Another useful clinical pulmonary test, and one that is also simple, is to record on a spirometer the forced expiratory vital capacity (FVC). Such a recording is shown in Figure 1A for a person with normal lungs and in Figure 1B for a person with partial airway obstruction. In performing the FVC maneuver, the person first inspires maximally to the TLC and then exhales into the spirometer with maximum expiratory effort as rapidly and as completely as possible. The total distance of the down slope of the lung volume record represents the FVC, as shown in the figure.
Fig1. Recordings during the forced vital capacity maneuver in a healthy person (A) and in a person with partial airway obstruction (B). (The “zero” on the volume scale is residual volume.) FEV1, forced expiratory volume during the first second; FVC, forced expiratory vital capacity.
Now, study the difference between the two records for (1) normal lungs and (2) partial airway obstruction. The total volume changes of the FVCs are not greatly different, indicating only a moderate difference in basic lung volumes in the two persons. There is, however, a major difference in the amounts of air that these persons can expire each second, especially during the first second. Therefore, it is customary to compare the recorded forced expiratory volume during the first second (FEV1) with the normal. In the normal person (see Figure 1A), the percentage of the FVC that is expired in the first second divided by the total FVC (FEV1/FVC%) is 80 percent. However, note in Figure 1B that, with airway obstruction, this value decreased to only 47 percent. In persons with serious airway obstruction, as often occurs with acute asthma, this value can decrease to less than 20 percent.
