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LEARNING OBJECTIVES

Learning Objectives

  • The student will be able to state normal intrapleural pressures from lung apex to base in the chest of upright and supine subjects, including the effects of gravity.

  • The student will be able to enumerate the physical factors affecting airway resistance and dynamic lung compliance in vivo.

  • The student will be able to distinguish normal, restrictive, and obstructive flow patterns and explain peripheral gas trapping by the concept of effort-independent, volume-dependent peak flow rates.

  • The student will be able to explain how alveolar distension is differentially affected by body position, residual volume (RV), and positive end-expiratory pressure (PEEP) ventilation.

The tissue elastic and surface tension recoil forces that provide inherent compliance to normal lung tissue were reviewed in Chaps. 4 and 5. For excised lungs, these forces can be examined as if they operated independently of constraints placed on lung expansion by bones and muscles of the thorax. However, the chest wall and airways strongly influence lung distension and the work of breathing (Fig. 6.1). Incorporating these factors into a broad model of ventilation will facilitate understanding of the major categories of lung disorders, the obstructive diseases and restrictive diseases.

FIGURE 6.1

The diaphragm is normally the main inspiratory motor during respiration; its caudal excursions create negative pressures that expand the lungs. The intercostals and other accessory muscles enlarge the anterior-posterior and lateral chest diameters. From Fox. Human Physiology, 10th ed.; 2008.

ELASTIC PROPERTIES OF THE INTACT THORAX

The chest wall and diaphragm normally oppose the recoil forces of lung tissue elasticity and surface tension. Thus, the negative PIP at functional residual capacity (FRC) (see Fig. 4.6) represents the equilibrium when these musculoskeletal elements are bowed inwardly by lung recoil. Since this balance is not intuitive, consider a bilateral pneumothorax when PIP = PB (Fig. 6.2). When PIP increases from −5 to 0 cm H2O, healthy lungs recoil toward their minimal volume, while the chest walls spring outward and the diaphragm drops toward the abdominal viscera. Indeed, the volume of the open chest when PIP = PB is about 80% of a subject’s normal total lung capacity (TLC), implying that musculoskeletal elements will oppose all forces attempting to make the chest either larger or smaller than about 80% of TLC.

FIGURE 6.2

At FRC, lung recoil forces are opposed by thoracic elements and PIP = −5 to −10 cm H2O. A pneumothorax allows lung to collapse and chest volume to approach TLC as ribs spring outward and the diaphragm moves caudally.

Thus, an in situ compliance curve for the entire lung-thorax system is the algebraic sum of ...

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