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...
ABSTRACT. The purpose of our study was to determine minimum information needed for rational management of inthe effects of insumation time (TI) and of tidal volume (VT) fants and children on mechanical ventilation. Because both on the passive deflation time constant ( 7 ) of juvenile rab-respiratory gas flow and volume are dependent on 7 , to provide bits. We sedated and paralyzed nine 7-wksld New Zealand optimal gas exchange it is necessary to match the mechanical rabbits (wt 1.2 0.2 SD kg) and placed them on a time-breathing device to the mechanical properties of the respiratory cycled, pressure-limited mechanical ventilator. Measure-system. ments of T and dynamic compliance of the respiratory Recently, experiments in healthy rabbits demonstrated that 7 system were made at nine ventilator settings. TI was ad-is influenced by TI (2). This observation could be explained by justed to 0.2,0.4, and 0.6 s and VT was adjusted to 10,15, the visoelastic properties of the respiratory system (3) or by a and 20 rnL so that the effects of TI and VT on T could be distribution of passive exhalation time constants (4); presently, determined independently. Respiratory system quasi-static there are no methods available for distinguishing if one of the and static compliances were also measured and served as two phenomena dominates. a basis for discussing the physiologic explanations for the The purpose of our study was to investigate the effects of V , effects of TI and VT on T. In general, increases in TI resulted as wdl as TI, on 7 of the lungs of small rabbits. As a basis for in increases in T and increases in VT also resulted in discussing the possible effects of stress relaxation and a distribuincreases in 7. Increasing TI from 0.2 to 0.6 s plus increas-tion of r on the measured r, we calculated the compliance of the ing V, from 10 to 20 mL resulted in an increase in T from respiratory system by three different methods. We measured 0.220 2 0.007 to 0.282 f 0.010 s (mean f SEM) ( p < Cdyn, Gsr and C,,.0.05, student Newman-Kuels), which is a 28% increase. At a VT of 10 mL, dynamic compliance increased from 1.95 f 0.10 to 2.14 & 0.10 mL1cm H z 0 (p < 0.05) when TI MATERIALS AND METHODSwas increased from 0.2 to 0.6 s; however, dynamic compliAnimal preparalion. We studied nine 7-wk-old New Zealand ance decreased 19% as VT was increased from 10 to 20 White rabbits weighing ktween 1 100 and 1 350 g ( 1239 * 230 mL. Quasi-static compliance increased from 2.04 f 0.07 g, mean ? SD). We anesthetized each rabbit with ketamine (30 to 2.17 f 0.12 mL/cm H20 { p < 0.05) as dellation volume mg/kg), acepromazine ( 1 mgjkg), and rompun (0.7 mgjkg) and was increased from 10 to 20 mL, yet static compliance then placed them in the supine position. Using lidocaine as a remained relatively constant at 2.6 mL/cm HIO in the local anesthetic, we then introduoed an endotracheal tube (3.0-volume range of 10 to 20 mL. We conclude that k t h TI mrn inner diameter) through a tracheostomy and tied sutures and VT affect T in the small, healthy rabbit and th...
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