EXPLANATION OF SPIROMETRY
through a mouthpiece. This flow can be integrated from the mouthpiece to achieve Flow vs. Time (FT), Volume vs. Time (VT), or Flow vs. Volume (FV) plots of the expira-tion. An example FV plot can be seen in Figure 2. From the plot, several quantities are measured:
(1) Forced Vital Capacity (FVC) is the total expelled volume during the expiration,
(2) Forced Expiratory Volume in one second (FEV1) is the volume exhaled in the first second,
(3) FEV1/FVC is simply the ratio of the aforementioned two measures, and,
(4) Peak Expiratory Flow (PEF) is the maximum flow velocity reached during the test.
The most common clinically-reported measures are FEV1, FVC, and FEV1/FVC, as they are used to quantify the de-gree of airflow limitation in chronic lung diseases such as asthma, COPD, and cystic fibrosis. In general, a healthy result is >80% of the predicted value based on height, age, and gender [16]. Abnormal values are [22]:
• Mild Lung Dysfunction: 60-79%
• Moderate Lung Dysfunction: 40-59%
• Severe Lung Dysfunction: below 40%
However, diagnosis from spirometry is more complicated than simple benchmarking—the shape of the flow curve is subjectively evaluated by a pulmonologist, who examines the descending limb of the Flow vs. Volume curve (i.e., after PEF, Figure 2). A linear slope indicates the absence of airflow limitation (i.e., normal lung function, solid line in Figure 2). A concave or “scooped” slope signifies airflow limitation (e.g., asthma or COPD, purple dashed line in Figure 2) due to differing time constants of exhaled air in different parts of the lung. The last curve (orange dashed line in Figure 2) is suggestive of restrictive lung disease such as that caused by respiratory muscle weakness or pul-monary fibrosis; it can be seen as a slight bowing of the curve, a plateau, and/or a decreased FVC.