FORCED VITAL CAPACITY, SLOW VITAL CAPACITY, OR INSPIRATORY VITAL CAPACITY - WHICH IS THE BEST MEASURE OF VITAL CAPACITY

Vital capacity can be measured as forced vital capacity (FVC), slow vi tal capacity (SVC), and inspiratory vital capacity (IVC). Although it is well known that the latter two are generally greater, a systematic comparison of the three in subjects with different degrees of airways obstruction has not been made. Sixty asthmatics and 20 normal subjects performed maneuvers for measurement of FVC, SVC, and IVC on a dry, ro lling-seal spirometer. The severity of airways obstruction in asthmati cs was classified as mild, moderate, and severe. There was no signific ant difference between FVC, SVC, and IVC in normal subjects. However, the three measurements of vital capacity were significantly different in all subgroups of asthmatics. FVC was smaller than both SVC and IVC. The differences were more marked in patients with moderate and severe degrees of airways obstruction.The differences between SVC and IVC we re small and clinically not important. Forced expiratory volume in 1 s ec (FEV1) expressed as percent of FVC, SVC, and IVC, was not different in normals and asthmatics with mild airways obstruction. The ratios w ere significantly different in asthmatics with moderate and severe air ways obstruction. FEV1/IVC ratio was the lowest in both the groups fol lowed by FEV1/SVC and FEV1/FVC. IVC and SVC are greater than FVC in pa tients with airways obstruction. This difference increases as the degr ee of obstruction increases. The difference between SVC or IVC and FVC serves as an indicator of air trapping. Both FVC and IVC could be mea sured and the largest VC used to calculate the FEV1/VC ratio because t his increases the sensitivity of spirometry in detecting airways obstr uction.