ABSTRACT'. The ability of tracheal smooth muscle tone to modulate the mechanical properties of neonatal airways was evaluated in six newborn lambs. Tracheal pressurevolume relationships, isovolumic compliance, hysteresis, and the relaxation time constant of the smooth muscle were evaluated as a function of incremental cholinergic stimulation. Tracheal active tensions were also determined at the graded levels of cholinergic stimulation. Data show that a maximal cholinergic stimulation resulted in a mean developed active tension value of 14.3 2 2.13 SEM x 10" dyneslcm. The resultant 55% decrease in tracheal compliance was linearly correlated to the increase in active tension linergic stimulation was utilized to alter smooth muscle tone and its effect on tracheal elastic and viscoelastic behavior were determined. The relationship between tracheal compliance, hysteresis, and relaxation time constant of the smooth muscle were evaluated as a function of the change in tracheal smooth muscle tension. M E T H O D SAnimal preparation. Six full-term newborn lambs, 1 to 3 days postpartum, were anesthetized (30 mg/kg intraperitoneal pentabarbital) and secured in the supine position. The ventral surface of the neck was dissected to reveal the upper trachea, and the exposed tracheal area proximal to the thoracic cage was transected and cannulated to allow spontaneous ventilation. Vascular integrity was maintained and subsequently verified by cholinergic stimulation. The left jugular vein was cannulated, and electrocardiographic leads were attached to the lamb.Proximal tracheal segments of 2.8 cm in length were measured and isolated. A plug was inserted in the most caudal end of the bypassed segment (Fig. 1). The lambs were then intubated with an endotracheal tube secured in place with ligatures at the cricoid cartilage. A rigid frame was affixed externally to the bypassed segments to maintain constant length during the experimental procedures.Tracheal smooth muscle tone plays an important and significant role in the conductance and the dimensions of the adult tracheobronchial tree (1). Coburn and Palombini (2) have demonstrated that graded increases in the active tension of the trachealis muscle results in a proportional decrease in the adult canine tracheal compliance. Other effects of smooth muscle tension on adult tracheal mechanics have been documented with respect to its viscoelastic properties (3), regulation (4), and structure-function (5). It is well accepted that neonatal airways are compliant structures (6, 7). Although these in vitro studies have provided valuable information concerning the passive properties of the airways, the effect of smooth muscle tone on the functional behavior of neonatal airways is not known. The relationship between smooth muscle contraction and mechanical function of the immature trachea has not been evaluated, hence it is unclear whether airway rigidity and function can be controlled at early stages of development.The purpose of this study was to evaluate the role of tracheal smooth muscl...

ABSTRAm. The effect of smooth muscle tone on the functional characteristics of neonatal airways was studied in six newborn lambs. Tracheal mechanics, resistance, and collapsibility were determined in tracheal segments of uniform length which were surgically isolated just caudal to the cricoid cartilage. Resistance to airflow through uncompressed (zero transmural pressure) and compressed (increased transmural pressure) trachea was evaluated over a range of physiological flows both before and after induced contraction of airway smooth muscle. Results demonstrate that neonatal tracheal smooth muscle does respond to cholinergic stimulation and that there was a significant decrease in airway compliance. Furthermore, the consequent increase in airway rigidity may play a role in resisting collapse or compression of the intrathoracic airway during expiration. (Pediatr Res 20: 492495, 1986) Structurally, neonatal airways are more compliant than adult airways (1-3). The effect of any transmural compression on these compliant structures could, therefore, lead to increased airflow resistance on expiration and consequently require additional work in breathing. Tracheal smooth muscle plays an important role in regulating the conducting airway volume and resistance in the adult (4). Furthermore, when the tracheal smooth muscle tone is increased (2, 5) adult airways (intrathoracic) have been shown to resist compression during expiration. However, although it is known that the trachealis muscle mass increases throughout postconceptional development, little quantitative data are available to confirm the relationship between smooth muscle contraction and mechanical function of the neonatal airways.This study was designed to evaluate the effect of neonatal airway smooth muscle tone on tracheal mechanics, resistance, and collapsibility. Resistance to airflow through uncompressed (zero transmural pressure) and compressed (increased transmural pressure) tracheae were examined over a range of physiological flows both before and after induced contraction of airway smooth muscle. Changes in airway resistance were associated with alterations in tracheal mechanics due to smooth muscle contraction. These studies should be useful in the understanding of how airway smooth muscle may increase the ability of immature airways to withstand compressive narrowing. tobarbital (20 mglkg). Each was secured in a supine position and the cervical trachea was exposed through a midline incision. The trachea was cannulated close to the thoracic inlet to establish a patent airway for spontaneous ventilation. Care was taken to preserve vascular and neural supply by utilizing blunt dissection. The left jugular vein was catheterized to administer drugs and dextrose infusion. The electrocardiogram was continuously monitored for lead 11. The lambs were maintained normothermic under a heat lamp. The bypassed tracheal segment was utilized to determine pressure-volume and pressure-flow relationships. Throughout the experiment, the segment was covered with moist d...