Chest
Volume 119, Issue 4, April 2001, Pages 1108-1113
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Clinical Investigations: Neuromuscular Disease
Assessing Inspiratory Muscle Strength in Patients With Neurologic and Neuromuscular Diseases: Comparative Evaluation of Two Noninvasive Techniques

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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 better index of inspiratory muscle strength than MIP in patients with neurologic and neuromuscular diseases (NNMD) with and without inspiratory muscle weakness.

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 measured 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) or higher (in 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 Pmosn, respectively) and in female subjects (27.5% vs 26.2%, respectively). There was no difference in the Pmosn/MIP ratios observed in the different diseases (one-way analysis of variance, F = 0.29, p = 0.91). In control subjects, 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 Pmosn/MIP ratio, suggesting an increasing difficulty in performing MIP as MIP values decreased. The majority of patients were between the prediction limits of the regression calculated for control subjects. At variance, patients with Duchenne dystrophy and low MIP were under the prediction limits of the regression calculated for control subjects, indicating a lower-than-expected Pmosn.

Conclusions

In patients with NNMD, irrespective of the etiology, we found the following: (1) Pmosn does not overcome the limitations of MIP measurement; (2) the two maneuvers are not interchangeable, but rather complement one another in the assessment of inspiratory muscle strength; (3) Pmosn may underestimate muscle strength as assessed by MIP in patients with NNMD with inspiratory muscle weakness; and (4) in patients with low MIP, the lower-than-expected Pmosn/MIP ratio confirms inspiratory muscle weakness.

Section snippets

Subjects

We studied 30 white patients (21 men [age range, 14 to 59 years] and 9 women [age range, 22 to 72 years]) with a number of NNMD (Table 1) referred to our laboratory because of suspected respiratory muscle weakness, and a group of 41 age-matched and sex-matched normal subjects (23 men [age range, 15 to 51 years] and 18 women [age range, 16 to 52 years]). In patients with quadriplegia, a high-level spinal lesion (C3 to C6) was found; spasticity was also present in quadriplegics and in multiple

Results

Demographic, clinical, and pulmonary function data, and Pmosn values of the 30 patients and means of control subjects are shown in Table 1. The values of MIP and Pmosn in male and female control subjects are shown in Table 2. In the control subjects as a whole, MIP tended to be higher than Pmosn (p < 0.06). In male control subjects (130.8 ± 25.3 cm H2O) but not in female control subjects (86.7 ± 23.8 cm H2O), MIP was significantly higher than Pmosn (110.0 ± 21.1 cm H2O in male subjects, 92.8 ±

Discussion

The main findings of the present study can be summarized as follows: (1) in control subjects, the variation coefficients for MIP and Pmosn were similar, and the Pmosn/MIP ratio was inversely related to MIP; and (2) the majority of patients were between the prediction limits calculated for control subjects, and most patients with Duchenne dystrophy were below them.

Before commenting on these findings, it is important to raise some methodologic considerations. First, differences in training and

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