What is the pathophysiology of restrictive lung disease?

Updated: Sep 16, 2020
  • Author: Jonathan Robert Caronia, DO; Chief Editor: John J Oppenheimer, MD  more...
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Answer

Air flows to and from the alveoli as lungs inflate and deflate during each respiratory cycle. Lung inflation is accomplished by a contraction of respiratory, diaphragmatic, and external intercostal muscles, whereas deflation is passive at rest. Functional reserve capacity (FRC) is the volume of air in the lungs when the respiratory muscles are fully relaxed and no airflow is present. The volume of FRC is determined by the balance of the inward elastic recoil of the lungs and the outward elastic recoil of the chest wall. Restrictive lung diseases are characterized by a reduction in FRC and other lung volumes because of pathology in the lungs, pleura, or  structures of the thoracic cage.

The distensibility of the respiratory system is called compliance. Compliance is the volume change produced by a change in the distending pressure. Lung compliance is independent of the thoracic cage, which is a semirigid container. The compliance of an intact respiratory system is an algebraic sum of the compliances of both of these structures. Therefore, it is influenced by any disease of the lungs, pleura, or chest wall.

In cases of intrinsic lung disease, the physiological effects of diffuse parenchymal disorders reduce all lung volumes by the excessive elastic recoil of the lungs,  relative to the outward recoil forces of the chest wall. Expiratory airflow is reduced in proportion to lung volume.

Arterial hypoxemia in disorders of pulmonary parenchyma is primarily caused by ventilation-perfusion mismatching, with further contribution from an intrapulmonary shunt. Decreased diffusion of oxygen rarely contributes to hypoxemia because sufficient time still exists for full equilibration of oxygen or carbon dioxide. However, if transit time is significantly shortened, as with exercise, this can highlight the pathology with significant exercise-induced desaturation.


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