conclude that Irs reflects actual changes in lung mass and thus hints at fluid accumulation within the lung.