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FVC to Slow Inspiratory Vital Capacity Ratio^*

A Potential Marker for Small Airways Obstruction

^* From the Departments of Pulmonology (Drs. Cohen, Postma, ten Hacken, Koëter, and Douma), Pulmonary Function (Ms. Vink-Klooster), and Lung Transplantation (Drs. van der Bij and Verschuuren), University Medical Center Groningen, University of Groningen, Groningen, the Netherlands.

Correspondence to: W. Rob Douma, MD, PhD, Secretariaat Longziekten UMCG, PO Box 30001, 9700 RB Groningen, the Netherlands; e-mail: w.r.douma{at}int.umcg.nl

Abstract

Background: The ratio of FVC to slow inspiratory vital capacity (SVC) has been reported to reflect small airways obstruction, but its validity as such is still unclear. The aim of this study was to assess the applicability of the FVC/SVC ratio as a marker of small airways function in patients with bronchiolitis obliterans syndrome (BOS) after lung transplantation (LTX), which is a disorder in which predominantly small airways obstruction causes progressive airflow limitation.

Methods: The FVC/SVC ratio was analyzed both cross-sectionally and longitudinally in 39 patients (26 men) with BOS after bilateral LTX (median age, 47 years; interquartile range [IQR], 35 to 54 years), and 36 bilateral lung transplant recipients without BOS (14 men; median age, 46 years; IQR, 41 to 53 years).

Results: The FVC/SVC ratio decreased significantly during follow-up in patients with BOS stages 1 and 2, by 2.2% and 4.4%, respectively, from baseline (p < 0.001). This decrease was not significantly associated with the decrease in FEV₁. The FVC/SVC ratio increased, though not significantly, in the group in which BOS did not develop by 1.1%, which is a significant difference from the average fall of 4.4% in the group in which BOS developed.

Conclusions: Significant, yet small decreases in FVC/SVC ratio occur in patients in whom BOS develops, independent from changes in FEV₁. At a group level, FVC/SVC ratio is able to detect small airways changes. These results merit prospective studies to determine the sensitivity of FVC/SVC ratio to quantifying small airways dysfunction at an individual level and in other airway diseases.

Key Words: FVC • inspiratory vital capacity • small airways obstruction • spirometry