Effect of device inhalational resistance on the three-dimensional configuration of the upper airway

A review of upper airway physiology relevant to the delivery and deposition of inhalation aerosols

Developing effective oral inhaled drug delivery treatment strategies for respiratory diseases necessitates a thorough knowledge of the respiratory system physiology, such as the differences in the airway channel's structure and geometry in health and diseases, their surface properties, and mechanisms that maintain their patency. While respiratory diseases, such as chronic obstructive pulmonary disease (COPD) and asthma and their implications on the lower airways have been the core focus of most of the current research, the role of the upper airway in these diseases is less known, especially in the context of inhaled drug delivery. This is despite the fact that the upper airway is the passageway for inhaled drugs to be delivered to the lower airways, and their replicas are indispensable in current standards, such as the cascade impactor experiments for testing inhaled drug delivery technology. This review provides an overview of upper airway collapsibility and their mechanical properties, the effects of age and gender on upper airway geometry, and surface properties. The review also discusses how COPD and asthma affect the upper airway and the typical inhalation flow characteristics exhibited by the patients with these diseases.

Characterization of dry powder inhaler performance through experimental methods

Experimental methods provide means for the quality control of existing DPIs and for exploring the influence of formulation and device parameters well in advance of clinical trials for novel devices and formulations. In this review, we examine the state of the art of in vitro testing of DPIs, with a focus primarily on the development of accurate in vitro-in vivo correlations. Aspects of compendial testing are discussed, followed by the influence of flow profiles on DPI performance, the characterization of extrathoracic deposition using mouth-throat geometries, and the characterization of regional thoracic deposition. Additional experimental methods that can inform the timing of bolus delivery, the influence of environmental conditions, and the development of electrostatic charge on aerosolized DPI powders are reviewed. We conclude with perspectives on current in vitro methods and identify potential areas for future investigation, including the estimation of variability in deposition, better characterization of existing compendial methods, optimization of formulation and device design to bypass extrathoracic deposition, and the use of novel tracheobronchial filters that aim to provide more clinically relevant measures of performance directly from in vitro testing.

The size and behavior of the human upper airway during inhalation of aerosols

INTRODUCTION

The mouth, the pharynx and the larynx are potential sites of aerosol deposition in the upper airway during inhalation of aerosolized drugs. The right angle bend of the lumen at the back of the mouth, the position of the tongue, the variable size and shape of the lumen in the pharynx and the larynx, and the breathing pattern could increase aerosol deposition in the upper airway and decrease lung deposition. Areas covered: In this review, the anatomy of the upper airway from the oral cavity to the glottis and the impact of mandibular protrusion and incisal opening on the size of the upper airway are highlighted. In addition, the impact of inhalation maneuvers, inhaler mouthpiece geometries and a stepped mouthpiece on the size of the upper airway are discussed. Expert opinion: The structure of the upper airway lumen does not have a fixed cross sectional area and is susceptible to both constriction and distension during inhalation. The size of the upper airway can be enlarged through mandibular protrusion and/or incisal opening which might decrease aerosol deposition in the upper airway and increase lung deposition.

Pediatric in vitro and in silico models of deposition via oral and nasal inhalation

Respiratory tract deposition models provide a useful method for optimizing the design and administration of inhaled pharmaceutical aerosols, and can be useful for estimating exposure risks to inhaled particulate matter. As aerosol must first pass through the extrathoracic region prior to reaching the lungs, deposition in this region plays an important role in both cases. Compared to adults, much less extrathoracic deposition data are available with pediatric subjects. Recently, progress in magnetic resonance imaging and computed tomography scans to develop pediatric extrathoracic airway replicas has facilitated addressing this issue. Indeed, the use of realistic replicas for benchtop inhaler testing is now relatively common during the development and in vitro evaluation of pediatric respiratory drug delivery devices. Recently, in vitro empirical modeling studies using a moderate number of these realistic replicas have related airway geometry, particle size, fluid properties, and flow rate to extrathoracic deposition. Idealized geometries provide a standardized platform for inhaler testing and exposure risk assessment and have been designed to mimic average in vitro deposition in infants and children by replicating representative average geometrical dimensions. In silico mathematical models have used morphometric data and aerosol physics to illustrate the relative importance of different deposition mechanisms on respiratory tract deposition. Computational fluid dynamics simulations allow for the quantification of local deposition patterns and an in-depth examination of aerosol behavior in the respiratory tract. Recent studies have used both in vitro and in silico deposition measurements in realistic pediatric airway geometries to some success. This article reviews the current understanding of pediatric in vitro and in silico deposition modeling via oral and nasal inhalation.

Assessing the performance of two dry powder inhalers in preschool children using an idealized pediatric upper airway model

High prevalence of pulmonary diseases in childhood requires inhalable medication even for young children. Little is known about the efficiency of aerosol therapy especially in preschool children. One factor which limits the lung dose is the upper airway geometry. Based on clinical data a recently developed idealized pediatric upper airway model (children 4-5 years) was used to investigate the performance of two dry powder inhalers (Easyhaler and Novolizer). In vitro investigations were first examined using steady flow rates and an inhalation volume of 1L. Chosen flow rates were 28, 41 and 60L/min (Easyhaler) and 45, 60 and 75L/min (Novolizer). Afterwards inhalation profiles simulated by an electronic lung were included. The investigations showed high amounts of drug particles (up to 80%) which were deposited in the upper airway model. The pulmonary deposition in vitro using the Easyhaler was about 28% (28-60L/min) and 22% (inhalation profile). Using the Novolizer in vitro pulmonary doses of 8-12% (45-75L/min) and about 5% (inhalation profile) were observed. The idealized model shows good performance reproducibility of dry powder inhalers. We have shown that age-dependent models might be appropriate tools for formulation and device development in pediatric age groups.

Studies of the Human Oropharyngeal Airspaces Using Magnetic Resonance Imaging IV—The Oropharyngeal Retention Effect for Four Inhalation Delivery Systems

Magnetic resonance imaging (MRI) of the oropharyngeal region from 20 adult volunteers using four model inhalation devices (varying mouthpiece diameters, airflow resistances) and tidal breathing was carried out. Statistical analysis (convex hull method) selected 12 scans from 80 data sets representing the extremes of all dimensions in the population. Twelve physical mouth-throat models were made by stereolithography using the exact scan data. The aim was to produce models with varying dimensions to span the adult population, and to investigate if oropharyngeal dimensions affected throat retention for different delivery systems. In an in vitro analysis, the models were used to determine the retention effect of the oropharyngeal airspaces when drug aerosols were administered from four inhalation delivery systems: a pressurised metered dose inhaler (pMDI), two different dry powder inhalers (DPIs A and B), and a nebulizer. The aims of this work were to determine the key parameters governing mouth-throat retention and whether retention was dependent on the delivery system used. Characterizing the throat models by measuring 51 different dimensional variables enabled determination of the most influential variables for dose retention for each inhalation delivery system. Throat model retention was found to be dependent on the delivery system (pMDI approximately DPI(A) > DPI(B) > Neb.). The most influential variable was the total throat model volume. Throat models representing high, median, and low oropharyngeal filtration in healthy adults have been identified.

Characteristics of the turbulent laryngeal jet and its effect on airflow in the human intra-thoracic airways

A computational fluid dynamics technique is applied to understand the relative importance of the upper and intra-thoracic airways and their role in determining central airflow patterns with particular attention paid to the importance of turbulence. The geometry of the human upper respiratory tract is derived from volumetric scans of a volunteer imaged via multidetector-row computed tomography. Geometry 1 consists of a mouthpiece, the mouth, the oropharynx, the larynx, and the intra-thoracic airways of up to six generations. Geometry 2 comprises only the intra-thoracic airways. The results show that a curved sheet-like turbulent laryngeal jet is observed only in geometry 1 with turbulence intensity in the trachea varying from 10% to 20%, whereas the turbulence in geometry 2 is negligible. The presence of turbulence is found to increase the maximum localised wall shear stress by three-folds. The proper orthogonal decomposition analysis reveals that the regions of high turbulence intensity are associated with Taylor-Görtler-like vortices. We conclude that turbulence induced by the laryngeal jet could significantly affect airway flow patterns as well as tracheal wall shear stress. Thus, airflow modeling, particularly subject specific evaluations, should consider upper as well as intra-thoracic airway geometry.

Air classifier technology (ACT) in dry powder inhalation

In this study, the in vitro fine particle deposition from a multi dose dry powder inhaler (Novolizer) with air classifier technology has been investigated. It is shown that different target values for the fine particle fraction (fpf<5 microm) of the same drug can be achieved in a well-controlled way. This is particularly relevant to the application of generic formulations in the inhaler. The well-controlled and predictable fpf is achieved through dispersion of different types of formulations in exactly the same classifier concept. On the other hand, it is shown that air classifier-based inhalers are less sensitive to the carrier surface and bulk properties than competitive inhalers like the Diskus. For 10 randomly selected lactose carriers for inhalation from four different suppliers, the budesonide fpf (at 4 kPa) from the Novolizer varied between 30 and 46% (of the measured dose; R.S.D.=14.2%), whereas the extremes in fpf from the Diskus dpi were 7 and 44% (R.S.D.=56.2%) for the same formulations. The fpf from a classifier-based inhaler appears to be less dependent of the amount of lactose (carrier) fines (<15 microm) in the mixture too. Classifier-based inhalers perform best with coarse carriers that have relatively wide size distributions (e.g. 50-350 microm) and surface discontinuities inside which drug particles can find shelter from press-on forces during mixing. Coarse carrier fractions have good flow properties, which increases the dose measuring accuracy and reproducibility. The fpf from the Novolizer increases with increasing pressure drop across the device. On theoretical grounds, it can be argued that this yields a more reproducible therapy, because it compensates for a shift in deposition to larger airways when the flow rate is increased. Support for this reasoning based on lung deposition modelling studies has been found in a scintigraphic study with the Novolizer. Finally, it is shown that this inhaler produces a finer aerosol than competitor devices, within the fpf<5 microm, subfractions of particles (e.g. <1, 1-2, 2-3, 3-4 and 4-5 microm) are higher.