Airway and tissue mechanics in anesthetized paralyzed children

To estimate the mechanical properties of the airways and respiratory tissues, respiratory system impedance (Zrs) was measured with low-frequency forced oscillations in 26 anesthetized, paralyzed children (aged 3 months-10 years) undergoing surgical correction of congenital heart diseases. Zrs was determined from the signals of tracheal flow and pressure between 0.4-12 Hz before surgery at zero mean transrespiratory pressure. The pulmonary (Z(L)) and chest wall (Z(W)) components of Zrs were also determined in 5 children by measuring esophageal pressure. A model containing frequency-independent resistance (R) and inertance (I), and coefficients of tissue-damping (G) and elastance (H), was fitted to the Zrs, Z(L), and Z(W) spectra. The total respiratory parameters normalized to body weights were 82.2 +/- 8.5 (SE) hPa x sec x l(-1) x kg, 0.152 +/- 0.05 hPa x sec(2) x l(-1) x kg, 293.8 +/- 20.0 hPa. l(-1) x kg, and 1,583 +/- 65.5 hPa x l(-1) x kg, for R, I, G, and H, respectively. The measurements of Z(L) and Z(W) revealed the dominance of the lungs in R (91 +/- 4.3%) and I (109 +/- 16%), and the major contribution of the lung parenchyma to G (61 +/- 7.3%) and H (66 +/- 7.4%) of the total respiratory system. It is concluded that anesthesia-paralysis provides an ideal condition for the measurement of low-frequency forced oscillatory impedance and its partitioning into airway and tissue components in mechanically ventilated children. The separation of pulmonary and chest wall mechanics demonstrates that airway properties can be estimated appropriately from Zrs data, while the chest wall may damp the changes in parenchymal properties.