Is the Slow Vital Capacity Clinically Useful to Uncover Airflow Limitation in Subjects With Preserved FEV/FVC Ratio?

Background: FVC may underestimate the slow vital capacity (SVC) due to early closure of the small airways at low lung volumes in the forced maneuver. It remains unclear whether using SVC instead of FVC in the FEV/vital capacity (VC) ratio increases the yield of spirometry in detecting airflow limitation or, alternatively, leads to a false-positive finding for obstruction.

Methods: This study included 13,893 adult outpatients with FEV/FVC and total lung capacity at or above the lower limit of normal. A cluster of clinical and physiological variables defined the probability of airway disease and dysfunction, respectively.

Results: The prevalence of "discordance" (preserved FEV/FVC but low FEV/SVC) was 20.4%: discordant subjects had lower mid-expiratory flows, higher airway resistance, worse gas trapping, and ventilation distribution abnormalities than "concordant" subjects (both ratios preserved) (P < .05). Regardless of sex, age < 60 years, BMI > 30 kg/m, and FEV > 70% predicted were associated with discordance (P < .001). Discordant subjects with preserved FEV/FVC but low FEV/SVC were four times more likely to be diagnosed with an obstructive airway disease by a respirologist compared with those with preserved FEV/SVC and FEV/FVC. The only exception was in the elderly subgroup with discordance (age > 70 years): only 10% of these subjects were subsequently diagnosed with an airway disease (P > .05).

Conclusions: Using SVC instead of FVC in the FEV/VC ratio enhances the yield of spirometry in detecting mild airflow obstruction in younger and obese subjects. The FEV/SVC ratio, however, should be used with caution in elderly subjects with preserved FEV/FVC because a low value may represent a false-positive finding for airflow limitation.