Airway Strain during Mechanical Ventilation in an Intact Animal Model

Abstract

Rationale: Mechanical ventilation with large tidal volumes causes ventilator-induced lung injury in animal models. Little direct evidence exists regarding the deformation of airways in vivo during mechanical ventilation, or in the presence of positive end-expiratory pressure (PEEP). Objectives: To measure airway strain and to estimate airway wall tension during mechanical ventilation in an intact animal model. Methods: Sprague-Dawley rats were anesthetized and mechanically ventilated with tidal volumes of 6, 12, and 25 cm 3 /kg with and without 10-cm H 2 O PEEP. Real-time tantalum bronchograms were obtained for each condition, using microfocal X-ray imaging. Images were used to calculate circumferential and longitudinal airway strains, and on the basis of a simplified mathematical model we estimated airway wall tensions. Measurements and Main Results: Circumferential and longitudinal airway strains increased with increasing tidal volume. Levels of mechanical strain were heterogeneous throughout the bronchial tree. Circumferential strains were higher in smaller airways (less than 800 mm). Airway size did not influence longitudinal strain. When PEEP was applied, wall tensions increased more rapidly than did strain levels, suggesting that a ''strain limit'' had been reached. Airway collapse was not observed under any experimental condition. Conclusions: Mechanical ventilation results in significant airway mechanical strain that is heterogeneously distributed in the uninjured lung. The magnitude of circumferential but not axial strain varies with airway diameter. Airways exhibit a ''strain limit'' above which an abrupt dramatic rise in wall tension is observed.

Lung and Systemic Inflammation in Preterm Lambs on Continuous Positive Airway Pressure or Conventional Ventilation

“…The initiation of ventilation with birth can cause nonuniform inflation and small airway stretch (5,6). Although the use of PEEP can improve oxygenation (28), it may not protect the lungs from injury (29).…”

“…Similarly, ventilation with lung volumes that approach or exceed total lung capacity will stretch the lungs and cause injury (3,4). The preterm fetal lung is particularly vulnerable to lung injury during the transition from a fluid-filled lung to airway breathing because surfactant deficiency increases the pressures required to open the lung, the lung aerates nonuniformly, and the delicate tissues of the preterm lung are easily stretched (5,6). The use of continuous positive airway pressure (CPAP) is being advocated for transitioning the preterm fetal lung to air breathing as a way to decrease lung injury (7,8).…”

Mechanical stretch decreases migration of alveolar epithelial cells through mechanisms involving Rac1 and Tiam1

“…This results in shunting of excessive tidal volume into the healthy lung, causing injury by either alveolar overdistention (10,15,16) or recruitment/derecruitment (8,58,59). In a recent study using an intact rat model, Sinclair et al (56) measured deformation of small airways in vivo during mechanical ventilation Fig. 3.…”

Injury and Inflammation from Resuscitation of the Preterm Infant

“…We think that much of the injury that occurs with the initiation of ventilation in the surfactant-deficient lung is small airway injury that then propagates to become diffuse lung injury. The preterm airways can stretch with routine ventilation pressures, and the stretch will be increased if lung parenchyma distal to the airway is collapsed because the airspace structures surrounding the airways are not acting interdependently to buffer local stretch [46] . Further, the airways and distal saccules/alveoli are filled with fluid which will form fluid columns and plugs with expiration.…”