Assessing inspiratory muscle strength in patients with neurologic and neuromuscular diseases - Comparative evaluation of two noninvasive techniques

Study objectives: Static mouth pressure during maximal inspiratory efforts is commonly used to evaluate inspiratory muscle strength. However, maximal inspiratory pressure (MIP) presents some potential limitations likely to be overcome by the measure of mouth pressure during a maximal sniff maneuver in patients with respiratory muscle weakness. The aim of the present study was to assess whether mouth pressure during sniff maneuver (Pmosn) is a bet ter index of inspiratory muscle strength than MIP in patients with neurolog ic and neuromuscular diseases (NNMD) with and without inspiratory muscle we akness. Subjects and measurements: Both MIP and Pmosn were measured in 30 patients affected by various types of NNMD and in 41 control subjects. Pmosn was mea sured with a 5-cm latex balloon-catheter system, the balloon being held in the oral cavity with the lips closed. Results: In control subjects, MIP was either similar (in female subjects) o r higher tin male subjects) than Pmosn, the variation coefficients for the two tests being similar both in male subjects (19.3% vs 19.1% for MIP and P mosn, respectively) and in female subjects (27.5% vs 20.2%, respectively). There was no difference in the Pmosn/MIP ratios observed in the different d iseases tone-way analysis of variance, F = 0.29, p = 0.91). In control subj ects, a significant inverse relationship between Pmosn/MIP ratio and MIP (r = - 0.66, p < 0.00001) was found ie, the lower the MIP, the higher the Pmo sn/MIB ratio, suggesting an increasing difficulty in performing MIP as MIP values decreased, The majority of patients were between the prediction limi ts of the regression calculated for control subjects. At variance, patients with Duchenne dystrophy and low MIP were under the prediction limits of th e regression calculated for control subjects, indicating a lower-than-expec ted Pmosn. Conclusions: In patients with NNMD, irrespective of the etiology, we found the following: (1) Pmosn does not overcome the limitations of MIP measureme nt; (2) the two maneuvers are not interchangeable, but rather complement on e another in the assessment of inspiratory muscle strength; (3) Pmosn may u nderestimate muscle strength as assessed by MIP in patients with NNMD with inspiratory muscle weakness; and (4) in patients with low MIP, the lower-th an-expected Pmosn/MIP ratio confirms inspiratory muscle weakness.