Background: The changes in inspiratory capacity (IC) over time in chronic obstructive pulmonary disease (COPD) patients are unknown. The Understanding Potential Long-term Impacts on Function with Tiotropium (UPLIFT®) trial included IC measurements.
Methods: IC analysis from UPLIFT® (N = 5992) was performed at 1 and 6 months, and every 6 months through 4 years. Annualized rate of decline in pre- and post-bronchodilator IC and mean differences at each time point were analyzed by mixed-effects models. The relationships between baseline IC and exacerbation rate and mortality were explored using Cox regression analysis.
Results: Baseline characteristics: age, 65 years; 75% men; post-bronchodilator forced expiratory volume in 1 second, 1.32 L (48% predicted); pre- and post-bronchodilator IC, 2.03 and 2.33 L. Mean IC rate of decline (mL/year) was 34 ± 2 (1.7% of baseline) and 50 ± 3 (2.1% of baseline) pre- and post-bronchodilator, respectively, without significant between-group differences. Morning pre-bronchodilator (trough) IC improved with tiotropium versus placebo: 124 mL (1 month), 103 mL (1 year), 107 mL (2 years), 98 mL (3 years), and 97 mL (4 years) (all p < 0.001). Post-bronchodilator improvements were similar between treatment groups. Lower baseline IC values were associated with reduced time to first exacerbation. For the lowest quartile (n = 1413) the values in months were 14.3 (11.7-17.0) for tiotropium and 10.3 (8.8-11.7) for controls (p < 0.01).
Conclusion: IC declines from approximately 34 to 50 mL/year in patients with stage II to IV COPD. Tiotropium treatment does not change the IC decline rate but provides 24-hour improvements in IC sustained over the long term. Trough IC differences suggest that tiotropium provides sustained decrease in end-expiratory lung volume.