The First Second Timed vital capacity and the course of obstructive lung diseases.

Previous work has shown that with increasing airway obstruction in chronic obstructive pulmonary disease, the decrement in the maximum midexpiratory flow (MMEF) is exponential. Therefore, with severe obstructive disease the decrease in the MMEF as the disease progresses is difficult to discern. This work points out that the fall in first second timed vital capacity (FEV1/FVC x 100) is linear through the course of the disease and, therefore, the decrement, in absolute terms, continues unaltered regardless of how severe the obstruction becomes. It is suggested that the FEV1/FVC x 100 might be more useful in monitoring the course of severe disease than the MMEF. However, it is stressed that both measurements are useful for large groups, but may not be useful in individual cases.