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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to define static lung compliance as determined using excised lungs inflated by negative pressure.

  • The student will be able to distinguish between tissue elastic recoil forces and surface tension recoil forces, and interpret hysteresis curves obtained during lung inflations with air, saline, or with air after saline lavage.

  • The student will be able to summarize the origin, cellular location, and primary constituents of pulmonary surfactant.

  • The student will be able to describe the major laboratory methods for analyzing amniotic fluid for pulmonary surfactant, including cutoff values for the foam stability index, the fluorescent polarization assay, and the lecithin/sphingomyelin ratio.

Lungs normally resist their own expansion throughout inspiration by means of inwardly directed recoil forces that would collapse them if left unopposed by the chest wall and diaphragm that are recoiling outward. Thus, lung volume at end-expiration and with the glottis open to atmospheric pressure represents the balance point of these directionally opposite forces, and was defined previously as functional residual capacity (FRC). The following discussion will summarize the features of healthy lung tissue that provide its inherent elastic recoil, and the role that pulmonary surfactant plays in modulating that elasticity so that breathing can be both effective and energetically feasible.

ELASTIC PROPERTIES OF LUNG TISSUE

The description in Chap. 4 of gas movement during ventilation introduced the concept of static elastic and surface tension recoil forces. These forces are visually evident on excised healthy lungs that quickly collapse to their minimal volume if the chest is opened to outside air, creating a pneumothorax. Such lungs normally retain air only in large airways and a few clusters of alveoli, having undergone atelectasis. The collapsed lungs are suspended by the trachea within a vacuum jar that can be evacuated to simulate negative intrapleural pressure (PIP) in vivo (Fig. 5.1). As jar pressure is made more negative in small stable increments, the lungs ‘inspire’ air and equilibrate at a new volume until jar pressure is altered again. In this stepwise way, stable changes in total lung volume per unit change in pressure are recorded when dynamic airway resistance is absent, and used to calculate their static compliance (ΔV/ΔP).

FIGURE 5.1

Static pressure-volume curves of excised lungs held at each "intrapleural" pressure for a few seconds before volume is measured. Curves are nonlinear and approach total lung capacity at the most negative pressures. See the text for additional details.

When negative pressure is first applied to the collapsed lungs, their volume does not change, that is, their compliance is zero. Then, as the lungs achieve a critical opening pressure (PCO) their compliance rapidly increases and the lungs fill easily. Thus, PCO is the minimal pressure required to open or ...

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