Pulmonary Function Testing

One of the most useful instruments for office practice is the spirometer. Today, the spirometer must find its rightful place alongside the sphygmomanometer, the electrocardiograph, and the ophthalmoscope. Abnormal spirometry is an indicator of increased risk for premature death from all causes. This indication has been known since the time of its invention in 1846 by John Hutchinson, a surgeon. Why has spirometry been so slow to be accepted in the mainstream of clinical practice? The author believes that spirometry has been couched in too much mystique. It also has required the careful study of structurefunction relationships of the human lung to understand what causes alterations in spirometric measurements. Longitudinal studies have helped clarify the clinical significance of tests purported to indicate early stages of chronic obstructive pulmonary disease (COPD). Also, in the past, inexpensive and user-friendly devices were not available for office and clinic use. All of this is changing rapidly. Now the primary care physician and his or her assistant can learn the basics of spirometry easily, which provides two main values—the forced vital capacity (FVC) and the forced expiratory volume in 1 second (FEVi). Reviewing how these values can be applied immediately to everyday practice is the purpose of this article. THE ESSENCE OF SPIROMETRY Spirometry simply measures airflow out of fully inflated lungs. The lungs are filled by muscular force, to expand the thorax. Full inhalation stretches the chest to its maximum. Following this, a full forced expiration rapidly empties the lungs into a device that records flow over time. Normal lungs empty in 6 seconds. Figure 1 represents the elastic forces, pulling outward from the thorax, balanced by the inward force of the lung's elasticity. The resting lung volume is known as the functional residual capacity (FRC). Although FRC can be determined indirectly by body plethysmograph or inert gas techniques, it is not measured by spirometry. Expiratory airflow is a function of elastic recoil of lungs and thorax, small airways function, large airways function, and interdependence between small airway and the surrounding alveolar attachments. These fundamental concepts are presented in Figure 2 in an expression analogous to Ohm's law.