The Value of Measuring Inspiratory Capacity in Subjects With Cystic Fibrosis

BACKGROUND: Inspiratory capacity (IC) is often overlooked as an important measured index of spirometry in patients with cystic fibrosis (CF). Abnormally low IC may indicate the onset of static/dynamic hyperinflation, which may be accompanied by dyspnea and an increase in the work of breathing. This cross-sectional study sought to determine whether measuring IC during spirometry, may add clinical value to FEV₁ measurements in CF subjects.

METHODS: Anthropometric, clinical, spirometry, and static lung volume data were gathered retrospectively from 98 of 165 subjects with CF (mean ± SD age 26.8 ± 11.0 y) registered in The Edmond and Lily Safra Children's Hospital, Sheba Medical Centre, Israel. We compared the IC (% predicted) to FEV₁, static lung volumes, and hospitalization days/year.

RESULTS: IC decreased alongside FEV₁ decline but at a slower pace (r² = 0.32). Incremental trapped air, as measured by residual volume (RV), and a rapid elevation in the ratio of RV to total lung capacity occurred when IC deteriorated below 60% predicted values. The unique combination of IC < 50% predicted and FEV₁ > 40% predicted induced an increase of up to 125 hospitalization days/year compared to subjects having IC > 50% predicted (up to 73 d/y, P < .001).

CONCLUSIONS: Measuring IC in CF subjects may reveal silent worsening of lung function as indicated by a decline in IC < 50% predicted while FEV₁ is still > 40% predicted. This condition may lead to inefficient breathing at high lung volumes, which may explain a subjective sensation of breathlessness and lead to an increase in hospitalization days/year.