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RESPIRATORY FUNCTION

The major classes of lung diseases include obstructive lung diseases (e.g., asthma, chronic obstructive pulmonary disease, and bronchiectasis), restrictive lung diseases (e.g., interstitial lung diseases, chest wall abnormalities, and neuromuscular diseases), and vascular abnormalities (e.g., pulmonary thromboembolism and pulmonary arterial hypertension). The respiratory system includes not only the lungs but also the chest wall, pulmonary circulation, and central nervous system. There are three key types of respiratory system physiologic disturbances that occur in varying combinations in different lung diseases: ventilatory function, pulmonary circulation, and gas exchange.

Disturbances in Ventilatory Function

Ventilation involves the delivery of gas to the alveoli. Pulmonary function tests are used to assess ventilatory function. The classification of lung volumes, which are measured with pulmonary function testing, is shown in Fig. 137-1. Spirometry involves forced exhalation from total lung capacity (TLC) to residual volume (RV); key measurements from a spirogram are the forced expiratory volume in 1 s (FEV1) and the forced vital capacity (FVC). Expiratory flow rates may be plotted against lung volumes to yield a flow-volume curve. Plateau of the inspiratory curve on the flow-volume loop suggests extrathoracic large airway obstruction, while plateau of the expiratory curve suggests intrathoracic large airway obstruction. Other lung volumes, including TLC and RV, are measured under static conditions using either helium dilution or body plethysmography. Lung volumes and flow rates are typically compared with population-based normal values that adjust for the age, height, sex, and race of the pt.

FIGURE 137-1

Spirogram of a slow vital capacity maneuver demonstrating various lung volumes.

There are two major patterns of abnormal ventilatory function detected by pulmonary function testing: restrictive and obstructive (Tables 137-1 and 137-2). The presence of obstruction is determined by a reduced ratio of FEV1/FVC (with abnormal often defined using a threshold of <0.7), and the severity of airflow obstruction is determined by the level of reduction of FEV1. With airflow obstruction, TLC may be normal or increased, and RV is typically elevated. With severe airflow obstruction, the FVC is often reduced.

TABLE 137-1COMMON RESPIRATORY DISEASES BY DIAGNOSTIC CATEGORIES
TABLE 137-2ALTERATIONS IN VENTILATORY FUNCTION IN DIFFERENT PULMONARY DISEASE CATEGORIES

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