Time Dependence of Lung Mechanics in Preterm Lambs

Abstract

ABSTRACT. The passive exhalation flow-volume plots of preterm lambs exhibit curvature. To explain this curvature, we proposed two mathematical models that could predict the measured passive exhalation flow-volume data well. One model takes into account the flow and volume dependence of resistance and compliance. The second model emphasizes the time dependence of lung mechanics and considers the respiratory system viscoelastic properties and the analogy of the lung to two electronic resistor-capacitor circuits connected in parallel. We attempted to determine which of the two models is more valid by analyzing passive exhalation flow-volume data that were obtained while briefly obstructing flow midway through deflation. In 14 preterm lambs (130 d gestation), the flow of exhaled gas increased from 76 + 35 mL/s when measured just before obstruction to 90 2 30 mL/s when measured immediately after release of the obstruction (p < 0.0001). This finding suggests that a time-dependent phenomenon was taking place during obstruction and is inconsistent with the model based upon the flow and volume dependence of resistance and compliance. We made similar measurements in four near-term (143-146 d gestation) and four full-term lambs (9-12 d of age). Their flow-volume curves were relatively linear, and they showed no increases in flow after removal of the obstruction. The results of this study strongly suggest that time-dependent phenomena caused the curvilinearity in the passive exhalation flow-volume plots of preterm lambs. We suspect that the time-dependent phenomena is associated with the premature lung and with parenchymal lung disease. The models of viscoelastance and of parallel inhomogeneities in the lung are consistent with our observations. (Pediatr Res 31: 276-279, 1992) Abbreviations 7, expiratory time constant C, compliance R, resistanceWe have previously reported that in preterm lambs (130 d gestation) the flow-volume plots during passive exhalation are curvilinear (with concavity toward the flow and volume axes (1). For the flow-volume curve to remain linear during exhalation, the product of C and R must remain constant; under these conditions, passive exhalation is well described using a single 7. To explain the departure from linearity in the flow-volume plots of the preterm lamb, we proposed two possible explanations. First, the respiratory system C or R might have been volumeand flow-dependent (2), causing the 7 to increase during exhalation or, second, time-dependent factors could cause T to increase during passive exhalation. The time-dependent factors that we considered were I) the viscoelastic (3) properties of the lung and chest wall, 2) parallel inhomogeneities (4) in the lung, and 3) the compliance of the large airways.To distinguish between volume-and flow-dependent and timedependent factors, we obstructed the flow briefly part way through exhalation. We reasoned that, if the curvature in the flow-volume plot was due to the volume and flow dependence of C and R, then the flow-volume plot after relea...