Acute oxygen supplementation does not relieve the impairment of respiratory muscle strength in hypoxemic COPD

Study objectives: To verify whether hypoxemia affects respiratory muscle strength in the absence of malnutrition and whether such effect, if present, is reversible with an acute oxygen supplementation. Design: Case series analysis, before-after trial. Setting: Outpatient pneumology departments of two university hospitals. Patients: One hundred twenty patients affected by COPD in stable conditions having actual to ideal body weight ratio of greater than or equal to 90%. Measurements and results: Maximal inspiratory pressure (MIP) was measured at functional residual capacity level in the whole sample of subjects and during oxygen supplementation in 58 patients having a PaO2 less than or equal to 60 mm Hg when breathing in room air. Predictors of MIP were assessed by a multivariate analysis. MIP values before and after oxygen supplementation were compared by a paired l test. MIP was independently correlated with FVC (p<0.001), PaO2 (p<0.01), and age (p<0.01). In the subgroup of hypoxemic patients, MIP values did not change significantly after oxygen supplementation (3.08+/-1.74 vs 3.03+/-1.91 kPa, t=0.43, not significant). Conclusions: Hypoxemia is an important negative correlate of MIP even in well-nourished COPD patients. Its effect is not reversible with an acute oxygen supplementation.