Form and function of the human nasal airways: biomechanical assessment

The nose preconditions air entering the respiratory system by warming, humidifying, and filtering the air and thus protecting the delicate lining of the lungs. The nose is an important sensory organ, and a portion of the inspired air is directed towards the upper portion of the interior nasal cavity where the olfactory receptors are located. In order to perform these different physiological functions, the internal nasal cavity possesses protrusions (turbinates) which interact with the jet of inspired air, producing complex flow patterns to improve heat and water exchange and retain samples of air for olfaction. Understanding the mechanisms governing the interaction between nasal geometry and airflow is not only of fundamental physiological importance, but of great potential benefit in health care, therapeutic drug delivery and environmental hazard assessment. Detailed in-vivo measurement of air-flow and particle deposition in humans is extremely difficult, and the large disparities in anatomical features between species means that extrapolation of the flow and transport characteristics from animal studies is of questionable validity. Computational modeling is thus needed to build predictive capabilities. This will help doctors to understand and assess nasal function in health and disease, they will provide a more rational basis for surgeons to plan and judge the success of interventions, they will allow scientists to investigate the mechanics of gas transport and particle deposition which is needed to improve drug delivery and environmental assessment, and they can be used for many other beneficial purposes. Recently computational techniques combined with in-vivo imaging have been developed which provide a means to model nasal air flow, and a number of studies have recently shed some light on the complex nature of flow in the nasal cavity. However no attempt has yet been made to develop and to apply systematic procedures to characterize the functional significance of the wide variation in normal nasal airway anatomy found in the population. Even for a given individual, there is a lack of detailed information on how changes in airway geometry affect the air flow and transport characteristics. Our first aim is to determine the geometric form of the nasal airway, initially from more than twenty individuals. We will develop and apply mathematical techniques to characterize the ways in which nasal geometry varies, and assess the significance of these variations for physiological performance. The initial data set will gradually grow through contributions from colleagues in other countries, and will provide a valuable resource to provide more accurate modeling and prediction of physiological function and for many applications. Our second aim is to investigate how the variation in nasal geometry in an individual affects flow and transport. There are several factors which produce intra-individual geometric changes, for example in most normal adults, each side of the nasal airways alternately congests and decongests to a varying extent in a process termed the nasal cycle, altering the flow (and often sensation), and rapid sniffing causes partial collapse of the external nose. We will apply novel imaging techniques to investigate these changes. By studying the flow through simulation and by experiments in replica models, we will then be able to understand the significance of such alterations in the conditions that determine the flow. Our third aim is to study the very complex physical processes which cause the air flow in the nose to become unstable at higher inspiratory rates, to determine how to model such processes, and to study their impact for transport in the nasal airways. Overall we aim to provide both the data and the techniques needed for the first systematic exploration of nasal air flow and transport across a broad range of individuals.

The research is intended to provide the first comprehensive study of air flow and transport in the nasal cavity, encompassing both inter- and intra-individual variations in the morphology of the airways. Firstly we aim to produce a database of reconstructed normal nasal airway geometries, derived by processing existing in-vivo CT and MR scans. Our initial database of 20-25 datasets will be extended through international collaboration. We will use procedures we have developed and are refining for compact topological representation to characterize the modes of variation in nasal morphology across the set of bi-lateral nasal airway reconstructions. Using computational simulations, and experiments in replica models for validation, we will determine ranges of relevant parameters such as pressure loss, regional wall shear and transport distributions, and regional deposition maps, in steady inspiratory flow. Secondly, we will investigate the significance of intra-individual variations in morphology for a smaller number (2-3) of representative geometries. Novel high-speed MR techniques adapted from cardiovascular imaging will be investigated to determine dynamic variations in internal passage caliber. Dynamic computational simulations, backed up by experiments in replica geometries with fixed boundaries will be applied to map variations in the above parameters due to alterations in temporal and spatial boundary conditions. Thirdly, we will investigate the complex interactions between the flow issuing from the nasal valve and the cavity which leads to the development of unsteadiness in the nasal flow at higher inspiratory rates. Using direct numerical simulation coupled with well controlled experiments in replica and appropriately simplified anatomical geometries, we will address a well-recognized deficiency in current modeling, and provide tools for a more comprehensive analysis of the data gathered in the first part.