Chest
Volume 130, Issue 6, December 2006, Pages 1834-1838
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Original Research: Prolonged Mechanical Ventilation
Effect of Home Mechanical Ventilation on Inspiratory Muscle Strength in COPD

https://doi.org/10.1378/chest.130.6.1834Get rights and content

Background

The mechanism responsible for chronic hypercapnic respiratory failure (HRF) in patients with COPD remains unclear. In this study, we tested the hypothesis that chronic HRF in patients with COPD is associated with low-frequency fatigue (LFF) of the diaphragm.

Methods

To test this hypothesis, we measured the twitch transdiaphragmatic pressure (Tw Pdi) elicited by stimulation of the phrenic nerves in 25 patients with chronic HRF (mean [± SD] Paco2, 55.2 ± 5.2 mm Hg) due to COPD before and 2 months after the initiation of noninvasive mechanical ventilation (NIV) [pressure-cycled ventilation with inspiratory positive airway pressure of 19.0 ± 2.5 cm H2O]. We reasoned that had LFF been present, Tw Pdi should rise after effective NIV.

Results

The treatment compliance with NIV was good (median of machine usage was 7.1 h per night). Paco2 decreased from 55.2 ± 5.2 to 48.8 ± 5.9 mm Hg (p < 0.001), and Pao2 increased from 53.1 ± 5.9 to 57.7 ± 7.0 mm Hg (p = 0.007). Mean Tw Pdi at baseline was 11.1 ± 6.6 cm H2O and after treatment was 11.7 ± 7.2 cm H2O (not significant). Also, maximal static inspiratory mouth pressure did not change significantly (44.3 ± 15.9 cm H2O vs 46.5 ± 19.7 cm H2O).

Conclusion

LFF of the diaphragm does not accompany chronic HRF in patients with COPD.

Section snippets

Patients

Thirty-four consecutive patients with COPD and chronic HRF electively admitted for establishment of NIV were enrolled in this study. Nine patients, however, dropped out during the adaptation period because they could not tolerate NIV. Since the study was conceived to test physiologic rather than clinical outcomes, data from these subjects were not further analyzed. Thus, this report contains the data of the remaining 25 patients (14 men; mean [± SD] body mass index, 22.5 ± 4.7 kg/m2; age, 60.0

Results

Blood gas and lung function data are presented in Table 1. All patients had chronic HRF with daytime hypercapnia and hypoxemia. Furthermore, vital capacity and FEV1 were reduced in keeping with the diagnosis of severe COPD. No change in FRC (measured as thoracic gas volume) was observed after NIV (Table 1).

Data from the hours counter on the machines used showed that the NIV patients had used the machines for a median of 7.1 h per night, reflecting good compliance. While by study design all

Discussion

The main finding of this study is that in patients with chronic HRF due to severe COPD, successful application of NIV does not result in an increase in Tw Pdi. This suggests that low-frequency diaphragm fatigue is not present in such patients prior to therapy. Discussion of the significance of these data will follow a critique of the methods.

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      First, it has been shown that the respiratory muscles of hypercapnic COPD patients work hard but are not fatigued; they seem to act as wise fighters, thereby deliberately keeping respiratory muscle work below the fatigue threshold at the expense of decreased tidal volumes and thus alveolar hypoventilation.13 Second, most studies on NIV in COPD have not shown any effects on maximal respiratory muscle pressures independent from changes in lung volumes,14 arguing against the hypothesis that if the muscles are rested, they should gain (reserve) capacity. As a consequence, instead of resting fatigued respiratory muscles, NIV might, among other things, lead to a decrease in respiratory muscle work through the induction of a more favorable breathing pattern, a pattern that can be maintained during the day.

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    The authors have no financial or other potential conflicts of interest to disclose.

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