Liquid plug flow in straight and bifurcating tubes

Practical dissolution dynamic nuclear polarization

This review article intends to provide insightful advice for dissolution-dynamic nuclear polarization in the form of a practical handbook. The goal is to aid research groups to effectively perform such experiments in their own laboratories. Previous review articles on this subject have covered a large number of useful topics including instrumentation, experimentation, theory, etc. The topics to be addressed here will include tips for sample preparation and for checking sample health; a checklist to correctly diagnose system faults and perform general maintenance; the necessary mechanical requirements regarding sample dissolution; and aids for accurate, fast and reliable polarization quantification. Herein, the challenges and limitations of each stage of a typical dissolution-dynamic nuclear polarization experiment are presented, with the focus being on how to quickly and simply overcome some of the limitations often encountered in the laboratory.

Magnetohydrodynamic Moving Liquid Plug Within a Microchannel: Analytical Solutions

The wide applications of plug flows in microscale in science and engineering help them attract a great deal recent interest. An analytical study is undertaken here to study the impacts of a transversely applied external uniform magnetic field affecting the motion of liquid in the plug in terms of hydrodynamic mixing properties. The well-known symmetric vortex structure occurring in a long plug with moderate aspect ratio is observed to be preserved, while the recirculation phenomenon is highly affected by the action of the magnetic field. The decelerating feature of Lorentz force on the liquid motion is illuminated by reducing the strength of the recirculating vortex moving towards the upper and lower walls. The effects of magnetic field on the flow resistance of the liquid plug as well as on the plug circulation rate and on the axial flux are also clarified. The liquid plug considered here is shown to be fully consistent with the continuous liquid flow in a channel whose exact solution is further extracted.

Surfactant delivery in rat lungs: Comparing 3D geometrical simulation model with experimental instillation

Surfactant Replacement Therapy (SRT), which involves instillation of a liquid-surfactant mixture directly into the lung airway tree, is a major therapeutic treatment in neonatal patients with respiratory distress syndrome (RDS). This procedure has proved to be remarkably effective in premature newborns, inducing a five-fold decrease of mortality in the past 35 years. Disappointingly, its use in adults for treating acute respiratory distress syndrome (ARDS) experienced initial success followed by failures. Our recently developed numerical model has demonstrated that transition from success to failure of SRT in adults could, in fact, have a fluid mechanical origin that is potentially reversible. Here, we present the first numerical simulations of surfactant delivery into a realistic asymmetric conducting airway tree of the rat lung and compare them with experimental results. The roles of dose volume (VD), flow rate, and multiple aliquot delivery are investigated. We find that our simulations of surfactant delivery in rat lungs are in good agreement with our experimental data. In particular, we show that the monopodial architecture of the rat airway tree plays a major role in surfactant delivery, contributing to the poor homogeneity of the end distribution of surfactant. In addition, we observe that increasing VD increases the amount of surfactant delivered to the acini after losing a portion to coating the involved airways, the coating cost volume, VCC. Finally, we quantitatively assess the improvement resulting from a multiple aliquot delivery, a method sometimes employed clinically, and find that a much larger fraction of surfactant reaches the alveolar regions in this case. This is the first direct qualitative and quantitative comparison of our numerical model with experimental studies, which enhances our previous predictions in adults and neonates while providing a tool for predicting, engineering, and optimizing patient-specific surfactant delivery in complex situations.

A Three-Dimensional Model of Human Lung Airway Tree to Study Therapeutics Delivery in the Lungs

Surfactant instillation into the lungs is used to treat several respiratory disorders such as neonatal respiratory distress syndrome (NRDS). The success of the treatments significantly depends on the uniformity of distribution of the instilled surfactant in airways. This is challenging to directly evaluate due to the inaccessibility of lung airways and great difficulty with imaging them. To tackle this problem, we developed a 3D physical model of human lung airway tree. Using a defined set of principles, we first generated computational models of eight generations of neonates' tracheobronchial tree comprising the conducting zone airways. Similar to native lungs, these models contained continuously-branching airways that rotated in the 3D space and reduced in size with increase in the generation number. Then, we used additive manufacturing to generate physical airway tree models that precisely replicated the computational designs. We demonstrated the utility of the physical models to study surfactant delivery in the lungs and showed the effect of orientation of the airway tree in the gravitational field on the distribution of instilled surfactant between the left and right lungs and within each lung. Our 3D lung airway tree model offers a novel tool for quantitative studies of therapeutics delivery.

Engineered Airway Models to Study Liquid Plug Splitting at Bifurcations: Effects of Orientation and Airway Size

Delivery of biological fluids, such as surfactant solutions, into lungs is a major strategy to treat respiratory disorders including respiratory distress syndrome that is caused by insufficient or dysfunctional natural lung surfactant. The instilled solution forms liquid plugs in lung airways. The plugs propagate downstream in airways by inspired air or ventilation, continuously split at airway bifurcations to smaller daughter plugs, simultaneously lose mass from their trailing menisci, and eventually rupture. A uniform distribution of the instilled biofluid in lung airways is expected to increase the treatments success. The uniformity of distribution of instilled liquid in the lungs greatly depends on the splitting of liquid plugs between daughter airways, especially in the first few generations from which airways of different lobes of lungs emerge. To mechanistically understand this process, we developed a bioengineering approach to computationally design three-dimensional bifurcating airway models using morphometric data of human lungs, fabricate physical models, and examine dynamics of liquid plug splitting. We found that orientation of bifurcating airways has a major effect on the splitting of liquid plugs between daughter airways. Changing the relative gravitational orientation of daughter tubes with respect to the horizontal plane caused a more asymmetric splitting of liquid plugs. Increasing the propagation speed of plugs partially counteracted this effect. Using airway models of smaller dimensions reduced the asymmetry of plug splitting. This work provides a step toward developing delivery strategies for uniform distribution of therapeutic fluids in the lungs.

Pulmonary fluid flow challenges for experimental and mathematical modeling

Modeling the flow of fluid in the lungs, even under baseline healthy conditions, presents many challenges. The complex rheology of the fluids, interaction between fluids and structures, and complicated multi-scale geometry all add to the complexity of the problem. We provide a brief overview of approaches used to model three aspects of pulmonary fluid and flow: the surfactant layer in the deep branches of the lung, the mucus layer in the upper airway branches, and closure/reopening of the airway. We discuss models of each aspect, the potential to capture biological and therapeutic information, and open questions worthy of further investigation. We hope to promote multi-disciplinary collaboration by providing insights into mathematical descriptions of fluid-mechanics in the lung and the kinds of predictions these models can make.

Computational analysis of microbubble flows in bifurcating airways: role of gravity, inertia, and surface tension

Although mechanical ventilation is a life-saving therapy for patients with severe lung disorders, the microbubble flows generated during ventilation generate hydrodynamic stresses, including pressure and shear stress gradients, which damage the pulmonary epithelium. In this study, we used computational fluid dynamics to investigate how gravity, inertia, and surface tension influence both microbubble flow patterns in bifurcating airways and the magnitude/distribution of hydrodynamic stresses on the airway wall. Direct interface tracking and finite element techniques were used to simulate bubble propagation in a two-dimensional (2D) liquid-filled bifurcating airway. Computational solutions of the full incompressible Navier-Stokes equation were used to investigate how inertia, gravity, and surface tension forces as characterized by the Reynolds (Re), Bond (Bo), and Capillary (Ca) numbers influence pressure and shear stress gradients at the airway wall. Gravity had a significant impact on flow patterns and hydrodynamic stress magnitudes where Bo > 1 led to dramatic changes in bubble shape and increased pressure and shear stress gradients in the upper daughter airway. Interestingly, increased pressure gradients near the bifurcation point (i.e., carina) were only elevated during asymmetric bubble splitting. Although changes in pressure gradient magnitudes were generally more sensitive to Ca, under large Re conditions, both Re and Ca significantly altered the pressure gradient magnitude. We conclude that inertia, gravity, and surface tension can all have a significant impact on microbubble flow patterns and hydrodynamic stresses in bifurcating airways.

Airway reopening through catastrophic events in a hierarchical network

When you reach with your straw for the final drops of a milkshake, the liquid forms a train of plugs that flow slowly initially because of the high viscosity. They then suddenly rupture and are replaced with a rapid airflow with the characteristic slurping sound. Trains of liquid plugs also are observed in complex geometries, such as porous media during petroleum extraction, in microfluidic two-phase flows, or in flows in the pulmonary airway tree under pathological conditions. The dynamics of rupture events in these geometries play the dominant role in the spatial distribution of the flow and in determining how much of the medium remains occluded. Here we show that the flow of a train of plugs in a straight channel is always unstable to breaking through a cascade of ruptures. Collective effects considerably modify the rupture dynamics of plug trains: Interactions among nearest neighbors take place through the wetting films and slow down the cascade, whereas global interactions, through the total resistance to flow of the train, accelerate the dynamics after each plug rupture. In a branching tree of microchannels, similar cascades occur along paths that connect the input to a particular output. This divides the initial tree into several independent subnetworks, which then evolve independently of one another. The spatiotemporal distribution of the cascades is random, owing to strong sensitivity to the plug divisions at the bifurcations.

Respiratory physiology on a chip

Our current understanding of respiratory physiology and pathophysiological mechanisms of lung diseases is often limited by challenges in developing in vitro models faithful to the respiratory environment, both in cellular structure and physiological function. The recent establishment and adaptation of microfluidic-based in vitro devices (μFIVDs) of lung airways have enabled a wide range of developments in modern respiratory physiology. In this paper, we address recent efforts over the past decade aimed at advancing in vitro models of lung structure and airways using microfluidic technology and discuss their applications. We specifically focus on μFIVDs covering four major areas of respiratory physiology, namely, artificial lungs (AL), the air-liquid interface (ALI), liquid plugs and cellular injury, and the alveolar-capillary barrier (ACB).

Epithelium damage and protection during reopening of occluded airways in a physiologic microfluidic pulmonary airway model

Airways of the peripheral lung are prone to closure at low lung volumes. Deficiency or dysfunction of pulmonary surfactant during various lung diseases compounds this event by destabilizing the liquid lining of small airways and giving rise to occluding liquid plugs in airways. Propagation of liquid plugs in airways during inflation of the lung exerts large mechanical forces on airway cells. We describe a microfluidic model of small airways of the lung that mimics airway architecture, recreates physiologic levels of pulmonary pressures, and allows studying cellular response to repeated liquid plug propagation events. Substantial cellular injury happens due to the propagation of liquid plugs devoid of surfactant. We show that addition of a physiologic concentration of a clinical surfactant, Survanta, to propagating liquid plugs protects the epithelium and significantly reduces cell death. Although the protective role of surfactants has been demonstrated in models of a propagating air finger in liquid-filled airways, this is the first time to study the protective role of surfactants in liquid plugs where fluid mechanical stresses are expected to be higher than in air fingers. Our parallel computational simulations revealed a significant decrease in mechanical forces in the presence of surfactant, confirming the experimental observations. The results support the practice of providing exogenous surfactant to patients in certain clinical settings as a protective mechanism against pathologic flows. More importantly, this platform provides a useful model to investigate various surface tension-mediated lung diseases at the cellular level.

Local interactions and the global organization of a two-phase flow in a branching tree

The transport of liquid plugs in a microfluidic branching tree is studied experimentally. The global flow pattern can be either symmetric or asymmetric, with daughter plugs dividing in synchrony or asynchrony as a function of the driving flow rate and the network geometry. For trees with narrowing channels, the plugs always reach the exits even at low flow rates. In contrast, only one path is opened in networks with widening channels when the flow rate is low. This behavior is explained by a comparison of the pressure drop necessary to drive viscocapillary motion of plugs in straight channels with the nonlinear pressure variations as a plug passes a bifurcation. A model is built, which predicts that only narrowing networks can be fully filled, while widening networks can never be fully invaded by a two-phase flow.

Propagation and breakup of liquid menisci and aerosol generation in small airways

BACKGROUND

Droplets exhaled during normal breathing and not associated with coughing may pose hazardous agents to infective diseases dissemination. The objective is to explore the physical mechanism, which may lead to droplets formation.

METHODS

We hypothesize that liquid menisci occlusions, which may form inside small airways, travel along the airway, may lose mass and finally disintegrate into small droplets. This hypothesis was numerically investigated applying physiologically plausible values of the phenomenological coefficients and geometrical conformations.

RESULTS

We show that three important dimensionless parameters control the motion and disintegration of menisci: the dimensionless mucus layer thickness, the dimensionless menisci initial thickness (all scaled by the airway radius), and the capillary number. Menisci traveling within airways may either remain at equilibrium or diminish or increase in size. Menisci that diminish in size may collapse into the mucus layer; form a large droplet that contains most of the menisci mass before disintegration; or form a larger number of small droplets (we show the forming of three or four droplets in a single occluded airway).

CONCLUSIONS

A critical capillary number for menisci at equilibrium could be defined. It was shown that menisci tend to diminish in size as the capillary number increases above the critical value, and a number of small droplets may be formed during normal breathing.

μ-PIV measurements of the ensemble flow fields surrounding a migrating semi-infinite bubble

Microscale particle image velocimetry (μ-PIV) measurements of ensemble flow fields surrounding a steadily-migrating semi-infinite bubble through the novel adaptation of a computer controlled linear motor flow control system. The system was programmed to generate a square wave velocity input in order to produce accurate constant bubble propagation repeatedly and effectively through a fused glass capillary tube. We present a novel technique for re-positioning of the coordinate axis to the bubble tip frame of reference in each instantaneous field through the analysis of the sudden change of standard deviation of centerline velocity profiles across the bubble interface. Ensemble averages were then computed in this bubble tip frame of reference. Combined fluid systems of water/air, glycerol/air, and glycerol/Si-oil were used to investigate flows comparable to computational simulations described in Smith and Gaver (2008) and to past experimental observations of interfacial shape. Fluorescent particle images were also analyzed to measure the residual film thickness trailing behind the bubble. The flow fields and film thickness agree very well with the computational simulations as well as existing experimental and analytical results. Particle accumulation and migration associated with the flow patterns near the bubble tip after long experimental durations are discussed as potential sources of error in the experimental method.

Unsteady propagation of a liquid plug in a liquid-lined straight tube

This paper considers the propagation of a liquid plug driven by a constant pressure within a rigid axisymmetric tube whose inner surface is coated by a thin liquid film. The Navier-Stokes equations are solved using the finite-volume method and the SIMPLEST algorithm. The effects of precursor film thickness, initial plug length, pressure drop across the plug, and constant surface tension on the plug behavior and tube wall mechanical stresses are investigated. As a plug propagates through a liquid-lined tube, the plug gains liquid from the leading front film, and it deposits liquid into the trailing film. If the trailing film is thicker (thinner) than the precursor film, the plug volume decreases (increases) as it propagates. For a decreasing volume, eventually the plug ruptures. Under a specific set of conditions, the trailing film thickness equals the precursor film thickness, which leads to steady state results. The plug speed decreases as the precursor film thins because the resistance to the moving front meniscus increases. As the pressure drop across the plug decreases, the plug speed decreases resulting in thinning of the trailing film. As the plug length becomes longer, the viscous resistance in the plug core region increases, which slows the plug and causes the trailing film to become even thinner. The magnitude of the pressure and shear stress at the tube inner wall is maximum in the front meniscus region, and it increases with a thinner precursor film. As the surface tension increases, the plug propagation speed decreases, the strength of the wall pressure in the front meniscus region increases, and the pressure gradient around the peak pressure becomes steeper.

Liquid and surfactant delivery into pulmonary airways

We describe the mechanisms by which liquids and surfactants can be delivered into the pulmonary airways. These are instilled and transported throughout the lung in clinical therapies such as surfactant replacement therapy, partial liquid ventilation and drug delivery. The success of these treatments is contingent on the liquid distribution and the delivery to targeted regions of the lung. The targeting of a liquid plug can be influenced by a variety of factors such as the physical properties of the liquid, the interfacial activity, the gravitational orientation, instillation method and propagation speed. We provide a review of experimental and theoretical studies that examine these effects in single tubes or channels, in tubes with single bifurcations and in the whole lung.

Transport of wetting liquid plugs in bifurcating microfluidic channels

A plug of wetting liquid is driven at constant pressure through a bifurcation in a microchannel. For a plug advancing in a straight channel, we find that the viscous dissipation in the bulk may be estimated using Poiseuille's law while Bretherton and Tanner's laws model the additional dissipation occurring at the rear and front interfaces. At a second stage, we focus on the behavior of the plug flowing through a T-junction. Experiments show the existence of a threshold pressure, below which the plug remains blocked at the entrance of the junction. Above this required pressure, the plug enters the bifurcation and either ruptures or splits into two daughter plugs, depending on the applied pressure and on the initial length of the plug. By means of geometrical arguments and the previously cited laws, we propose a global model to predict the transitions between the three observed behaviors.

Effects of Inertia and Gravity on Liquid Plug Splitting at a Bifurcation

Liquid plugs may form in pulmonary airways during the process of liquid instillation or removal in many clinical treatments. During inspiration the plug may split at airway bifurcations and lead to a nonuniform final liquid distribution, which can adversely affect treatment outcomes. In this paper, a combination of bench top experimental and theoretical studies is presented to study the effects of inertia and gravity on plug splitting in an airway bifurcation model to simulate the liquid distributions in large airways. The splitting ratio, Rs, is defined as the ratio of the plug volume entering the upper (gravitationally opposed) daughter tube to the lower (gravitationally favored) one. Rs is measured as a function of parent tube Reynolds number, Rep; gravitational orientations for roll angle, ϕ, and pitch angle, γ; parent plug length LP; and the presence of pre-existing plug blockages in downstream daughter tubes. Results show that increasing Rep causes more homogeneous splitting. A critical Reynolds number Rec is found to exist so that when Rep⩽Rec, Rs=0, i.e., no liquid enters the upper daughter tube. Rec increases while Rs decreases with increasing the gravitational effect, i.e., increasing ϕ and γ. When a blockage exists in the lower daughter, Rec is only found at ϕ=60deg in the range of Rep studied, and the resulting total mass ratio can be as high as 6, which also asymptotes to a finite value for different ϕ as Rep increases. Inertia is further demonstrated to cause more homogeneous plug splitting from a comparison study of Rs versus Cap (another characteristic speed) for three liquids: water, glycerin, and LB-400X. A theoretical model based on entrance flow for the plug in the daughters is developed and predicts Rs versus Rep. The frictional pressure drop, as a part of the total pressure drop, is estimated by two fitting parameters and shows a linear relationship with Rep. The theory provides a good prediction on liquid plug splitting and well simulates the liquid distributions in the large airways of human lungs.

Biofluid Mechanics of the Pulmonary System

Presents an overview of leading areas of discovery in bio-fluid mechanics related to the pulmonary system, with particular reference to the airways. Areas briefly reviewed include airway gas dynamics, impedance studies, collapsible-tube studies, and airway liquid studies. Emphasis is placed on promising further directions, such as analysis of interacting fluid-mechanical or fluid-structure phenomena, multi-scale modeling across widely varying length and time scales, and integration of advanced simulations into respiratory investigation and pulmonary medicine.

Effect of gravity on liquid plug transport through an airway bifurcation model

Many medical therapies require liquid plugs to be instilled into and delivered throughout the pulmonary airways. Improving these treatments requires a better understanding of how liquid distributes throughout these airways. In this study, gravitational and surface mechanisms determining the distribution of instilled liquids are examined experimentally using a bench-top model of a symmetrically bifurcating airway. A liquid plug was instilled into the parent tube and driven through the bifurcation by a syringe pump. The effect of gravity was adjusted by changing the roll angle (phi) and pitch angle (gamma) of the bifurcation (phi = gamma =0 deg was isogravitational). Phi determines the relative gravitational orientation of the two daughter tubes: when phi not equal to 0 deg, one daughter tube was lower (gravitationally favored) compared to the other. Gamma determines the component of gravity acting along the axial direction of the parent tube: when gamma not equal to 0 deg, a nonzero component of gravity acts along the axial direction of the parent tube. A splitting ratio Rs, is defined as the ratio of the liquid volume in the upper daughter to the lower just after plug splitting. We measured the splitting ratio, Rs, as a function of: the parent-tube capillary number (Cap); the Bond number (Bo); phi; gamma; and the presence of pre-existing plugs initially blocking either daughter tube. A critical capillary number (Cac) was found to exist below which no liquid entered the upper daughter (Rs = 0), and above which Rs increased and leveled off with Cap. Cac increased while Rs decreased with increasing phi, gamma, and Bo for blocked and unblocked cases at a given Cap > Ca,. Compared to the nonblockage cases, Rs decreased (increased) at a given Cap while Cac increased (decreased) with an upper (lower) liquid blockage. More liquid entered the unblocked daughter with a blockage in one daughter tube, and this effect was larger with larger gravity effect. A simple theoretical model that predicts Rs and Cac is in qualitative agreement with the experiments over a wide range of parameters.

Bubble splitting in bifurcating tubes: a model study of cardiovascular gas emboli transport

The transport of long gas bubbles, suspended in liquid, through symmetric bifurcations, is investigated experimentally and theoretically as a model of cardiovascular gas bubble transport in air embolism and gas embolotherapy. The relevant dimensionless parameters in the models match the corresponding values for arteries and arterioles. The effects of roll angle (the angle the plane of the bifurcation makes with the horizontal), capillary number (a dimensionless indicator of flow), and bubble volume (or length) on the splitting of bubbles as they pass through the bifurcation are examined. Splitting is observed to be more homogenous at higher capillary numbers and lower roll angles. It is shown that, at nonzero roll angles, there is a critical value of the capillary number below which the bubbles do not split and are transported entirely into the upper branch. The value of the critical capillary number increases with roll angle and parent tube diameter. A unique bubble motion is observed at the critical capillary number and for slightly slower flows: the bubble begins to split, the meniscus in the lower branch then moves backward, and finally the entire bubble enters the upper branch. These findings suggest that, in large vessels, emboli tend to be transported upward unless flow is unusually strong but that a more homogeneous distribution of emboli occurs in smaller vessels. This corresponds to previous observations that air emboli tend to lodge in the upper regions of the lungs and suggests that relatively uniform infarction of tumors by gas embolotherapy may be possible.

Cellular stress failure in ventilator-injured lungs

The clinical and experimental literature has unequivocally established that mechanical ventilation with large tidal volumes is injurious to the lung. However, uncertainty about the micromechanics of injured lungs and the numerous degrees of freedom in ventilator settings leave many unanswered questions about the biophysical determinants of lung injury. In this review we focus on experimental evidence for lung cells as injury targets and the relevance of these studies for human ventilator-associated lung injury. In vitro, the stress-induced mechanical interactions between matrix and adherent cells are important for cellular remodeling as a means for preventing compromise of cell structure and ultimately cell injury or death. In vivo, these same principles apply. Large tidal volume mechanical ventilation results in physical breaks in alveolar epithelial and endothelial plasma membrane integrity and subsequent triggering of proinflammatory signaling cascades resulting in the cytokine milieu and pathologic and physiologic findings of ventilator-associated lung injury. Importantly, though, alveolar cells possess cellular repair and remodeling mechanisms that in addition to protecting the stressed cell provide potential molecular targets for the prevention and treatment of ventilator-associated lung injury in the future.

Multi-branching flows from one mother tube to many daughters or to a network

Multiply branching fluid flows are modelled in two contexts. The first (type I) is for one-to-many branching. Computations are described for flow through a channel, with fully developed motion upstream, which branches abruptly into a number of subchannels downstream. The differences in pressure between the upstream end of the channel and the downstream ends of the subchannels are substantial. Comparisons with recent analytical predictions show fair agreement for Reynolds numbers in the low tens and above. The second context (type II) has successive generations of bifurcation in a network. Modelling, computations and analysis include the effects of many bifurcations.

Steady propagation of a liquid plug in a two-dimensional channel

In this study, we investigate the steady propagation of a liquid plug within a two-dimensional channel lined by a uniform, thin liquid film. The Navier-Stokes equations with free-surface boundary conditions are solved using the finite volume numerical scheme. We examine the effect of varying plug propagation speed and plug length in both the Stokes flow limit and for finite Reynolds number (Re). For a fixed plug length, the trailing film thickness increases with plug propagation speed. If the plug length is greater than the channel width, the trailing film thickness agrees with previous theories for semi-infinite bubble propagation. As the plug length decreases below the channel width, the trailing film thickness decreases, and for finite Re there is significant interaction between the leading and trailing menisci and their local flow effects. A recirculation flow forms inside the plug core and is skewed towards the rear meniscus as Re increases. The recirculation velocity between both tips decreases with the plug length. The macroscopic pressure gradient, which is the pressure drop between the leading and trailing gas phases divided by the plug length, is a function of U and U2, where U is the plug propagation speed, when the fluid property and the channel geometry are fixed. The U2 term becomes dominant at small values of the plug length. A capillary wave develops at the front meniscus, with an amplitude that increases with Re, and this causes large local changes in wall shear stresses and pressures.

The Effect of Airway Wall Motion on Surfactant Delivery

Soluble surfactant and airway surface liquid transport are examined using a mathematical model of Marangoni flows which accounts for airway branching and for cyclic airway stretching. Both radial and longitudinal wall strains are considered. The model allows for variation of the amplitude and frequency of the motion, as may occur under a variety of ventilatory situations occurring during surfactant replacement therapy. The soluble surfactant dynamics of the thin fluid film are modeled by linear sorption. The delivery of surfactants into the lung is handled by setting the proximal boundary condition to a higher concentration compared to the distal boundary condition. Starting with a steady-state, nonuniform, surfactant distribution, we find that transport of surfactant into the lung is enhanced for increasing strain amplitudes. However, for fixed amplitude, increasing frequency has a smaller effect. At small strain amplitudes, increasing frequency enhances transport, but at large strain amplitudes, increasing cycling frequency has the opposite effect.

Effect of ventilation rate on instilled surfactant distribution in the pulmonary airways of rats

Liquid can be instilled into the pulmonary airways during medical procedures such as surfactant replacement therapy, partial liquid ventilation, and pulmonary drug delivery. For all cases, understanding the dynamics of liquid distribution in the lung will increase the efficacy of treatment. A recently developed imaging technique for the study of real-time liquid transport dynamics in the pulmonary airways was used to investigate the effect of respiratory rate on the distribution of an instilled liquid, surfactant, in a rat lung. Twelve excised rat lungs were suspended vertically, and a single bolus (0.05 ml) of exogenous surfactant (Survanta, Ross Laboratories, Columbus, OH) mixed with radiopaque tracer was instilled as a plug into the trachea. The lungs were ventilated with a 4-ml tidal volume for 20 breaths at one of two respiratory rates: 20 or 60 breaths/min. The motion of radiodense surfactant was imaged at 30 frames/s with a microfocal X-ray source and an image intensifier. Dynamics of surfactant distribution were quantified for each image by use of distribution statistics and a homogeneity index. We found that the liquid distribution depended on the time to liquid plug rupture, which depends on ventilation rate. At 20 breaths/min, liquid was localized in the gravity-dependent region of the lung. At 60 breaths/min, the liquid coated the airways, providing a more vertically uniform liquid distribution.