Alternative model of respiratory tissue viscoplasticity

After-effects of thixotropic conditionings on operational chest wall and compartmental volumes of patients with Parkinson's disease

Individuals with Parkinson's disease (PD) present respiratory dysfunctions, mainly due to decreased chest wall expansion, which worsens with the course of the disease. These findings contribute to the restrictive respiratory pattern and the reduction in chest wall volume. According to literature, inspiratory muscle thixotropic conditioning maneuvers may improve lung volumes in these patients. The study aimed to determine the after-effects of respiratory muscle thixotropic maneuvers on breathing patterns and chest wall volumes of PD. A crossover study was performed with twelve patients with PD (8 males; mean age 63.9±8.8 years, FVC%pred 89.7±13.9, FEV1%pred 91.2±15, FEV1/FVC%pred 83.7±5.7). Chest wall volumes were assessed using OEP during thixotropic maneuvers. Increases in EIVCW (mean of 126mL, p = 0.01) and EEVCW (mean of 150mL, p = 0.005) were observed after DITLC (deep inspiration from total lung capacity) due to increases in pulmonary (RCp) and abdominal (RCa) ribcage compartments. Changes in ICoTLC (inspiratory contraction from TLC) led to significant EIVCW (mean of 224mL, p = 0.001) and EEVCW (mean of 229mL, p = 0.02) increases that were mainly observed in the RCp. No significant changes were found when performing DERV (deep expiration from residual volume) and ICoRV (Inspiratory contraction from RV). Positive correlations were also observed between the degree of inspiratory contraction during ICoTLC and EEVRCp (rho = 0.613, p = 0.03) and EIVRCp (rho = 0.697, p = 0.01) changes. Thixotropy conditioning of inspiratory muscles at an inflated chest wall volume increases EIVCW and EEVCW in the ten subsequent breaths in PD patients. These maneuvers are easy to perform, free of equipment, low-cost, and may help patients improve chest wall volumes during rehabilitation.

Physiology of the alveolar surface network

The alveolar surface network (ASN) is the totally fluid intraacinar conformation of the alveolar surface liquid (ASL) continuum circulating, both in series and in parallel, through ultrathin (to <7 nm) molecular conduits formed by appositions of unit bubbles of alveolar gas. The ASN is the analogue of foam in vitro. Appositions of unit bubble films, namely foam films, include (a) bubble-to-bubble at the alveolar entrance, across alveolar ducts, and at pores of Kohn ('classical foam films'); (b) bubble-to-epithelial cell surface ('cell-surface foam film'); and (c) bubble-to-open surface liquid layer of the terminal conducting airways ('surface foam film'). These appositions of monolayer bubble films create (a) 'macrochannels' ('pressure points', 'reservoirs') that modulate ASL transfers, volume and flow throughout the acinus and between acinar surface and both the interstitium and the terminal conducting airways surfaces, and (b) 'microchannels' along the broadest surfaces of the appositions. 'Microchannels', which are expectedly bilayer, serve several functions, including (a) virtually frictionless orientation of unit bubbles and ASL to fill the acinar air space; (b) virtually unrestricted diffusion of respiratory gases; (c) architectural support ('infrastructure') against the 'mass' and 'recoil' force of the interstitium; and (d) provision of 'gate' and 'bridge' dynamics that further modulate and direct ASL circulation. The physiological and anatomical boundary between acinar ASN and the bubble-free open liquid surfaces of the conducting airways is marked by the surface foam film. The ASN operates as outlined above in all regions of the lung, at all lung volumes, beginning at the onset of air-breathing at birth and continuing throughout life. Reports of its discovery (Pulmonary Physiology of the Fetus, Newborn and Child (1975) 116; Pediatr. Res. 12 (1978) 1070) and subsequent confirmatory research including the adult lung are summarized in this review by progressive development of each function. These functions, which are normal for a relatively dry foam such as the ASN (where gas:liquid volume ratio is >99:1) cannot be duplicated by the conventional theories and models of an open 'alveolar lining layer'. The unfortunate research technologies upon which these theories and models have been formulated have, indeed, obfuscated recognition of the ASN in vivo. They are also presented and critiqued in this review.

Thixotropy of rib cage respiratory muscles in normal subjects

In this study, we searched for signs of thixotropic behavior in human rib cage respiratory muscles. If rib cage respiratory muscles possess thixotropic properties similar to those seen in other skeletal muscles in animals and humans, we expect resting rib cage circumference would be temporarily changed after deep rib cage inflations or deflations and that these aftereffects would be particularly pronounced in trials that combine conditioning deep inflations or deflations with forceful isometric contractions of the respiratory muscles. We used induction plethysmography to obtain a continuous relative measure of rib cage circumference changes during quiet breathing in 12 healthy subjects. Rib cage position at the end of the expiratory phase (EEP) was used as an index of resting rib cage circumference. Comparisons were made between EEP values of five spontaneous breaths immediately before and after six types of conditioning maneuvers: deep inspiration (DI); deep expiration (DE); DI combined with forceful effort to inspire (FII) or expire (FEI); and DE combined with forceful effort to inspire (FIE) or expire (FEE), both with temporary airway occlusion. The aftereffects of the conditioning maneuvers on EEP values were consistent with the supposition that human respiratory muscles possess thixotropic properties. EEP values were significantly enhanced after all conditioning maneuvers involving DI, and the aftereffects were particularly pronounced in the FII and FEI trials. In contrast, EEP values were reduced after DE maneuvers. The aftereffects were statistically significant for the FEE and FIE, but not DE, trials. It is suggested that respiratory muscle thixotropy may contribute to the pulmonary hyperinflation seen in patients with chronic obstructive pulmonary disease.

Theoretical study of inspiratory flow waveforms during mechanical ventilation on pulmonary blood flow and gas exchange

A lumped two-compartment mathematical model of respiratory mechanics incorporating gas exchange and pulmonary circulation is utilized to analyze the effects of square, descending and ascending inspiratory flow waveforms during mechanical ventilation. The effects on alveolar volume variation, alveolar pressure, airway pressure, gas exchange rate, and expired gas species concentration are evaluated. Advantages in ventilation employing a certain inspiratory flow profile are offset by corresponding reduction in perfusion rates, leading to marginal effects on net gas exchange rates. The descending profile provides better CO2 exchange, whereas the ascending profile is more advantageous for O2 exchange. Regional disparities in airway/lung properties create maldistribution of ventilation and a concomitant inequality in regional alveolar gas composition and gas exchange rates. When minute ventilation is maintained constant, for identical time constant disparities, inequalities in compliance yield pronounced effects on net gas exchange rates at low frequencies, whereas the adverse effects of inequalities in resistance are more pronounced at higher frequencies. Reduction in expiratory air flow (via increased airway resistance) reduces the magnitude of upstroke slope of capnogram and oxigram time courses without significantly affecting end-tidal expired gas compositions, whereas alterations in mechanical factors that result in increased gas exchanges rates yield increases in CO2 and decreases in O2 end-tidal composition values. The model provides a template for assessing the dynamics of cardiopulmonary interactions during mechanical ventilation by combining concurrent descriptions of ventilation, capillary perfusion, and gas exchange.

The alveolar surface network: a new anatomy and its physiological significance

It is generally held that the terminal lung unit (TLU) is an agglomeration of alveoli that opens into the branching air spaces of respiratory bronchioles, alveolar ducts, and alveolar sacs and that these structures are covered by a continuous thin liquid layer bearing a monomolecular film of surfactants at the open gas-liquid interface. The inherent structural and functional instability given TLUs by a broad liquid surface layer of this nature has been mitigated by the discovery that the TLU surface is in fact an agglomeration of bubbles, a foam (the alveolar surface network) that fills the TLU space and forms ultrathin foam films that 1) impart infrastructural stability to sustain aeration, 2) modulate circulation of surface liquid, both in series and in parallel, throughout the TLU and between TLUs and the liquid surface of conducting airways, 3) modulate surface liquid volume and exchange with interstitial liquid, and 4) sustain gas transfer between conducting airways and pulmonary capillaries throughout the respiratory cycle. The experimental evidence, from discovery to the present, is addressed in this report. Lungs were examined in thorax by stereomicroscopy immediately from the in vivo state at volumes ranging from functional residual capacity to maximal volume (Vmax). Lungs were then excised; bubble topography of all anterior and anterolateral surfaces was reaffirmed and also confirmed for all posterior and posterolateral surfaces. The following additional criteria verify the ubiquitous presence of normal intraalveolar bubbles. 1) Bubbles are absent in conducting airways. 2) Bubbles are stable and stationary in TLUs but can be moved individually by gentle microprobe pressure. 3) Adjoining bubbles move into the external medium through subpleural microincisions; there is no free gas, and vacated spaces are rendered airless. Adjacent bubbles may shift position in situ, while more distal bubbles remain stationary. 4) The position and movement of "large" bubbles identifies them as intraductal bubbles. 5) Transection of the lung reveals analogous bubble occurrence and history in central lung regions. 6) Bubbles become fixed in place and change shape when the lung is dried in air; the original shape and movement are restored when the lung is rewet. 7) All exteriorized bubbles are stable with lamellar (film) surface tension near zero. 8) Intact lungs prepared and processed by the new double-embedding technique reveal the intact TLU bubbles and bubble films. Lungs were also monitored directly by stereomicroscopy to establish their presence, transformations, and apparent function from birth through adulthood, as summarized in the following section.

ANATOMY

Intraalveolar bubbles and bubble films (the unit structures of the alveolar surface network) have been found in all mammalian species examined to date, including lambs, kids, and rabbit pups and adult mice, rats, rabbits, cats, and pigs. Rabbits were used for the definitive studies. 1) A unit bubble occupies each alveolus and branching airway of the TLU; unit bubbles in clusters correspond with alveolar clusters. 2) The appositions of unit bubble lamellae (films) form a network of liquid channels within the TLUs. The appositions are bubble to bubble (near alveolar entrances, at pores of Kohn, and between ductal bubbles), bubble to epithelial cell surface, and bubble to surface liquid of conducting airways. They rapidly form stable Newtonian black foam films (approximately 7 nm thick) under hydrodynamic conditions expected in vivo. 3) Lamellae of the foam films and bubbles tend to exclude bulk liquid and thus maintain near-zero surface tension. At the same time, the foam film formations--abetted by the constant but small retractive force of tissue recoil--stabilize unit bubble position within the network. 4) Unit bubble mobility in response to applied force increases as liquid accumulates within the network (e.g. (ABSTRACT TRUNCATED)

Airway mechanics, gas exchange, and blood flow in a nonlinear model of the normal human lung

A model integrating airway/lung mechanics, pulmonary blood flow, and gas exchange for a normal human subject executing the forced vital capacity (FVC) maneuver is presented. It requires as input the intrapleural pressure measured during the maneuver. Selected model-generated output variables are compared against measured data (flow at the mouth, change in lung volume, and expired O2 and CO2 concentrations at the mouth). A nonlinear parameter-estimation algorithm is employed to vary selected sensitive model parameters to obtain reasonable least squares fits to the data. This study indicates that 1) all three components of the respiratory model are necessary to characterize the FVC maneuver; 2) changes in pulmonary blood flow rate are associated with changes in alveolar and intrapleural pressures and affect gas exchange and the time course of expired gas concentrations; and 3) a collapsible midairway segment must be included to match airflow during a forced expiration. Model simulations suggest that the resistances to airflow offered by the collapsible segment and the small airways are significant throughout forced expiration; their combined effect is needed to adequately match the inspiratory and expiratory flow-volume loops. Despite the limitations of this lumped single-compartment model, a remarkable agreement with airflow and expired gas concentration measurements is obtained for normal subjects. Furthermore, the model provides insight into the important dynamic interactions between ventilation and perfusion during the FVC maneuver.

Intraalveolar bubbles and bubble films: III. Vulnerability and preservation in the laboratory

BACKGROUND

Having confirmed (Scarpelli et al. 1996. Anat. Rec. 244:344-357 and 246:245-270) the discovery of intraalveolar bubbles and films as the normal anatomical infrastructure of aerated alveoli at all ages, we now address three questions. Why have these structures been so elusive? Visible in fresh lungs from the in vivo state, can they be preserved by known laboratory methods? Can they be preserved intact for study in tissue sections?

METHODS

Lungs of adult rabbits and pups were examined in thorax directly from the in vivo state to confirm normal bubbles both at functional residual capacity and at maximal volume; other lungs were permitted to deflate naturally to minimal volume. The fate of bubbles in situ (either intact, transected, or diced lung tissue) and of isolated bubbles was assessed (1) during conventional histopreparative processing, (2) during inflation-deflation after degassing, (3) after drying in air, (4) during and after quick freezing in liquid N2, and (5) after preservation in fixed and stained tissue sections prepared by a new double-impregnation procedure in which glutaraldehyde-fixed tissue was preembedded in agar, dehydrated and clarified chemically, embedded in paraffin, sectioned, and stained. Control studies included both blocking of bubble formation by rinsing the air spaces with Tween 20 prior to double impregnation and preparation of normal tissue without preembedding in agar.

RESULTS

(1) Each of the following procedures in conventional processing dislocated and disrupted bubbles and films: osmium tetroxide and glutaraldehyde:formaldehyde:tannic acid mixture fixation; chemical dehydration (70-100% ethanol) and clarification (xylene and acetone); and embedding in paraffin or epoxy resin. Transection and dicing of the tissue aggravated the untoward effects. In contrast, bubbles and films remained stable in either glutaraldehyde or formaldehyde, which, however, did not protect against the other agents. (2) Degassing destroyed all bubbles as expected; however, bubbles and films re-formed immediately with reinflation. (3) Topography of fixed bubbles and films was retained after air drying. The dry polygonal configuration reverted to spherical-oval either in saline solution or in 50% ethanol, whereas vulnerability to upgraded ethanol concentrations was unchanged. (4) Normal topography and shape appeared to be retained during quick freezing and after thawing. (5) Intraalveolar and intraductal bubbles and films were preserved and photographed in sections from tissue prepared by the double-impregnation procedure; they were not seen either when bubble formation had been blocked (double-impregnation procedure) or when preembedding in agar had been omitted.

CONCLUSIONS

(1) Whether or not fixed in glutaraldehyde or formaldehyde, preservation of intraalveolar and intraductal bubbles and films is not to be expected in tissue prepared by conventional histopreparative procedures, whereas product artifacts may be expected from bubble rupture in situ. (2) Degassing cannot be recommended for studies of alveolar structure-function interrelations because all natural bubbles are disrupted in the process, and bubble re-formation may not parallel their "natural history" in vivo. (3) Compared with glutaraldehyde or formaldehyde fixation, air drying offers no added protection against the untoward effects of conventional processing. (4) Quick-frozen tissue is equally at risk. (5) A new double-impregnation procedure does preserve bubbles and films during processing, sectioning, and staining.

Respiratory impedances and acinar gas transfer in a canine model for emphysema

We examined how the changes in the acini caused by emphysema affected gas transfer out of the acinus (Taci) and lung and chest wall mechanical properties. Measurements were taken from five dogs before and 3 mo after induction of severe bilateral emphysema by exposure to papain aerosol (170-350 mg/dose) for 4 consecutive wk. With the dogs anesthetized, paralyzed, and mechanically ventilated at 0.2 Hz and 20 ml/kg, we measured Taci by the rate of washout of 133Xe from an area of the lung with occluded blood flow. Measurements were repeated at positive end-expiratory pressures (PEEP) of 10, 5, 15, 0, and 20 cmH2O. We also measured dynamic elastances and resistances of the lungs (EL and RL, respectively) and chest wall at the different PEEP and during sinusoidal forcing in the normal range of breathing frequency and tidal volume. After final measurements, tissue sections from five randomly selected areas of the lung each showed indications of emphysema. Taci during emphysema was similar to that in control dogs. EL decreased by approximately 50% during emphysema (P < 0.05) but did not change its dependence on frequency or tidal volume. RL did not change (P > 0.05) at the lowest frequency studied (0.2 Hz), but in some dogs it increased compared with control at the higher frequencies. Chest wall properties were not changed by emphysema (P > 0.05). We suggest that although large changes in acinar structure and EL occur during uncomplicated bilateral emphysema, secondary complications must be present to cause several of the characteristic dysfunctions seen in patients with emphysema.

Effects of PEEP on respiratory mechanics are tidal volume and frequency dependent

How the effects of frequency, tidal volume (VT) and PEEP interact to determine the mechanical properties of the respiratory system is unclear. Airway flow and airway and esophageal pressures were measured in ten intubated, anesthetized/paralyzed patients during mechanical ventilation at 10-30 breaths/min and VT of 250-800 ml. From these measurements, Fourier transformation was used to calculate elastance (E) and resistance (R) of the total respiratory system (subscript rs), lungs (subscript L) and chest wall (subscript cw) at 5, 10 and 0 cm PEEP. As PEEP increased from 0-5 cmH2O, all elastances and resistances decreased (P < 0.05). Increasing PEEP to 10 cmH2O decreased EL, Rrs, and RL further (P < 0.05). The changes in Ers, EL, Rrs and RL caused by PEEP were less (P < 0.05) as VT increased, while changes in Rrs, RL and Ers were less (P < 0.05) as frequency increased. VT dependences in Ers and Rrs were enhanced (P < 0.05) at 0 cmH2O PEEP. The ratio of EL to chest wall elastance was not affected by PEEP (P > 0.05), but increased (P < 0.05) with increasing VT at 5 and 10 cmH2O PEEP. We conclude that it is critical to standardize ventilatory parameters when comparing groups of patients or testing clinical intervention efficacy and that the differential effects on the lungs and chest wall must be considered in optimizing the application of PEEP.

Intraalveolar bubbles and bubble films: II. Formation in vivo through adulthood

BACKGROUND

Intraalveolar bubbles and bubble films have been shown to be part of the normal alveolar architecture in vivo from birth through the first 2 days of extrauterine life of rabbit pups (Scarpelli et al., 1996a. Anat. Rec. 244:344-357). The intraluminal boundary between air-way free gas and alveolar bubbles at the level of respiratory bronchioles is established within 1 hour after birth. We now examine the lung through the rest of development, namely, 2 weeks, 1, 2, and 3 months, and adulthood.

METHODS

In quick succession in anesthetized spontaneously breathing rabbits, the abdominal aorta was transected and trachea was occluded either after an end-tidal exhalation at functional residual capacity (FRC) or after volume expansion in vivo by a single inflation from FRC to 20 or 25 cm H2O pressure (V20, V25). Immediately the thorax was opened and lungs were examined (anterior, anterolateral) through a dissecting stereomicroscope while still in the chest, unperturbed (pleural surface temperature 34 degrees C). Heart and lungs were then removed en bloc and re-examined (anterior, lateral, posterior) to confirm that architecture had not changed (22-27 degrees C). After these immediate examinations, lungs were entered into one of the protocols enumerated in Results.

RESULTS

Immediate examination revealed bubbles in all aerated subpleural and deep ("central") alveoli from apex to base at all ages and temperatures. Bubbles were confirmed from two views (top and tangential) and from their individual mobility in response to gentle microprobe pressure. A "common bubble" (> 30 microns to approximately 120 microns inside diameter at FRC) appeared to occupy a single alveolus, sometimes arranged in clusters and collectively accounting for approximately 84% of the total bubble population. Few "large bubbles" appeared to be intraductal. We concluded that "small bubbles" (< or = 30 microns; approximately 16% of the total population) were contracted common bubbles. The free gas-bubble film boundary of the airways was at the level of respiratory bronchioles. Subsequent protocols: (1) Common bubbles moved out of adjoining tissue following subpleural incision. Adjacent bubbles either moved into vacated spaces or into the outside liquid medium. Large bubble(s) followed common bubbles out of the tissue. Small bubbles were less mobile and distal common bubbles did not move. The sequence of bubble movement at V25 was the same. Isolated bubbles had normal surfactant content and surface tension according to "Pattle's stability ratio." Transection revealed analogous conditions in central alveoli. (2) Bubble size increased during inflation from FRC to V25. Airless spaces were aerated with bubbles during inflation. (3) The bubble surface was compressed during deflation to 81% of maximal volume (Vmax) and below, including deflation to minimal volume (Vmin). (4) Bubble/alveolar shape changed from spherical-oval to polygonal when the pleural surface dried at FRC and V25. The original shape was restored when the surface was re-wet. Dry tissue showed but did not emit bubbles when cut; re-wet tissue did. (5) Lung liquid content and volume-pressure were normal at FRC. (6) As expected, conventionally fixed, dehydrated, and embedded sections showed no bubbles.

CONCLUSIONS

Bubbles and bubble films are fundamental to normal architecture of aerated alveoli at all lung volumes from birth through adulthood. As infrastructure, they sustain aeration and resist deformation. With ductal films, they may be expected to form an alveolar surface liquid (foam film) network (Scarpelli, 1988. Surfactants and the Lining of the Lung) that modulates liquid balance principally at Plateau borders. They expand and contract respectively during inflation and deflation, maintaining their closed film integrity. Films are compressed to "film collapse" in situ during deflation from volumes well above FRC to Vmin. At these volumes, intact films sustain aeration; some may disperse into t

Harmonic distortion from nonlinear systems with broadband inputs: applications to lung mechanics

We present a simple index, extended harmonic distortion (kd), to represent the degree of system nonlinearity under sparse pseudorandom noise inputs (SPRN). The frequencies in a SPRN waveform are neither harmonics nor sums or differences of the other component frequencies. Expressed by percentage, the kd is the square root of the ratio of output power at non-input frequencies to the total output power. We evoke three simple corrections to recover the true kd under imperfect SPRN inputs. Simulations on two block-structured nonlinear models (Wiener and Hammerstein) demonstrate the necessity and effectiveness of these corrections especially for the Wiener-type nonlinearity. By applying kd to pressure-flow data of dog lungs, we found that the nonlinear harmonic interactions from a lung arise primarily from its tissues most likely the processes governing the tissue stiffness. This nonlinearity, assessed from kd, is stronger at higher tidal volumes and enhanced (but to a lesser degree) during bronchoconstriction. We conclude that since the kd approach avoids the necessity for multiple-input measurements and lengthy data records, it may be useful for tracking the dynamic variations in nonlinearities of a physiological system.

Influence of flow pattern on the parameter estimates of a simple breathing mechanics model

The first-order model of breathing mechanics is widely used in clinical practice to assess the viscoelastic properties of the respiratory system. Although simple, this model takes the predominant features of the pressure-flow relationship into account but gives highly systematic residuals between measured and model-predicted variables. To achieve a better fit of the entire data set, an approach hypothesizing deterministic time-variations of model parameters, summarized by information-weighted histograms was recently proposed by Bates and Lauzon. The present study uses flow and pressure data measured in intensive care patients to evaluate the real potential of this approach in clinical practice. Information-weighted histograms of the model parameters, estimated by an on-line identification algorithm, were first constructed by taking into account the parameter percentage standard deviations. Then, the influence of the respiratory flow pattern on the calculated histograms was evaluated by the Kolmogorov-Smirnov statistical test. The results show that the method gives good reproducibility under stable experimental conditions. In addition, for a given airflow waveform, an increase in respiratory frequency shifts the histograms representing time-varying viscous properties strongly versus lower values, whereas it shifts the histograms representing time-varying elastic properties slightly versus higher values. On the other hand, the same histograms were highly dependent on the airflow waveform, especially for the viscous properties. Even in a limited experimental work, in all the conditions considered, the method provides results which agree well with the physiological knowledge of nonlinear and multicompartment behavior of respiratory mechanics.

Contribution of viscoelastic stress to the rate-dependence of pulmonary dynamic elastance

To further characterize the contribution of the stresses accumulated during inflation in viscoelastic elements of the lungs to the rate-dependence of pulmonary dynamic elastance, we analyzed the changes in the pressures measured at the airway opening and in subpleural air spaces during airway occlusions performed at constant inflation rates of 5, 10, 20, and 40 ml/(kg sec) in 13 anesthetized piglets (mean age = 7 days). The analysis was repeated after saline lavage of the lungs and during intravenous infusion of histamine in 7 and 4 of the piglets, respectively. Viscoelastic stresses dissipated as stress relaxation were solely responsible for the differences between dynamic and static elastance before and after lung lavage and for more than 40% of this difference during histamine infusion (the remainder probably being caused by ventilation inequalities). The viscoelastic contribution to dynamic elastance increased by more than two-fold after lung lavage and was independent of inflation rate and only minimally dependent upon inflation volume. Our results demonstrate that viscoelastic stresses are primarily responsible for the dynamic stiffening of piglet lungs at low rates of inflation. They also support the notion that viscoelastic and elastic stresses are coupled as the lungs inflate.