Mechanics of the respiratory system during high frequency ventilation.

No rational approach has evolved for selecting operating conditions for clinical application of high-frequency ventilation (HFV). To this end, we divide our discussion of HFV into considerations of mechanics versus transport, and treat the latter as a constraint. After describing some of the phenomena that influence distending pressure (and its distribution) expressed across pulmonary tissues, we address the pressure costs per unit ventilation and the factors that influence them.

Thoracic gas volume in rabbits by low-frequency ambient pressure changes.

R. Peslin et al. measured thoracic gas volume (TGV) in adults using a new method employing low-frequency ambient pressure changes (APC) (J.

Differential responses of tissue viscance and collateral resistance to histamine and leukotriene C4.

Alterations in tissue viscance (Vti) and collateral resistance (Rcoll) are both used as indexes of peripheral lung responses. However, it is not known whether the two responses reflect the effects of activation of the same contractile elements. We measured differential responses in Vti and Rcoll to histamine and leukotriene (LT) C4 to determine whether each evoked a similar pattern of response.

Relationship between maximal expiratory flows and lung volumes in growing humans.

We examined airway vs. lung parenchymal growth, as inferred from maximal expiratory flows (MEF) and lung volumes (V), respectively, to determine whether the interindividual variability of airway size (inferred from MEF) changes during lung growth and whether a young child with large (or small) airways for his parenchymal size (inferred from V) maintains relatively large (or small) airways for his lung size as he grows to adulthood. Serial measurements of MEF and V were obtained from a cohort of healthy 6- to 27-yr-old males (n = 26) and females (n = 21) over a period of 18 yr.

Lung inflation does not increase maximal expiratory flow during induced obstruction in the dog.

A deep inflation (DI) reverses induced bronchoconstriction in normal human subjects whether assessed by airway resistance before and after a DI or by isovolumic maximal expiratory flows (Vmax) from partial expiratory flow-volume (PEFV) vs. maximum expiratory flow-volume (MEFV) maneuvers. These observations suggest that with induced constriction the hysteresis of airways exceeds that of the parenchyma.

Bird lung models show that convective inertia effects inspiratory aerodynamic valving.

We assessed various aerodynamic factors which might influence inspiratory valve function in the avian lung. During inspiration, no flow enters the proximal segments of the ventrobronchi connecting the primary bronchus to cranial sacs. Instead, all flow in the primary bronchus continues through the mesobronchus.

Interrupter resistance elucidated by alveolar pressure measurement in open-chest normal dogs.

The interrupter method for measuring respiratory system resistance involves rapidly interrupting flow at the mouth while measuring the pressure just distal to the point of interruption. The pressure signal observed invariably exhibits two distinct phases. The first phase is a very rapid jump, designated delta Pinit, which occurs immediately on interruption of flow.

Axial dispersion in respiratory bronchioles and alveolar ducts.

The mixing of gases in the pulmonary acinus was characterized by analyzing axial gas dispersion during steady flow in models of respiratory bronchioles and alveolar ducts. An analysis (method of moments) developed for addressing dispersion in porous media was used to derive an integral expression for the axial dispersion coefficient (D*). Evaluation of D* required solving the Navier-Stokes equations for the flow field and a convection-diffusion type equation arising from the analysis.

Inspiratory valving in avian bronchi: aerodynamic considerations.

The presence of unidirectional flow in the avian lung is thought to be effected by aerodynamic 'valves'. First we review the history of this hypothesis and summarize existing evidence. Second, we present a semiquantitative treatment of the various fluid dynamic factors that may be involved in directing fluid flow.

In vivo estimation of tracheal distensibility and hysteresis in normal adults.

We used the acoustic reflection technique to measure the cross-sectional area of tracheal and bronchial airway segments of eight healthy adults. We measured airway area during a slow continuous expiration from total lung capacity (TLC) to residual volume (RV) and during inspiration back to TLC. Lung volume and esophageal pressure were monitored continuously during this quasi-static, double vital capacity maneuver.

Airway size is related to sex but not lung size in normal adults.

Within individuals, lung size as assessed by total lung capacity (TLC) or vital capacity (VC) appears to be unrelated to airway size as assessed physiologically by maximum expiratory flows (MEF). Green et al. (J.

Heterogeneous lung emptying during forced expiration.

Several lines of evidence suggest that the healthy mammalian lung empties homogeneously during a maximally forced deflation. Nonetheless, such behavior would appear to be implausible if for no other reason than that airway structure is known to be substantially heterogeneous among parallel pathways of gas conduction. To resolve this paradox we reexamined the degree to which lung emptying is homogeneous, and considered mechanisms that might control differential regional emptying.

Effect of mouthpiece, noseclips, and head position on airway area measured by acoustic reflections.

To investigate whether it is possible to simplify the methodology of measuring airway area by acoustic reflections, we measured upper airway area in 10 healthy subjects during tidal breathing according to seven different protocols. Three protocols employed custom-made bulky mouthpiece with or without nose-clips, two protocols used a scuba-diving mouthpiece and cotton balls placed in the nostrils instead of noseclips, and two protocols employed neck flexion and extension. We found no significant difference in average pharyngeal, glottic, and tracheal areas for any of the protocols except for neck flexion, which was associated with a significantly lower mean pharyngeal area.

Rib cage versus abdominal displacement in rabbits during forced oscillations to 30 Hz.

We measured relative displacement of the rib cage (RC) and abdomen (ABD) in 12 anesthetized rabbits during forced oscillations. Sinusoidal volume changes were delivered through a tracheostomy at frequencies from 0.5 to 30 Hz and measured by body plethysmography.

Dimensional analysis does not determine a mass exponent for metabolic scaling.

In several recent article, Heusner used dimensional reasoning to derive important biological conclusions regarding the scaling of metabolism with body mass [Respir. Physiol. 48: 13-25, 1982; J.

Factors influencing mechanical performance of neonatal high-frequency ventilators.

Factors influencing the mechanical performance of neonatal high-frequency ventilators of diverse design were assessed under controlled conditions. Each of eight ventilators was coupled to in vitro models of the neonatal respiratory system simulating disease of varying severity. The principal performance characteristics examined were frequency dependence and load dependence of tidal volume delivered, peak inspiratory flow rate, and waveforms of pressure at either end of the endotracheal tube.

Interdependent regional lung emptying during forced expiration: a transistor model.

We recognized similarities between isovolume pressure-flow curves of the lung and emitter-collector voltage-current characteristics of bipolar transistors, and used this analogy to model expiratory flow limitation in a two-generation branching network with parallel nonhomogeneity. In this model, each of two bronchi empty parenchymal compliances through a common trachea, and each branch includes resistances upstream and downstream of a flow-limiting site. Properties of each airway are specified independently, allowing simulation of differences between the tracheal and bronchial generations and between the parallel bronchial paths.

Heterogeneity of mean alveolar pressure during high-frequency oscillations.

Mean alveolar pressure may exceed mean airway pressure during high-frequency oscillations (HFO). To assess the magnitude of this effect and its regional heterogeneity, we studied six excised dog lungs during HFO [frequency (f) 2-32 Hz; tidal volume (VT) 5-80 ml] at transpulmonary pressures (PL) of 6, 10, and 25 cmH2O. We measured mean pressure at the airway opening (Pao), trachea (Ptr), and four alveolar locations (PA) using alveolar capsules.

Interdependence of regional expiratory flow.

Flows from different lung regions interact at the junctions of the bronchial tree, and flow from each region depends on the driving pressures for other regions. At each junction, flow from the region with the higher driving pressure is favored. As a result there is a limit on the difference in alveolar pressures that can develop during expiratory flow from a lung with regional differences in lung compliance and airway resistance.

Regional alveolar pressure during periodic flow. Dual manifestations of gas inertia.

We measured pressure excursions at the airway opening and at the alveoli (PA) as well as measured the regional distribution of PA during forced oscillations of six excised dog lungs while frequency (f[2-32 Hz]), tidal volume (VT [5-80 ml]), and mean transpulmonary pressure (PL [25, 10, and 6 cm H2O]) were varied. PA's were measured in four alveolar capsules glued to the pleura of different lobes. The apex-to-base ratio of PA's was used as an index of the distribution of dynamic lung distension.

Alveolar pressure magnitude and asynchrony during high-frequency oscillations of excised rabbit lungs.

One possible advantage of high-frequency ventilation (HFV) over conventional mechanical ventilation is that adequate pulmonary ventilation may be established with lower pressure swings. Pressure swings measured at the airway opening may not accurately reflect pressure swings in the alveoli, however. Furthermore, little is known about the synchrony of alveolar filling during HFV.

Nonhomogeneity of lung response to inhaled histamine assessed with alveolar capsules.

To assess the homogeneity of airway responses to inhaled histamine we examined regional alveolar pressure excursions (PA) arising from small-amplitude oscillations applied at the airway opening (Pao). In five anesthetized and vagotomized dogs the sternum was split and the anterior right lung field exposed. PA was sampled using four capsules affixed to the right apical and middle lobes while lung impedance (ZL) and airway impedances (Zaw) were measured during conventional tidal breathing and during forced oscillations (2-60 HZ at 10 cmH2O distending pressure).

Alveolar pressure nonhomogeneity during small-amplitude high-frequency oscillation.

In six excised canine lungs, regional alveolar pressures (PA) were measured during small-amplitude high-frequency oscillations applied at the airway opening. Both the regional distribution of PA's and their relationship to pressure excursions at the airway opening (Pao) were assessed in terms of amplitude and phase. PA was sampled in several capsules glued to the pleural surface and communicating with alveolar gas via pleural punctures.

Reproducibility and accuracy of airway area by acoustic reflection.

To determine the accuracy and reproducibility of measurements of airway area by acoustic reflection (AAAR) we made repeated measurements of tracheal areas in human volunteers, glass airway models, and excised canine tracheae. In 10 adult males, the mean ratio of tracheal AAAR to tracheal areas determined roentgenographically was not significantly different from one (1.06 +/- 0.

Respiratory system impedance from 4 to 40 Hz in paralyzed intubated infants with respiratory disease.

To describe the mechanical characteristics of the respiratory system in intubated neonates with respiratory disease, we measured impedance and resistance in six paralyzed intubated infants with respiratory distress syndrome, three of whom also had pulmonary interstitial emphysema. We subtracted the effects of the endotracheal tube after showing that such subtraction was valid. Oscillatory flow was generated from 4 to 40 Hz by a loudspeaker, airway pressure was measured, and flow was calculated from pressure changes in an airtight enclosure mounted behind the flow source (speaker plethysmograph).