D6 Spirometry

Topic

Spirometry measures respiratory volumes and flows using a spirometer—either a differential flow sensor or a displacement sensor. Through these measurements, spirometry directly quantifies standard parameters of ventilatory mechanics.

The first measurement is airway resistance: spirometry calculates this based on the forced expiratory volume in the first second (FEV1)—that is, the amount of air a person forcefully exhales during the first second of a forced expiration—and the flow-volume pattern, which tracks how the speed of exhaled air changes as the lung empties. The lower the FEV1 is relative to expected values, the higher the airway resistance detected by spirometry.

The second measurement is lung compliance: spirometry estimates this based on forced vital capacity (FVC)—the total volume of air a person forcefully exhales after a maximal inhalation. The lower the FVC is relative to expected values, the lower the lung compliance detected by spirometry.

With airway resistance and lung compliance measured, spirometry serves as the standard diagnostic tool for classifying and quantifying respiratory issues: when airway resistance rises above expected levels, spirometry classifies the problem as an obstructive pattern; when lung compliance falls below expected levels, it classifies it as a restrictive pattern; and when neither airway resistance nor lung compliance deviates from expected values—yet mobilized volumes remain low—the respiratory issue points to respiratory muscle weakness rather than a problem with the airways or the lung itself. By converting airway resistance and lung compliance into figures comparable to normal ventilatory mechanics parameters, spirometry makes it possible not only to detect the presence of a respiratory problem but also to classify its type and quantify the extent of the deviation from normal.

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