Effect of Ventilation on Mechanical Properties and Pressure-Flow Relationships of Immature Airways

Abstract

ABSTRACT. Ventilation of immature airways has been shown to result in pressure-induced deformation and alteration of mechanical properties. These changes in mechanical properties may alter the effect of compressive pressures on pressure-flow relationships. To test this hypothesis, unventilated (Group I, n = 8) and ventilated (group 11, n = 8; mean pressures of 8-12 cm H 2 0 for 2 h duration) tracheal segments were excised from preterm lambs approximately 118 days gestation and mounted in a Krebsfilled chamber for determination of compliances and pressure-flow relationships. Compliance data were obtained for both the inflation loop (inflation compliance) and the collapsing loop (collapsing compliance) of the pressure-volume curve for each segment. Supported by NIH Grant NHLBI HL 3203 1. 1compliance, airway compression, and resistance to airflow has been studied extensively in adult airways (l-3), to a lesser degree in newborn airways (4), and there is no research with respect to the premature airway. Prolonged mechanical ventilation has little effect on adult airways but has been shown to effect the dimensions (5-8) and mechanical properties (5, 9) of preterm and newborn airways. However, analysis of these mechanical properties in preterm and newborn airways extended only to the compliance as determined by the inflation loop of the P-V curve. This provided no insight into airway rigidity under compression. It is therefore unclear how mechanical ventilation affects the immature airway's ability to withstand compressive collapse and thereby influences pressure-flow relationships.This study was designed to evaluate the effects of ventilation on the mechanical properties and pressure-flow relationships of preterm lamb tracheae. Differences in compliances defined by both the inflation and collapsing loops of the P-V curve of ventilated and unventilated tracheal segments were examined. Resistance to airflow over a physiological range of flows and compressive pressures was also determined in both groups, and intergroup differences were associated with observed differences in compliances. This study may help to explain how ventilation influences the ability of the premature airway to withstand compressive collapse, and further aid in the understanding of flow limitation and gas-trapping observed in infants who have been mechanically ventilated. METHODSThe apparatus used in these experiments (Fig.

In Vivo Mechanical Properties of the Developing Airway

“…In the present study, the 53% decrease in tracheal compliance in lambs is similar to that seen in the rabbit pup studied over a similar developn~ental range of 0.67 gestation to term (8). Age-related differences in airway compliance may explain in part many of the clinical observations associated with the premature infant such as high resistance to airflow, flow limitation at low efforts, and reduced expiratory reserve (12,13,18). In addition, the compliant airways of premature infants appear highly susceptible to alterations in their dimensions (5,12) and mechanical properties (12) when chronically subjected to positive pressure ventilation.…”

“…Age-related differences in airway compliance may explain in part many of the clinical observations associated with the premature infant such as high resistance to airflow, flow limitation at low efforts, and reduced expiratory reserve (12,13,18). In addition, the compliant airways of premature infants appear highly susceptible to alterations in their dimensions (5,12) and mechanical properties (12) when chronically subjected to positive pressure ventilation. Morphometric changes characteristic of acquired tracheomegaly and bronchopulmonary dysplasia may represent attempts by the immature airway to minimize wall stress during ventilation, similar to changes exhibited by the vascular wall consequent to hypertension.…”

“…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)(6)(7)(8)(9)(10)(11)(12)(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).…”

The Impact of Mechanical Ventilation on Immature Airway Smooth Muscle: Functional, Structural, Histological, and Molecular Correlates

“…Clinically, this can result in elevated dead space ventilation and a higher work of breathing. Functionally, mechanical ventilation alters airway mechanics, creating an organ that is more compliant to compressive forces [4] . This produces an increase in airway collapsibility during forced expirations, such as crying or coughing.…”

New aspects of airway mechanics in pre-term infants

“…Since increased transmural pressures may affect airway compliance [20,35], infants who had received mechanical ventilation were excluded from this study. Only one infant received CPAP therapy (20 h; far,15157 Hz), while two had brief supplemental oxygen (1 day, far,15195 Hz and 11 h, far,15177 Hz).…”