Differences Between Slow and Fast Vital Capacities in Patients with Obstructive Disease

Abstract

From 200 consecutive patients with obstructive lung disease who were undergoing clinical lung function testing, we selected 15 in whom the slow vital capacity (SVC) was at least 0.35 L larger than the forced vital capacity (FVC). In 11 of these patients, this difference was not accountable in terms of the duration of the expiratory maneuver, though both FVC and SVC were dynamically determined in that expiratory flow rates did not attain zero for 1 second before either SVC or FVC were terminated by the subsequent inspiration. The 11 patients were a heterogeneous group, both in terms of diagnosis (4 had asthma) and severity of obstruction. In all patients at low lung volumes, expiratory flow rates were greater during SVC than FVC. The most likely explanation for these findings was the development of airway closure at higher lung volumes during FVC than during SVC, but we were unable to demonstrate this. Bronchodilator increased both SVC and FVC but did not change the difference between them. In six patients, external resistances were employed to see if differences between SVC and FVC were flow or effort dependent: in two, the difference was flow dependent while in four it was effort dependent We conclude that reproducible time independent differences between SVC and FVC occasionally occur in patients with obstructive disease but their mechanism is unclear.