Measures of Pulmonary Volumes

Spirometry is the measurement of the volume of air moving into or out of the airways. In this process, various ventilatory maneuvers are undertaken that

Figure 6 Spirometric representation of lung volumes.

permit an estimation of pulmonary volumes and capacities (Fig. 6). Such measurements are valuable for diagnosis of airway disease because pathological conditions can modify specific pulmonary volumes. Definitions of the specific lung volumes are provided in Table 4. Measurements of lung volumes are

Table 4 Definitions of Lung Volumes

Volume

Definition

Tidal volume (TV) Inspiratory reserve volume (IRV) Expiratory reserve volume (ERV) Residual volume (RV) Inspiratory capacity Functional residual capacity (FRC) Vital capacity

Total lung capacity

Air inspired or expired during a normal breath

Volume maximally inspired after a normal tidal inspiration Volume maximally expired after a normal tidal expiration Volume remaining in the airways after completion of a maximal expiratory effort Volume inspired maximally after a normal expiration; = TV + IRV Volume remaining in the airways after a normal expiration; = ERV + RV Volume maximally expired from the lungs after a maximal inspiration; = IRV + TV + ERV Volume in the airways after a maximal inspiration; = IRV + TV + ERV + RV

Lung volumes can be determined by spirometry and reflect the volume of air remaining in the airways after various inspiratory or expiratory maneuvers. Lung capacities encompass two or more lung volumes (as shown by the provided formulas).

generally normalized for a subject's body size (weight, height, or surface area), age, and gender. This process permits a comparison with standardized or predicted lung volumes, thereby allowing identification of lung pathophysiol-ogies using a simple procedure. Some examples of the way in which airway disorders alter lung volumes are described in the following. During an episode of airway obstruction (as in asthmatic bronchospasm), expiration of air is difficult, and air becomes trapped in the lower airways. This results in an increase in the residual volume and functional residual capacity and a decrease in vital capacity. In conditions that adversely affect respiratory muscles, such as poliomyelitis and spinal cord injuries, voluntary control of inspiratory or expiratory movement is diminished (or absent) and vital capacity is reduced.

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