In Vivo Mechanical Properties of the Developing Airway

Abstract

ABSTRACT. The inherent mechanical characteristics of the airways are determined in part by their elastic and viscoelastic properties. As compliant structures during early development, the airways are susceptible to significant distention and collapse, depending on the proportionality between airway volume and transmural pressure. To characterize the age-related changes in airway mechanical properties, the elastic and viscoelastic behavior of in vivo tracheal segments were evaluated in preterm and newborn lambs over a wide range of developmental age (108 to 154 days postconceptional age). Tracheal pressure-vol relationships and concomitant airway compliance measurements were used to determine elastic behavior. Calculations of the tracheal relaxation time constant on the same tracheal segments were used to evaluate airway viscoelastic behavior. Data demonstrated a significant (p < 0.01) correlation with developmental age. With increasing age, the airways were found to be less compliant, and the tracheal relaxation time constant was observed to decrease. The difference in elastic properties of the trachea, in vivo compared to in vituo, suggest that neural-humoral and surrounding connective tissue factors may affect the elasticity of the developing airway. Although the modulating effects of smooth muscle tone and supporting connective tissue assist in the control of airway dimension and resistance to airflow in the intact airway, the age-related differences in the elastic properties may be a factor that predisposes the more immature airway to positive pressure-induced damage. increased dead space and gas trapping (3-6). These findings underscore the need for determining the functional characteristics of immature airways. Numerous investigators have characterized the elastic and viscoelastic properties of the airways by using a variety of experimental techniques, in a number of species, and at different stages of development (5-13). To date, only one study, using in vitro rabbit tracheae, has examined airway mechanical properties throughout a range of early development according to a consistent protocol and experimental technique (5). Although this study has provided valuable information concerning age-related differences in airway mechanics, it is difficult to extrapolate these data to the fully innervated and perfused in vivo trachea. Furthermore, the in vitro preparation cannot address the influence of neuralhumoral and supporting structures on the in situ trachea, which may also be age-related.The in vivo study of airway physiology in the premature animal has been limited by the inability to maintain physiologic gas exchange and acid-base status that potentially could influence airway properties (14). Recent advances in liquid ventilation techniques have enabled us to establish a stable and viable preterm animal preparation to study physiologic development beginning at the early stages of gestation (15). Using this technology, we evaluated airway compliance and the relaxation time constant of in vivo tracheal seg...

New aspects of airway mechanics in pre-term infants

“…This suggests that increased airway wall compliance due to immaturity must have a significant influence on far,1 and hence n in pre-term infants. The latter would be consistent with structural findings [22,34].…”

“…One hypothesis is that the structural immaturity of the airway walls results in highly compliant airways, crucially determining flow limitation in immature airways of prematurely born infants. The high compliance of the airway walls in immature animals has been demonstrated in vitro [22,34], but the situation might be different in vivo and/or in human infants, depending on the strength of airway-parenchyma attachments and the relationship between lung and chest wall compliance [24]. This relationship will be further complicated by the tendency of young infants to dynamically elevate their endexpiratory volume, thereby further regulating airway patency and elastic recoil, and by changes in sleep state, which may affect both lung volume and upper airway tone [16,23].…”

“…It has been shown in an animal model, that structural differences of immature airways may result in a markedly increased airway wall compliance [20]. In addition, developmental changes in the properties of the lung parenchyma [21,22], dynamic control of endexpiratory lung volume [23], high chest wall compliance [24] and diminished airway-tissue coupling [25] influence elastic recoil of the respiratory system with subsequent impact on functional airway diameter and airway wall mechanics. Due to the complexity of these interactions, it is extremely difficult to assess the potential impact of altered airway wall mechanics on measured values of resistance or forced expiratory flows in pre-term infants during the first months of life.…”

Developmental Changes in Tracheal Structure

“…Several investigators have examined the mechanical properties of the developing airways, both in humans and in various animal species (1)(2)(3)(4)(5)(6). These studies indicate that the airways from preterm neonates are highly compliant and collapsible structures that become stiffer and better able to withstand pressure-induced deformation with increasing maturity.…”

Structure-Function of Airway Generations 0 to 4 in the Preterm Lamb

“…DISCUSSION The trachea has been used as a model of airway function in adult (1 6, 17, 3 1, 32), newborn (5,19,33), preterm (6, 1 1, 14), and developmental (2, 4,8,12,15,18) studies in a variety of animal species. The trachea has a unique anatomical position and structure that may qualify it to perform a unique function.…”