Incomplete forced expiration - estimating vital capacity by a mathematical method.

BACKGROUND Vital capacity is a key parameter in the determination of lung function, usually assessed by means of a forced expiration maneuver. This maneuver can be exhausting, and patients often cannot complete it. OBJECTIVES This study evaluates a method to estimate forced vital capacity (FVC) based on the extrapolation of volume-time curves from forced expiration. METHODS The algorithm was applied to 2,363 volume-time curves from patients with and without respiratory disease. 416 of these spirograms originated from incomplete maneuvers. For each spirogram, estimated (FVC(est)) and measured FVC were compared with inspiratory vital capacity. RESULTS Reliable FVC(est) were obtained for 82% of all and for 76% of the incomplete maneuvers. Regardless of the category of respiratory disease and acceptability of forced expiration, FVC(est) were close to inspiratory vital capacities. CONCLUSIONS When assessing the lung function of patients who cannot complete forced expiration, this method could help to reduce the duration of maneuvers required to provide a reliable estimate for vital capacity.