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.