SVC vs. FVC in FEV1/FVC Ratio: A Comparison

The forced maneuver used in forced vital capacity (FVC) could underreport the slow vital capacity (SVC) as a result of early closure of the small airways at low lung volumes. The use of SVC as opposed to FVC in the forced expiratory volume at 1 second (FEV₁)/VC-ratio is unexplored, so it is uncertain what the outcome would be: an increased yield of spirometry in detecting airflow limitation or a false-positive obstruction result. A new study sought to determine what the outcome would be in different patient populations. A total of 13,893 adult outpatients with FEV₁/FVC and total lung capacity at or above the lower limit of normal were evaluated for the study. Clinical variables were used to define the probability of airway disease, and for dysfunction, physiological variables were used. Discordance was defined as preserved FEV₁/FVC but low FEV₁/SVC and was present in 20.4% of patients. Discordant patients, compared to concordant ones, had lower mid-expiratory flows, higher airway resistance, worse gas trapping, and ventilation distribution abnormalities. When controlling for variables including sex, age, body mass index, and FEV₁, more than 70% predicted were associated with discordance. Discordant also patients had significantly greater odds of receiving an obstructive airway disease diagnosis compared to patients with preserved FEV₁/FVC and FEV₁/SVC, with the only exception being discordant patients aged > 70 years, of whom only 10% received an airway disease diagnosis. Read more