Time dependence of lung mechanics in preterm lambs.

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 +/- 30 mL/s when measured immediately after release of the obstruction (p less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)

Passive deflation time constant of 7-week-old rabbits.

The purpose of our study was to determine the effects of insufflation time (TI) and of tidal volume (VT) on the passive deflation time constant (tau) of juvenile rabbits. We sedated and paralyzed nine 7-wk-old New Zealand rabbits (wt 1.2 +/- 0.2 SD kg) and placed them on a time-cycled, pressure-limited mechanical ventilator. Measurements of tau and dynamic compliance of the respiratory system were made at nine ventilator settings. TI was adjusted to 0.2, 0.4, and 0.6 s and VT was adjusted to 10, 15, and 20 mL so that the effects of TI and VT on tau could be determined independently. Respiratory system quasi-static and static compliances were also measured and served as a basis for discussing the physiologic explanations for the effects of TI and VT on tau. In general, increases in TI resulted in increases in tau and increases in VT also resulted in increases in tau. Increasing TI from 0.2 to 0.6 s plus increasing VI from 10 to 20 mL resulted in an increase in tau from 0.220 +/- 0.007 to 0.282 +/- 0.010 s (mean +/- SEM) (p less than 0.05, student Newman-Kuels), which is a 28% increase. At a VT of 10 mL, dynamic compliance increased from 1.95 +/- 0.10 to 2.14 +/- 0.10 mL/cm H2O (p less than 0.05) when TI was increased from 0.2 to 0.6 s; however, dynamic compliance decreased 19% as VT was increased from 10 to 20 mL.(ABSTRACT TRUNCATED AT 250 WORDS)