Passive Deflation Time Constant of 7-Week-Old Rabbits

Abstract

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...