Functionally relevant threshold of inspiratory muscle strength in patients with chronic obstructive pulmonary disease.

OBJECTIVES: We aimed to identify the inspiratory muscle strength thresholds below which exercise capacity is compromised in men with chronic obstructive pulmonary disease (COPD). METHODS: We measured the maximum static inspiratory mouth pressure (PImax) and the percentage of predicted values (%PImax) in 113 patients with COPD. Six-minute walk distance (6-MWD) was measured as an indicator of functional exercise capacity, and a 6-MWD of <350 m was defined as functional exercise intolerance. Thresholds were determined as values with high specificity (>0.90) and maximal sensitivity. Statistical significance was set at P < 0.01. RESULTS: The data of 96 patients (74 ± 6 years old; forced expiratory volume in 1-s [FEV1], 56.5 ± 26.2% predicted) were analyzed; three women and 14 participants with missing data were excluded. Multivariate logistic regression models identified significant associations of PImax (odds ratio at 99% confidence interval [CI]: 0.95 [0.92-0.98]) and %PImax (odds ratio at 99% CI: 0.97 [0.95-0.99]) with 6-MWD, after adjustments for height and FEV1. C-statistics showed that the area under the curves (99% CI) of PImax and %PImax were comparable (0.87 [0.77-0.96] and 0.83 [0.72-0.94]). The thresholds of PImax and %PImax were 45.1 cmH2O and 66%; PImax and %PImax also had moderate positive likelihood ratios of 4.44 and 5.00. CONCLUSIONS: Thresholds of inspiratory muscle strength in men with COPD could help clinicians evaluate whether their patient's inspiratory muscle strength is inadequate to achieve a 6-MWD of ≥350 m, and identify patients who should be targeted for inspiratory muscle training.