Mucus transport in the tracheobronchial tree of normal and bronchitic rats

Physiology and pathophysiology of human airway mucus

The mucus clearance system is the dominant mechanical host defense system of the human lung. Mucus is cleared from the lung by cilia and airflow, including both two-phase gas-liquid pumping and cough-dependent mechanisms, and mucus transport rates are heavily dependent on mucus concentration. Importantly, mucus transport rates are accurately predicted by the gel-on-brush model of the mucociliary apparatus from the relative osmotic moduli of the mucus and periciliary-glycocalyceal (PCL-G) layers. The fluid available to hydrate mucus is generated by transepithelial fluid transport. Feedback interactions between mucus concentrations and cilia beating, via purinergic signaling, coordinate Na+ absorptive vs Cl- secretory rates to maintain mucus hydration in health. In disease, mucus becomes hyperconcentrated (dehydrated). Multiple mechanisms derange the ion transport pathways that normally hydrate mucus in muco-obstructive lung diseases, e.g., cystic fibrosis (CF), chronic obstructive pulmonary disease (COPD), non-CF bronchiectasis (NCFB), and primary ciliary dyskinesia (PCD). A key step in muco-obstructive disease pathogenesis is the osmotic compression of the mucus layer onto the airway surface with the formation of adherent mucus plaques and plugs, particularly in distal airways. Mucus plaques create locally hypoxic conditions and produce airflow obstruction, inflammation, infection, and, ultimately, airway wall damage. Therapies to clear adherent mucus with hydrating and mucolytic agents are rational, and strategies to develop these agents are reviewed.

Regional Differences in Mucociliary Clearance in the Upper and Lower Airways

As the nasal cavity is the portal of entry for inspired air in mammals, this region is exposed to the highest concentration of inhaled particulate matter and pathogens, which must be removed to keep the lower airways sterile. Thus, one might expect vigorous removal of these substances via mucociliary clearance (MCC) in this region. We have investigated the rate of MCC in the murine nasal cavity compared to the more distal airways (trachea). The rate of MCC in the nasal cavity (posterior nasopharynx, PNP) was ∼3–4× greater than on the tracheal wall. This appeared to be due to a more abundant population of ciliated cells in the nasal cavity (∼80%) compared to the more sparsely ciliated trachea (∼40%). Interestingly, the tracheal ventral wall exhibited a significantly lower rate of MCC than the tracheal posterior membrane. The trachealis muscle underlying the ciliated epithelium on the posterior membrane appeared to control the surface architecture and likely in part the rate of MCC in this tracheal region. In one of our mouse models (Bpifb1 KO) exhibiting a 3-fold increase in MUC5B protein in lavage fluid, MCC particle transport on the tracheal walls was severely compromised, yet normal MCC occurred on the tracheal posterior membrane. While a blanket of mucus covered the surface of both the PNP and trachea, this mucus appeared to be transported as a blanket by MCC only in the PNP. In contrast, particles appeared to be transported as discrete patches or streams of mucus in the trachea. In addition, particle transport in the PNP was fairly linear, in contrast transport of particles in the trachea often followed a more non-linear route. The thick, viscoelastic mucus blanket that covered the PNP, which exhibited ∼10-fold greater mass of mucus than did the blanket covering the surface of the trachea, could be transported over large areas completely devoid of cells (made by a breach in the epithelial layer). In contrast, particles could not be transported over even a small epithelial breach in the trachea. The thick mucus blanket in the PNP likely aids in particle transport over the non-ciliated olfactory cells in the nasal cavity and likely contributes to humidification and more efficient particle trapping in this upper airway region.

Molecular Dynamics Simulations to Explore the Structure and Rheological Properties of Normal and Hyperconcentrated Airway Mucus

We develop the first molecular dynamics model of airway mucus based on the detailed physical properties and chemical structure of the predominant gel-forming mucin MUC5B. Our airway mucus model leverages the LAMMPS open-source code [https://lammps.sandia.gov], based on the statistical physics of polymers, from single molecules to networks. On top of the LAMMPS platform, the chemical structure of MUC5B is used to superimpose proximity-based, non-covalent, transient interactions within and between the specific domains of MUC5B polymers. We explore feasible ranges of hydrophobic and electrostatic interaction strengths between MUC5B domains with 9 nanometer spatial and 1 nanosecond temporal resolution. Our goal here is to propose and test a mechanistic hypothesis for a striking clinical observation with respect to airway mucus: a 10-fold increase in non-swellable, dense structures called flakes during progression of cystic fibrosis disease. Among the myriad possible effects that might promote self-organization of MUC5B networks into flake structures, we hypothesize and confirm that the clinically confirmed increase in mucin concentration, from 1.5 to 5 mg/mL, alone is sufficient to drive the structure changes observed with scanning electron microscopy images from experimental samples. We post-process the LAMMPS simulated datasets at 1.5 and 5 mg/mL, both to image the structure transition and compare with scanning electron micrographs and to show that the 3.33-fold increase in concentration induces closer proximity of interacting electrostatic and hydrophobic domains, thereby amplifying the proximity-based strength of the interactions.

Dysfunctional Bronchial Cilia Are a Feature of Chronic Obstructive Pulmonary Disease (COPD)

Impaired mucociliary clearance may increase COPD exacerbation risk. We aimed to compare bronchial ciliary function and epithelial ultrastructure of COPD patients to healthy controls and explore its relationship to exacerbator phenotypes (frequent [FE] and infrequent [IFE] exacerbator). In this cross-sectional study, 16 COPD patients and 12 controls underwent bronchial brushings. Ciliary beat frequency (CBF) and dyskinesia index (DI; % of dyskinetic cilia) were assessed using digital high-speed video microscopy, and epithelial ultrastructure using transmission electron microscopy (TEM). Bronchial epithelium in COPD showed lower CBF and higher DI, compared to controls (median [IQR] CBF: 6.8 (6.1-7.2) Hz vs 8.5 (7.7-8.9) Hz, p<0.001 and DI: 73.8 (60.7-89.8) % vs 14.5 (11.2-16.9) %, p<0.001, respectively). This was true for FE and IFE phenotypes of COPD, which were similar in terms of bronchial CBF or DI. Subgroup analyses demonstrated lower CBF and higher DI in FE and IFE COPD phenotypes compared to controls, irrespective of smoking status. TEM showed more loss of cilia, extrusion of cells, cytoplasmic blebs and dead cells in COPD patients versus controls. Profound dysfunction of bronchial cilia is a feature of COPD irrespective of exacerbation phenotype and smoking status, which is likely to contribute to poor mucus clearance in COPD.Supplemental data for this article is available online at https://doi.org/10.1080/15412555.2021.1963695 .

Effect of hypertonic saline on mucociliary clearance and clinical outcomes in chronic bronchitis

Background

Mucus dehydration and impaired mucus clearance are common features of cystic fibrosis (CF) and chronic obstructive pulmonary disease (COPD). In CF, inhaled hypertonic saline (HS) improves lung function and produces sustained increases in mucociliary clearance (MCC). We hypothesised that administration of HS (7% NaCl) twice daily for 2 weeks would improve clinical outcomes and produce sustained increases in MCC in COPD subjects with a chronic bronchitis (CB) phenotype.

Methods

Twenty-two CB subjects completed a double-blinded, crossover study comparing inhaled HS to a hypotonic control solution (0.12% saline) administered via nebuliser twice daily for 2 weeks. Treatment order was randomised. During each treatment period, symptoms and spirometry were measured. MCC was measured at baseline, shortly after initial study agent administration, and approximately 12 h after the final dose.

Results

HS was safe and well tolerated but overall produced no significant improvements in spirometry or patient-reported outcomes. CB subjects had slower baseline MCC than healthy subjects. The MCC rates over 60 min (Ave60Clr) in CB subjects following 2 weeks of HS were not different from 0.12% saline but were slower than baseline (Ave60Clr was 9.1±6.3% at baseline versus 5.3±6.9% after HS; p<0.05). Subgroup analyses determined that subjects with residual baseline central lung clearance (14 subjects) had improved spirometry and symptoms following treatment with HS, but not 0.12% saline, treatment.

Conclusions

Inhaled HS appeared to be safe in a general CB population. A specific phenotypic subgroup may benefit from HS but requires additional study.

Quantitative Assessment of Regional Mucociliary Clearance in Smokers with Mild-to-Moderate Chronic Obstructive Pulmonary Disease and Chronic Bronchitis from Planar Radionuclide Imaging

Background: Mucociliary clearance (MCC) rate from the lung has been shown to be reduced in chronic obstructive pulmonary disease (COPD). This study investigates the value of regional clearance measurements in assessing MCC in mild-to-moderate disease. Methods: Measurement of lung MCC using planar gamma camera imaging was performed in three groups: (i) healthy nonsmoking controls (NSCs) (n = 9), (ii) smoking controls (SCs) who were current smokers with normal lung function (n = 10), and (iii) current smokers with mild-to-moderate COPD and bronchitis (n = 15). The mean (±standard deviation) forced expiratory volumes at 1 second (FEV1) for the three groups were 109 (± 18), 94 (± 5), and 78 (± 12), respectively. After inhalation of a technetium-99m labeled aerosol, planar imaging was performed over 4 hours and then at 24 hours. Both lung clearance and tracheobronchial clearance (TBC) (normalized to 24 hours clearance) were calculated for inner and outer lung zones. Inner zone clearance was corrected for input from the outer zone. A novel parameter, the bronchial airways clearance index (BACI), which combined clearance data from both zones, was also evaluated. Regional results were compared with whole lung clearance in the same subjects. Results: Corrected inner zone clearance at 3 hours was not reduced compared with NSC in either SCs or COPD. Outer zone clearance was higher in COPD than in the other groups. Corrected inner zone TBC showed significant reductions in SC and COPD compared with NSC. BACI was significantly reduced in COPD compared with NSC and also correlated with FEV1. The mean BACI for SC was also reduced compared with NSC, but the distribution of results was bimodal, with a significant proportion of subjects having values in the NSC range. Conclusions: Regional MCC demonstrated differences between NSCs, SCs, and subjects with mild-to-moderate COPD, which were not apparent with whole lung measurements.

Mucociliary Transport in Healthy and Cystic Fibrosis Pig Airways

Cystic fibrosis (CF) lung disease is the major cause of morbidity and mortality in people with CF. Abnormal mucociliary transport has been the leading hypothesis for the underlying pathogenesis of CF airway disease. However, this has been difficult to investigate at very early time points. A porcine CF model, which recapitulates many features of CF disease in humans, enables studies to be performed in non-CF and CF pigs on the day that they are born. In newborn CF pigs, we found that under basal conditions, mucociliary transport rates in non-CF and CF pigs are similar. However, after cholinergic stimulation, which stimulates submucosal gland secretion, particles become stuck in the CF airways owing to a failure of mucus strands to release from submucosal glands. In this review, we summarize these recent discoveries and also discuss the morphology, composition, and function of mucins in the porcine lung.

Nanoparticles that do not adhere to mucus provide uniform and long-lasting drug delivery to airways following inhalation

Mucoadhesive particles (MAP) have been widely explored for pulmonary drug delivery because of their perceived benefits in improving particle residence in the lungs. However, retention of particles adhesively trapped in airway mucus may be limited by physiologic mucus clearance mechanisms. In contrast, particles that avoid mucoadhesion and have diameters smaller than mucus mesh spacings rapidly penetrate mucus layers [mucus-penetrating particles (MPP)], which we hypothesized would provide prolonged lung retention compared to MAP. We compared in vivo behaviors of variously sized, polystyrene-based MAP and MPP in the lungs following inhalation. MAP, regardless of particle size, were aggregated and poorly distributed throughout the airways, leading to rapid clearance from the lungs. Conversely, MPP as large as 300 nm exhibited uniform distribution and markedly enhanced retention compared to size-matched MAP. On the basis of these findings, we formulated biodegradable MPP (b-MPP) with an average diameter of <300 nm and examined their behavior following inhalation relative to similarly sized biodegradable MAP (b-MAP). Although b-MPP diffused rapidly through human airway mucus ex vivo, b-MAP did not. Rapid b-MPP movements in mucus ex vivo correlated to a more uniform distribution within the airways and enhanced lung retention time as compared to b-MAP. Furthermore, inhalation of b-MPP loaded with dexamethasone sodium phosphate (DP) significantly reduced inflammation in a mouse model of acute lung inflammation compared to both carrier-free DP and DP-loaded MAP. These studies provide a careful head-to-head comparison of MAP versus MPP following inhalation and challenge a long-standing dogma that favored the use of MAP for pulmonary drug delivery.

Airway Gland Structure and Function

Submucosal glands contribute to airway surface liquid (ASL), a film that protects all airway surfaces. Glandular mucus comprises electrolytes, water, the gel-forming mucin MUC5B, and hundreds of different proteins with diverse protective functions. Gland volume per unit area of mucosal surface correlates positively with impaction rate of inhaled particles. In human main bronchi, the volume of the glands is ∼ 50 times that of surface goblet cells, but the glands diminish in size and frequency distally. ASL and its trapped particles are removed from the airways by mucociliary transport. Airway glands have a tubuloacinar structure, with a single terminal duct, a nonciliated collecting duct, then branching secretory tubules lined with mucous cells and ending in serous acini. They allow for a massive increase in numbers of mucus-producing cells without replacing surface ciliated cells. Active secretion of Cl(-) and HCO3 (-) by serous cells produces most of the fluid of gland secretions. Glands are densely innervated by tonically active, mutually excitatory airway intrinsic neurons. Most gland mucus is secreted constitutively in vivo, with large, transient increases produced by emergency reflex drive from the vagus. Elevations of [cAMP]i and [Ca(2+)]i coordinate electrolyte and macromolecular secretion and probably occur together for baseline activity in vivo, with cholinergic elevation of [Ca(2+)]i being mainly responsive for transient increases in secretion. Altered submucosal gland function contributes to the pathology of all obstructive diseases, but is an early stage of pathogenesis only in cystic fibrosis.

Duration of action of hypertonic saline on mucociliary clearance in the normal lung

Inhalation of hypertonic saline (HS) acutely enhances mucociliary clearance (MC) in both health and disease. In patients with cystic fibrosis (CF), repeated use of HS causes a sustained improvement in MC as well as clinical benefit. The pharmacodynamic duration of activity on MC may be an important determinant of its therapeutic potential in other airways diseases. Before moving toward testing the clinical benefits of HS for non-CF indications, we sought to assess the duration of pharmacodynamic effects of HS in healthy subjects by performing radiotracer clearance studies at baseline, 30-min post-HS administration, and 4-h post-HS administration. Indeed, acceleration of MC was observed when measured 30 min after HS inhalation. This acceleration was most pronounced in the first 30 min after inhaling the radiotracer in the central lung region (mean Ave30Clr = 15.5 vs. 8.6% for 30-min post-HS treatment vs. mean baseline, respectively, P < 0.005), suggesting that acute HS effects were greatest in the larger bronchial airways. In contrast, when MC was measured 4 h after HS administration, all indices of central lung region MC were slower than at baseline: Ave30Clr = 5.9% vs. 8.6% (P = 0.10); Ave90Clr = 12.4% vs. 16.8% (P < 0.05); clearance through 3 h = 29.4 vs. 43.7% (P < 0.002); and clearance through 6 h = 39.4 vs. 50.2% (P < 0.02). This apparent slowing of MC in healthy subjects 4-h post-HS administration may reflect depletion of airway mucus following acute HS administration.

A periciliary brush promotes the lung health by separating the mucus layer from airway epithelia

Mucus clearance is the primary defense mechanism that protects airways from inhaled infectious and toxic agents. In the current gel-on-liquid mucus clearance model, a mucus gel is propelled on top of a "watery" periciliary layer surrounding the cilia. However, this model fails to explain the formation of a distinct mucus layer in health or why mucus clearance fails in disease. We propose a gel-on-brush model in which the periciliary layer is occupied by membrane-spanning mucins and mucopolysaccharides densely tethered to the airway surface. This brush prevents mucus penetration into the periciliary space and causes mucus to form a distinct layer. The relative osmotic moduli of the mucus and periciliary brush layers explain both the stability of mucus clearance in health and its failure in airway disease.

Tracheobronchial clearance in health and disease: with special reference to interciliary fluid

Rats were exposed to influenza B virus for 2 hours. Mucociliary function in the isolated airway preparation of these animals was studied from the first day after exposure and at regular intervals for up to 190 days. Transmission electron microscopy (TEM) studies were done at times corresponding to the mucociliary function studies. TEM studies confirmed the results of the mucociliary function studies: pathological changes were first apparent three days after exposure and began in the distal bronchioles. The morphological studies further demonstrated that the infection progressed proximally to include the entire tracheobronchial tree by the 18th day. The alterations observed included an increase in the number of goblet cells, a decrease in the number of microvilli and a reduction in the amount of interciliary fluid. The last two changes were closely correlated with the severity of the infection. The abnormalities found in mucociliary function included hypersecretion of mucus, inactive and hypoactive zones, synchronization of cilia beating, and a sticking together of the cilia tips.

Embryonic chicken trachea as a new in vitro model for the investigation of mucociliary particle clearance in the airways

Mucociliary clearance (MC) is an important defense mechanism of the respiratory system to eliminate inhaled and possibly noxious particles from the lung. Although the principal mechanics of MC seem to be relatively clear there are still open questions regarding the long-term clearance of particles. Therefore, we have developed a new set-up based on embryonic chicken trachea (ECT) to investigate mucociliary particle clearance in more detail. ECT was placed in an incubation chamber after carbon particles were applied and tracked using optical microscopy. The aim of the study was to validate this model by investigating the impact of temperature, humidity and drugs on particle transport rates. Particles were transported reproducibly along the trachea and clearance velocity (2.39 +/- 0.25) mm/min was found to be in accordance to data reported in literature. Variation in temperature resulted in significantly reduced MC: (0.40 +/- 0.12) mm/min (20 degrees C); (0.42 +/- 0.10) mm/min (45 degrees C). Decreasing humidity (99-60%) had no significant effect on MC, whereas reduction to 20% humidity showed a significant influence on particle clearance. The use of different cilio- and muco-active drugs (propranolol, terbutalin, N-acetylcysteine) resulted in altered MC according to the pharmacological effect of the substances: a concentration dependent decrease of MC was found for Propranolol. From our results we conclude that this model can be employed to investigate MC of particles in more detail. Hence, the model may help to understand and identify decisive physico-chemical parameters for MC and to answer open questions regarding the long-term clearance phenomenon.

Respiratory gas conditioning and humidification

Respiratory gas conditioning and humidification are important but poorly understood aspects of mechanical ventilation. The physiologic principles and the best methods to achieve appropriate gas conditioning are addressed in this article.

Stochastic modeling predictions for the clearance of insoluble particles from the tracheobronchial tree of the human lung

Bronchial clearance of deposited particles was simulated using a stochastic model of the tracheobronchial tree. The clearance model introduced in this study considers (1) a continuous decrease of the mucus thickness from the trachea to the terminal bronchioles according to a linear or an exponential function, (2) the possibility of mucus discontinuities, which are mainly found in intermediate and distal airways of the tracheobronchial compartment, (3) mucus production in proximal airways, (4) a slow bronchial clearance phase due to the capture of a defined particle fraction f (s) in the periciliary sol phase, and (5) an eventual delay of the mucociliary transport at carinal ridges of airway bifurcations. Based on the concept of mucus volume conservation in single bifurcations, a reduction of the thickness of the mucus blanket from proximal to distal airways causes a significant increase of the mucus velocities in small ciliated airways compared to other stochastic modeling predictions assuming a constant thickness of the mucus layer throughout the conducting airways. This effect is further enhanced by the consideration of mucus discontinuities. In contrast, the ability of bronchial airways to produce a certain volume of mucus has a decreasing effect on the mucus velocities. In all generated clearance velocity models, mucociliary clearance is completely terminated within 24 h after exposure, consistent with the experimental evidence. Implementation of a slow bronchial clearance phase predicts a long-term retention fraction, which is fully cleared from the lung after several weeks. For 1-microm MMAD particles, 24-h retention varies between 0.42 and 0.52, in line with the suggestions of the ICRP. Mucus delay at carinal ridges only affects short-term clearance by increasing the retained particle fraction at a given time, while long-term retention is not influenced.

Mucociliary and long-term particle clearance in the airways of healthy nonsmoker subjects

Spherical monodisperse ferromagnetic iron oxide particles of 1.9-microm geometric and 4.2-microm aerodynamic diameter were inhaled by 13 healthy nonsmoking subjects using the shallow bolus technique. The bolus width was 100 ml, and the penetration front depth was 150 +/- 27 ml. The mean flow rate during inhalation and exhalation was 250 ml/s. The Fowler dead space and the phase 1 dead space of the airways were 282 +/- 49 and 164 +/- 34 ml, respectively. Deposition was below 20% without breath holding and 51 +/- 8% after an 8-s breath-holding time. We attempted to confine the bolus deposition to the bronchial airways by limiting the bolus front depth to the phase 1 dead space volume. Particle retention was measured by the magnetopneumographic method over a period of 9 mo. Particle clearance from the airways showed a fast and a slow phase; 49 +/- 9% followed the fast phase with a mean half-time of 3.0 +/- 1.6 h and characterized the mucociliary clearance. The remaining fraction was cleared slowly with a half-time of 109 +/- 78 days. The slow clearance phase was comparable to clearance measurements from the lung periphery of healthy nonsmokers, which allowed macrophage-dependent clearance mechanisms of the slow cleared fraction to be taken into account. Despite the fact that part of the slowly cleared particles may originate from peripheral deposition, the data demonstrate that mucociliary clearance does not remove all particles deposited in the airways and that a significant fraction undergoes long-term retention mechanisms, the origin of which is still under discussion.

Stochastic model of particle clearance in human bronchial airways

A stochastic bronchial clearance model, based on a stochastic morphometric model of the human bronchial tree, has been developed, which simulates the combined action of fast and slow bronchial clearance mechanisms by Monte Carlo methods. To model fast bronchial clearance, mucus velocities in individual airways were based on a correlation between mucus velocity and airway diameter, considering conservation of mucus flow. In addition, mucus transport was assumed to be delayed at bronchial bifurcation zones. The size dependence of the slow bronchial clearance phase was considered by a linear relationship between the slow bronchial clearance fraction, f(s), and the geometric particle diameter, derived from bolus inhalation experiments. Potential variations of f(s) from proximal to distal airway generations were simulated by five different scenarios, which allocated slow bronchial clearance to successively peripheral bronchial regions. Alveolar clearance, which contributes only to longterm particle retention, was modeled by transfer rates supplied by the ICRP respiratory tract model. To test the different components of the clearance model, modeling predictions were compared with experimental retention data from bolus inhalation experiments, using various particle sizes and bolus front depths, as well as from slow inhalation experiments, with a flow rate of only 0.045 L sec(-1). The overall good agreement between modeling results and experimental data indicate that the present model correctly predicts bronchial clearance, suggesting that slow bronchial clearance mechanisms are most effective in smaller bronchial airways.

Respiratory gas conditioning in infants with an artificial airway

There is a strong physiological rationale for delivering the inspiratory gas at or close to core body temperature and saturated with water vapour to infants with an artificial airway undergoing long-term mechanical ventilatory assistance. Cascade humidifiers with heated wire ventilatory circuitry may achieve this goal safely. Whenever saturated air leaves the humidifier chamber at 37 degrees C and condensate accumulates in the circuit, the gas loses humidity and acquires the potential to dry airway secretions near the tip of the endotracheal tube. Heat and moisture exchangers and hygroscopic condenser humidifiers with or without bacterial filters have become available for neonates. They can provide sufficient moisture output for short-term ventilation without excessive additional dead space or flow-resistive load for term infants. Their safety and efficacy for very low birthweight infants and for long-term mechanical ventilation has not been established conclusively. A broader application of these inexpensive and simple devices is likely to occur with further design improvements. When heated humidifiers are appropriately applied, water or normal saline aerosol application offers no additional significant advantage in terms of inspiratory gas conditioning and may impose a water overload on the airway or even systemically. Although airway irrigation by periodic bolus instillation of normal saline solution prior to suctioning procedures is widely practised in neonatology, virtually no data exist on its safety and efficacy when used with appropriately humidified inspired gas. There is no evidence that conditioning of inspired gas to core body temperature and full water vapour saturation may promote nosocomial respiratory infections.

A typical path model of tracheobronchial clearance of inhaled particles in rats

A mathematical description of particle clearance from the ciliated conducting airways (tracheobronchial region) of the lungs in rats was developed, assuming that particles on the mucus blanket behave as a fluid and adhere to principles of fluid flow described by the continuity equation. Effective particle transport velocities for given generations of airways were estimated from reported tracheal mucus velocities. Using typical rat airway geometry and estimated particle transport velocities, solutions of sets of rate equations for transport from each generation of airways were summed to estimate total particle clearance from the tracheobronchial region of the lung as a function of time. Aerosol particle size distribution (MMAD ranging from 0.1 to 4.2 microm, and sigma(g) from 1 to 2.7) and concentration data from several investigators were used to predict short-term, tracheobronchial clearance (retention) in rats up to 24 h following exposure. Comparisons between predicted and observed retention showed an average difference between model predictions, and observed fractional retention of initial lung or body burden was 4.9%, with a tendency toward underprediction of clearance of particles >3.0 microm.

Exogenous surfactant therapy and mucus rheology in chronic obstructive airway diseases

Exogenous surfactant is a specialized biomaterial used for substitution of the lipoprotein mixture normally present in the lungs-pulmonary surfactant. Respiratory Distress Syndrome is a disease of preterm infants mainly caused by pulmonary immaturity as evidenced by a deficiency of mature lung surfactant. Pulmonary surfactant is known to stabilize small alveoli and prevent them from collapsing during expiration. However, apart from alveoli, surfactant also lines the narrow conducting airways of the tracheobronchial tree. This paper reviews the role of this surfactant in the airways and its effect on mucus rheology and mucociliary clearance. Its potential role as a therapeutic biomaterial in chronic obstructive airway diseases, namely asthma, chronic bronchitis, and respiratory manifestations of cystic fibrosis, are discussed. This paper also attempts to elucidate the exact steps in the pathogenic pathway of these diseases which could be reversed by supplementation of exogenous surfactant formulations. It is shown that there is great potential for the use of present day surfactants (which are actually formulated for use in Respiratory Disease Syndrome) as therapy in the aforementioned diseases of altered mucus viscoelasticity and mucociliary clearance. However, for improved effectiveness, specific surfactant formulations satisfying certain specific criteria should be tailor-made for the clinical condition for which they are intended. The properties required to be fulfilled by the optimal exogenous surfactant in each of the above clinical conditions are enumerated in this paper.

Mucus quality on horse tracheal epithelium: microscopic grading based on transparency

The aim of this ex-vivo study on excised tracheas of healthy horses was to characterise the microscopic heterogeneity of mucus quality by a visual grading system based on transparency and to determine whether differences in mucus quality, assessed by a visual grading system, influence tracheal mucus velocity (TMV). Small pieces of each trachea were mounted into a humidified chamber under a microscope. Mucus quality was visually subdivided into four grades (MG) and ciliary beat frequency and TMV were determined. Mucus on excised horse tracheal epithelium does not form a homogenous layer. We observed flakes and streams of a great heterogeneity, which by the characteristic of transparency can be qualified and quantified. Visual characterisation of mucus was able to explain a considerable part of TMV variation. Therefore, it can be considered as a suitable non-invasive method for the evaluation of mucus quality and transport effectiveness.

Relationship between the humidity and temperature of inspired gas and the function of the airway mucosa

OBJECTIVE

To review the available literature on the relationship between the humidity and temperature of inspired gas and airway mucosal function.

DATA SOURCES

International computerized databases and published indices, experts in the field, conference proceedings, bibliographies.

STUDY SELECTION/DATA EXTRACTION

Two hundred articles/texts on respiratory tract physiology and humidification were reviewed. Seventeen articles were selected from 40 articles for inclusion in the published data verification of the model. Selection was by independent reviewers. Extraction was by consensus, and was based on finding sufficient data.

DATA SYNTHESIS

A relationship exists between inspired gas humidity and temperature, exposure time to a given humidity level, and mucosal function. This relationship can be modeled and represented as an inspired humidity magnitude vs. exposure time map. The model is predictive of mucosal function and can be partially verified by the available literature. It predicts that if inspired humidity deviates from an optimal level, a progressive mucosal dysfunction begins. The greater the humidity deviation, the faster the mucosal dysfunction progresses.

CONCLUSIONS

A model for the relationship between airway mucosal dysfunction and the combination of the humidity of inspired gas and the duration over which the airway mucosa is exposed to that humidity is proposed. This model suggests that there is an optimal temperature and humidity above which, and below which, there is impaired mucosal function. This optimal level of temperature and humidity is core temperature and 100% relative humidity. However, existing data are only sufficient to test this model for gas conditions below core temperature and 100% relative humidity. These data concur with the model in that region. No studies have yet looked at this relationship beyond 24 hrs. Longer exposure times to any given level of inspired humidity and inspired gas temperatures and humidities above core temperature and 100% relative humidity need to be studied to fully verify the proposed model.

Ciliary ultrastructure and beating activity in rat and guinea-pig respiratory mucosa

1. The rat and the guinea-pig are commonly used animals when the effects of drugs on ciliary activity in respiratory airways are studied. There are few data concerning the possible differences in ciliary function between these two animals. 2. Using a photodetector method we measured the ciliary beating frequency (CBF) from the upper part of the trachea, the lower part of the trachea and the distal part of the main bronchi (subsegmental bronchi) of rat and guinea-pig respiratory tract. In addition, the structure of the cilia was studied using scanning electron microscopy (SEM). 3. CBF in the rat respiratory tract was significantly lower than in the guinea-pigs. In the upper trachea, the CBF for rat was 12.7 beats/s and 15.3 beats/s for guinea-pig. The respective values were 9.2/16.0 beats/s in the lower part of the trachea and 6.9/13.8 beats/s in subsegmental bronchi. In both rats and guinea-pigs CBF was lower in the subsegmental bronchi than in the trachea (rat: 25.0-45.7%, guinea-pig: 9.8-13.8%). 4. In addition to higher CBF, the quality of the photo-electrical signal was better from guinea-pig tissues, probably as a result of the larger amounts of ciliated cells and longer cilia of guinea-pig respiratory epithelial compared to those in rat mucosa. 5. SEM showed that the rat cilia were on average shorter (3.6 vs 4.3 microns) and thinner (0.19 vs 0.22 microns) than those of the guinea-pig. Rat mucosa was markedly less ciliated than the respiratory mucosa of the guinea-pig.(ABSTRACT TRUNCATED AT 250 WORDS)

Modeling the concentration of ethanol in the exhaled breath following pretest breathing maneuvers

A previously developed mathematical model that describes the relationship between blood alcohol (ethanol) concentration and the concentration of alcohol in the exhaled breath at end-exhalation (BrAC) has been used to quantitate the effect of pretest breathing conditions on BrAC. The model was first used to "condition" the airways with different breathing maneuvers prior to simulating a single exhalation maneuver, the maneuver used in standard breath alcohol testing. On inspiration, the alcohol in the air reaches local equilibrium with the alcohol in the bronchial capillary bed prior to entering the alveolar region. On expiration, approximately 50% of the alcohol absorbed on inspiration is desorbed back to the airways. BrAC correlates with the amount of alcohol that is desorbed to the airways. The six pretest breathing conditions and the percent change in BrAC relative to the control maneuver were: hyperventilation (-4.4%), hypoventilation (3.7%), hot-humid air (-2.9%), hot-dry air (0.66%), cold-humid air (0.13%), and cold-dry air (0.53%). The mechanism underlying these responses is not due to changes in breath temperature, but, rather to changes in the axial profile of alcohol content in the mucous lining of the airways.

The role of airway mucus in pulmonary toxicology

Airway mucus is a complex airway secretion whose primary function as part of the mucociliary transport mechanism is to to serve as renewable and transportable barrier against inhaled particulates and toxic agents. The rheologic properties necessary for this function are imparted by glycoproteins, or mucins. Some respiratory disease states, e.g., asthma, cystic fibrosis, and bronchitis, are characterized by quantitative and qualitative changes in mucus biosynthesis that contribute to pulmonary pathology. Similar alterations in various aspects of mucin biochemistry and biophysics, leading to mucus hypersecretion and altered mucus rheology, result from inhalation of certain air pollutants, such as ozone, sulfur dioxide, nitrogen dioxide, and cigarette smoke. The consequences of these pollutant-induced alterations in mucus biology are discussed in the context of pulmonary pathophysiology and toxicology.

Ciliary beat frequency at six levels of the respiratory tract in cow, dog, guinea-pig, pig, rabbit and rat

1. The ciliary beat frequency (CBF) of six animal species from six regions of the respiratory tract were measured: inferior turbinate, nasopharynx, the upper part of trachea from first to second cartilage, the lower part of trachea, main bronchus and subsegmental bronchi. Cow, pig, dog, rabbit, guinea-pig and rat were studied. 2. There were no significant differences in the CBF values between cow, pig and dog, and the ciliary activity was essentially the same (11.3-16.9 Hz) in all parts of the respiratory tract. 3. In the rat, the CBF was slower, especially in subsegmental bronchi (6.8 Hz). 4. In general, CBF was higher in upper than lower airways, with the exception of guinea-pig. 5. Signal quality was the highest in guinea-pig tissue, whereas rat tissue produced the signal with the widest variation in CBF and the highest proportion of distorted waveforms. 6. Therefore, for studying drug effects on ciliary activity, guinea-pig seems to be a more suitable animal than the more commonly used rabbit or rat. In drug comparisons, the same part of the respiratory tract and the same animal species should be used.

In vitro transport of carbon in the trachea of veal calves

Carbon transport was studied in vitro in preparations of trachea and bronchus from veal calves. The mean velocity varied from 4.2 mm/min in the main bronchi to 6.3 mm/min in the ventral trachea. In some locations no transport was observed. Stereomicroscopic evaluation of Alcian blue-phloxine stained mucosal surfaces revealed differences in the appearance of the mucus layer between locations with and without mucus transport. It is concluded that quality and integrity of the mucus layer play an important role in mucus transport.

Cholinergic and adrenergic stimulation of mucociliary transport in the rat trachea

Mucociliary transport in the rat trachea was monitored through a surgical window for approximately one hour. Rates were grouped according to 5-min intervals which were examined with analyses of variance. Rates were consistent within each rat, although inter-animal variability was pronounced. Unstimulated transport involved minimal amounts of mucus and averaged 4.5 +/- 1.4 mm/min. Pilocarpine, presumably a secretagogue, caused a sustained (55 +/- 10 min) increase to 5.7 +/- 1.6 mm/min. Isoproterenol, presumably a cilioexcitatory agent, caused a brief (14 +/- 6 min) peak in transport at 6.4 +/- 2.1 mm/min. There were slow and fast animals which remained relatively slow or fast during and after treatment, indicating that each rat had an individual base-line determined by factors other than those susceptible to stimulation. Transport ceased almost at once throughout the airway, as mucus became scanty although ciliary activity was wide-spread. It was concluded that both types of response revealed different mechanisms for the physiological modulation of mucociliary transport.

Marked goblet cell hyperplasia with mucus accumulation in the airways of patients who died of severe acute asthma attack

To examine the changes in airways in bronchial asthma (BA) during an asthma attack causing death, we performed morphometric analysis of autopsied lungs from three outpatients who died of severe acute asthma attacks (group A) and compared these to five patients who died of non-status asthmaticus (group B). Controls (group NL) were four patients who died of diseases other than respiratory disorders. Area proportions of bronchial glands to bronchial wall (gland [percent]) and of goblet cells to total epithelial layer (goblet [percent]) and the intraluminal amount of mucus in the airways (MOR) were measured in a paraffin section. There were no significant differences in age, sex, smoking history, duration of BA history, and dosage of glucocorticoids received between groups A and B. Although both groups A and B showed significantly larger values of gland (percent) in the central airways and of inflammatory cell numbers in the airway walls than did group NL, no significant differences were observed between groups A and B. In contrast, markedly significant increases in goblet (percent) and in MOR were observed in group A compared to groups B and NL. These increases in group A were more dominant in the peripheral airway: 30-fold and threefold increases of group B in goblet (percent) and MOR, respectively. Furthermore, MOR significantly correlated with goblet (percent) in the peripheral airways (p less than 0.05). These findings suggest that a marked increase in goblet cells of the airways is a feature characteristic of patients with BA who die of a severe acute attack.

Release of mucus glycoconjugates by Pseudomonas aeruginosa rhamnolipid into feline trachea in vivo and human bronchus in vitro

Pseudomonas aeruginosa colonizes the lower respiratory tracts of patients with severe bronchiectasis, including cystic fibrosis, a condition associated with increased airway mucus output. We have shown that an extract containing chloroform-soluble extracellular products of P. aeruginosa releases glycoconjugates into the cat trachea in vivo. This activity was not related to pyocyanin, a major component of the extract, but was associated with the rhamnolipids. Purified monorhamnolipid (100 micrograms/ml) released radiolabeled and periodic acid-Schiff (PAS)-reactive glycoconjugates (delta 3H = +490 +/- 70%, delta 35S = +170 +/- 40%, delta PAS = +8.6 +/- 1.7 micrograms/min; n = 6, P less than 0.02 for each). Dirhamnolipid (200 micrograms/ml) was also effective (delta 3H = +640 +/- 70%, delta 35S = +130 +/- 20%, delta PAS = +9.3 +/- 1.5 micrograms/min; n = 6, P less than 0.02 for each). Monorhamnolipid (100 micrograms/ml) also released 35S-labeled and PAS-reactive glycoconjugates from human bronchial tissue in vitro (delta 35S = +189 +/- 47%, delta PAS = +26.3 +/- 8.5 micrograms/min; n = 7, P less than 0.001 versus control tissues in which no stimulus was given). The cat tracheal glycoconjugates released by the rhamnolipids differed from those released by pilocarpine 50 microM, in having a higher 3H:35S ratio (P less than 0.001). After gel chromatography on a Sepharose CL-4B column, the void volume fractions of the glycoconjugates also had different profiles in a cesium chloride density gradient. Those released by rhamnolipid banded at 1.62 g/ml, while those released by pilocarpine banded mainly at 1.50 g/ml, with some of the higher density material also present.(ABSTRACT TRUNCATED AT 250 WORDS)

Mucociliary clearance from central airways in patients with excessive sputum production with and without primary ciliary dyskinesia

Studies of mucociliary clearance were made on 17 patients with bronchiectasis and excessive sputum production. Tracheal mucus velocity was measured. Five patients with primary ciliary dysfunction and 12 who had no clinical evidence of this defect were studied. The mean (+/- SD) tracheal mucus velocity of the five patients with proved or presumptive primary ciliary dysfunction was 2.9 +/- 0.2 mm per minute and for the 12 patients without ciliary dysfunction, 7.9 +/- 1.4 mm per minute. (The previously determined rate for normals was 4.7 +/- 1.3 mm per minute.) In four of five patients with ciliary dysfunction, CBF was below 7.5 Hz while in the one patient of the 12 without ciliary dysfunction CBF was at the lower limit of normal values. Thus, mucociliary clearance responds to load, depends partly on CBF, and has a built-in control system and the means for compensating for ciliary dysfunction.

Mucosal inflammation in asthma

Over the past decade, it has become increasingly recognized that airways inflammation is one of the major components of asthma. Until recently, measurements of bronchial responsiveness and mediators of allergic reactions were the only methods of studying pathogenetic mechanisms in asthma. With improved diagnostic procedures such as fiberoptic bronchoscopy, it has become possible to investigate these mechanisms and the resulting inflammatory changes in situ. BAL has highlighted the presence of mast cells and eosinophils and has given proof of their mediator participation in airways inflammation and hyperresponsiveness. Endobronchial biopsies have so far yielded results that are similar to those obtained from postmortem studies, although it appears that there are varying degrees of inflammation in living asthmatics. Even in mild disease, the histopathologic features of bronchial asthma are consistent with chronic inflammation. Indirect evidence obtained from allergen challenge leading to increased bronchial hyperresponsiveness during LAR, and direct evidence of inflammatory cells and their mediators in the airway mucosa and lumen after allergen challenge argue for an active role of cells in bringing about inflammatory changes. At present, however, it is not possible to relate precisely the findings obtained by bronchoscopy to the clinical presentation and progression of asthma. Cell activation with production of potent mediators of inflammation may be more relevant to inflammation than the simple presence of these cells in the airways. Almost all the inflammatory cells present in the bronchial wall and lumen have been implicated in the pathogenesis of mucosal inflammation in asthma, but with our current state of knowledge, none can be singled out as the most important contributor. The mast cell was the first to be investigated in depth, and despite the accumulation of large amounts of data concerning its ultrastructure and function, it remains uncertain to what extent this cell is involved in inflammatory responses. Thus, while its main role appears to be that of initiator of allergen-induced responses, the eosinophil has attracted more attention as a proinflammatory cell rather than as an antiinflammatory cell with a capacity to be selectively recruited from the circulation in response to IgE-dependent signals. The eosinophil secretes potent mediators that cause damage to the bronchial epithelium and lead to bronchoconstriction. The role of other cells is at present not as well defined.(ABSTRACT TRUNCATED AT 400 WORDS)

Inhibition and recovery of mammalian respiratory ciliary function after formaldehyde exposure

Formaldehyde (HCHO) has been reported to impair mucociliary clearance. The present investigation using rabbit and porcine tracheal explants in vitro examined (1) the impairment of ciliary activity, an essential component of mucociliary clearance; (2) the reversibility of ciliary dysfunction after HCHO exposure; and (3) the mechanism by which ciliary activity is reduced by HCHO. HCHO treatment of rabbit tracheal rings significantly decreased zones of active ciliated epithelium in direct proportion to concentration and exposure duration. There was also a significant concentration-dependent reduction of ciliary beat frequency. Removal of HCHO permitted recovery of zones of ciliary activity to normal beat frequencies; greater inhibitory concentrations of HCHO required greater time for return of function. Treatment of porcine tracheae with increasing concentrations of HCHO for time periods inhibitory to rabbit ciliary activity correspondingly reduced the yield of cilia extractable from treated epithelium. Furthermore, the specific activity of ATPase of extracted ciliary axonemes was diminished with increasing HCHO concentration, indicating loss of function. A recovery period following identical exposures of the porcine tracheae to the lower HCHO concentrations resulted in normal yields of functionally intact ciliary axonemes. Similarly, a recovery period after the highest HCHO concentration produced more functional axonemes than obtained from exposed tracheae without a recovery period, although less than normal yields. Therefore, ciliary dysfunction elicited by a defined range of HCHO concentrations is reversible. The yield and functional integrity of ciliary axonemes from epithelium exposed to HCHO with a recovery period are significantly greater than those without such a recovery period, suggesting an alteration and subsequent repair of epithelial surface components following HCHO exposure.

Series distribution of mucociliary clearance of magnetized iron particles in anesthetized dogs

Clearance of magnetic iron particles placed in the trachea, peripheral airway and alveoli was studied in anesthetized dogs. A retrograde catheter was inserted into a peripheral airway 2 mm in diameter. A thin vinyl tube was inserted into the peripheral airway from the chest wall through the retrograde catheter and 0.05 ml of saline containing 30 mg of iron oxide (Fe3O4) was instilled into either the peripheral airway or the trachea. Further, 30 mg of Fe3O4 solution was instilled with an injection needle into the alveoli. After magnetization from outside the chest wall, remanent magnetic fields (RMF) generated in the trachea, peripheral airway and alveoli were sequentially measured with a flux-gate magnetometer. The decay of RMF immediately after sequential magnetization shows clearance of Fe3O4 particles. Relaxation of RMF just before sequential magnetization is related to random misalignment of the particles. The fastest clearance was observed in the trachea, followed by the peripheral airway and the slowest was that in the alveoli. Exposure to isoproterenol resulted in increased clearance in the trachea and peripheral airway. However, relaxation was of similar magnitude and was not altered by isoproterenol throughout the entire airway. These results suggest that mechanisms responsible for clearance and relaxation in vivo are different and that clearance accelerates toward the central airway from alveoli and relaxation may be due to properties of the airway surface lining layer.

Orientation of respiratory tract cilia in patients with primary ciliary dyskinesia, bronchiectasis, and in normal subjects

A reproducible technique, utilising a graphics tablet and a personal computer for measuring ciliary orientation from electron micrographs of ciliated epithelium, was assessed. Ciliary deviation was measured in 47 normal subjects (mean ciliary deviation +/- 1 SD was 14.6 (3.3 degrees)), in eight patients with bronchiectasis and normal ciliary function (15.1 (6.5 degrees], and in seven patients with primary ciliary dyskinesia (38.7 (7.8 degrees); significantly greater than the first two groups). Measurements of ciliary deviation at the tip, base, and basal feet showed very little variation along the ciliary shaft in all three groups, suggesting that valid measurements of ciliary deviation can be made at any level of the cilium. Mean ciliary deviation in normal subjects was always less than 30 degrees; all patients with a mean ciliary deviation of greater than 30 degrees had recurrent respiratory tract disease. Four of seven patients with primary ciliary dyskinesia had ciliary disorientation; in one this was the only defect. Measurements of inter- and intraobserver variability using this method showed a maximum difference between observations of 4.1 degrees. It is suggested that ciliary orientation should be measured in patients suspected of having defective ciliary function or structure, or both.

Mucociliary transport and epithelial morphology with elongation and collapse in rat trachea

Mucociliary clearance in the trachea depends on a close relationship between structure and function of the components of its mucociliary system. Current explanations of normal mucociliary function denote narrow tolerances to change in factors such as the depth of secretions lining the lumen and the distribution of cilia. This study centered on changes in airway length under physiological conditions in the rat. Following extension of the head, the length of the trachea increased 50% without change in diameter. The rate of mucociliary clearance did not change with head position, averaging 3.79 +/- 1.03 mm/min in the flexed and 3.81 +/- 1.10 mm/min in the extended position. Extension of the head caused surface epithelial cells to elongate longitudinally and to decrease in height. These changes were greater in ciliated cells than in mucus-containing cells, thus accentuating the degree of protrusion of the secretory cells into the lumen of the stretched trachea. It is proposed that, during head extension, the relative protrusion of secretory cells into the lumen added bulk to the volume otherwise occupied by luminal secretions, and that these consistent changes in cell geometry served to adjust the height of ciliary tips within the periciliary fluid, thus mitigating fluctuations in the depth of the fluid. It was concluded that the mucociliary system of tracheas subjected to substantial change in configuration supported clearance because spatial relationships of essence to mucociliary function were maintained.

The propulsion of mucus by cilia

The presence of cilia on epithelia of the respiratory tract was reported more than 150 yr ago, and the two-layer model of mucus transport was put forward more than 50 yr ago. However, it is only in the last 10 yr or so that the motion of mucus-propelling cilia of the mammalian respiratory system has been adequately described, and fluid dynamic studies have developed far enough to allow descriptions of the mechanisms by which ciliary movement is coupled to mucus transport. In this review, scientific developments on the study of cilia and mucus, and interactions between them, are drawn together to further understanding of mucociliary clearance mechanisms of the respiratory tract. The study of the cilia incorporates a discussion of the internal mechanics and biochemistry of the ciliary axoneme, the physical principles of the beat pattern, and the (weak) metachronal coordination of cilia in the lung. Mucus rheology plays a central role in mucociliary transport with the rheologic properties of the mucus determining the effective functioning of this clearance mechanism. Theoretical models provide information on the mechanical principles of the beat pattern as well as providing reliable estimates of the transport rates. Although airflow is not thought to contribute to mucus transport in the normal state, high frequency ventilation and coughing may make significant contributions.

Effects of acute and chronic dimethylamine exposure on the nasal mucociliary apparatus of F-344 rats

Dimethylamine (DMA) is a highly water soluble gas with many industrial applications. Male F-344 rats were exposed to 175 ppm DMA 6 hr per day for 1, 2, 4, or 9 days or 2 years. Gross changes in nasal structure were recorded, effects of DMA on the mucociliary apparatus were assessed using video analysis, and tissues were evaluated for histopathology. In vitro nasal mucociliary flow patterns, mucus flow rates, and ciliary activity were studied and recorded for video motion analysis. There were distinct and generally consistent differences in the shape of the naso-, maxillo-, and ethmoid turbinates between young and old animals. Acute and chronic DMA exposures resulted in erosion of the anterior margins of the naso- and maxilloturbinates and fenestration of the adjacent septum. Ciliastasis and mucostasis were observed only on the anteromedial aspect of the maxilloturbinate. In the chronically exposed rats, mucociliary activity was present in areas adjacent to erosions of the turbinates and septum. Abnormal mucus flow patterns, including altered or reversed direction of flow and "whirlpool-like" formation, were observed in all treated rats, but were more severe following chronic exposure. There was a good correlation between the distribution of responses as assessed by histopathology and abnormal mucociliary function at all time points. In conclusion, the mucociliary apparatus continues to function in the nasal passages of rats having localized destruction of nasal epithelium, induced by DMA exposure, and this clearance system responds to alterations of nasal structure by modification of mucus flow patterns.