Routine spirometry, obtained with the patients seated in a comfortable armchair, was measured as previously described. 13,16,22 Functional residual capacity (FRC) was measured by the helium dilution technique. The normal values for lung volumes were those of the European Community for Coal and Steel. 23 Arterial blood gas tensions were measured with a blood gas analyzer (IL-1304; Instrumentation Laboratory; Milan, Italy).

Protocol All subjects were tested in the morning. Before the experiment, the subjects were well acquainted with the laboratory and equipment. An arterial blood sample and lung function tests were performed, and then changes in volume, flow, and Ppl were recorded. Finally, the respiratory muscle strength tests were performed in each patient.

Data Analysis
Volume and time components of the respiratory cycle, RL, and Eldyn were averaged in each patient over 30 consecutive breaths. Eldyn was expressed both as an actual value and as a percent of Pplsn, an index of the balance between the elastic load of the lung relative to the maximal inspiratory force available. Single and stepwise multiple regression analyses were performed to assess relationships between variables. The statistical analysis we carried out considers the dependency of a variable (eg, the level of PaCO₂) on a series of independent variables. The effect of each variable on PaCO₂ was evaluated independent of the effect of all other variables. In a multivariate analysis, a rule of thumb is to limit the number of variables as a function of the number of patients.28 Thus, multiple regression analysis with stepwise selection of the independent variables was carried out relating PaCO₂ to functional variables. The proportion of total variance in the dependent variable accounted for by the predicted variables is reported as the square of the correlation coefficient (r²). Single regression analysis was performed using Pearson’s single correlation coefficient. Comparisons between groups were made using the Mann-Whitney U test. A value of p < 0.05 was considered as the threshold of statistical significance. Data are presented as mean ± SD unless otherwise specified. .

Clinical, anthropometric, and respiratory function characteristics of the patients (and control subjects) are shown in Table 1. As shown in Table 1, vital capacity (VC) was reduced in 11 patients, as was total lung capacity in 9. The means of MIP (47.8 ± 28.3 cm H2O; range, 11 to 127 cm H2O; p = 0.00001) and MEP (49.5 ± 26.2 cm H2O; range, 15 to 104 cm H2O; p = 0.00002) were significantly lower than in control subjects. In 11 patients and 10 patients, the values of MIP and MEP, respectively, were lower than the mean – 2 SD of the value calculated for the control subjects. Arterial blood gases were normal in all but eight patients in whom PaCO2 was considered to be high (>= 45 mm Hg) and in three patients in whom PaCO2 was low (< 80 mm Hg). In some of the patients, and particularly in patients 12 and 14, in whom PaCO2 was normal, a high level of ventilation was found.

We have found that in patients with NMD, Eldyn (%Pplsn) is the strongest predictor of the variance in PaCO2. Increased Eldyn (%Pplsn) was associated with a decreased TI, which truncates VT, thereby leading to chronic CO2 retention (PaCO2).