Forced Expiratory Vital Capacity and Forced Expiratory Volume
Another exceedingly useful clinical pulmonary test, and one that is also simple, is to make a record on a spirometer of the forced expiratory vital capacity(FVC). Such a record is shown in Figure 42–3A for aperson with normal lungs and in Figure 42–3B for a person with partial airway obstruction. In performing the FVC maneuver, the person first inspires maximally to the total lung capacity, then exhales into the
spirometer with maximum expiratory effort as rapidly and as completely as possible. The total distance of the downslope of the lung volume record represents the FVC, as shown in the figure.
Now, study the difference between the two records(1) for normal lungs and (2) for partial airway obstruc-tion. 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 thatthese persons can expire each second, especially duringthe 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 42–3A), the percentage of the FVC that is expired in the first second divided by the total FVC (FEV1/FVC%) is 80 per cent. However, note in Figure 42–3B that, with airway obstruction, this value decreased to only 47 per cent. In serious airway obstruction, as often occurs in acute asthma, this can decrease to less than 20 per cent.