Breathing In: The Determinants of Lung Volume
The measurement of lung volume is as old as the spirometer. Measurements of lung volume and its subdivisions yield critical diagnostic information about the principle cause of disease, either restrictive with decreased volume or obstructive with increased volume. There are numerous simple and complex techniques for measuring lung volume including simple spirometry, body plethysmography, and the CT scan.
The interpretation of lung volume measurements requires knowledge of the interaction of the respiratory muscles on the chest wall and the static elastic forces at work. A step-by-step series of assessments and considerations are presented aimed at extracting the maximal amount of useful information from the measurement of the lung volume subdivisions (i.e., volumes and capacities). The volumes and capacities that are most useful for detailed assessment of the effect of disease on the lung are; residual volume (RV), functional residual capacity (FRC), and total lung capacity (TLC). These three measure the boundaries of the operational change in volume from the normal functional volume to the extremes of vital capacity. They also yield the maximum amount of information. Four cases illustrate the utility of measuring lung volumes and the approach to interpretation.
KeywordsLung volume Lung capacity Obstruction Restriction Boyle’s law Plethysmograph Gas trapping
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