Inspiratory muscle dysfunction mediates and predicts a disease continuum of hypercapnic failure in chronic obstructive pulmonary disease

Introduction Advanced chronic obstructive pulmonary disease (COPD) is associated with chronic hypercapnic failure. The present work aimed to comprehensively investigate inspiratory muscle function as a potential key determinant of hypercapnic respiratory failure in patients with COPD. Methods Prospective patient recruitment encompassed 61 stable subjects with COPD across different stages of respiratory failure ranging from normocapnia to isolated nighttime hypercapnia and daytime hypercapnia. Arterialized blood gas analyses and overnight transcutaneous capnometry were used for patient stratification. Assessment of respiratory muscle function encompassed bodyplethysmography, maximum inspiratory pressure (MIP), diaphragm ultrasound, and transdiaphragmatic pressure recordings following cervical magnetic stimulation of the phrenic nerves (twPdi) and a maximum sniff manoeuvre (Sniff Pdi). Results Twenty patients showed no hypercapnia, 10 had isolated nocturnal hypercapnia, and 31 daytime hypercapnia. Bodyplethysmography clearly distinguished patients with and without hypercapnia, but did not discriminate patients with isolated nocturnal hypercapnia from those with daytime hypercapnia. In contrast to ultrasound parameters and transdiaphragmatic pressures only MIP reflected the extent of hypercapnia across all three stages. MIP values below -48 cmH2O predicted nocturnal hypercapnia (area under the curve=0.733, p=0.052). Conclusion In COPD, inspiratory muscle dysfunction contributes to progressive hypercapnic failure. In contrast to invasive tests of diaphragm strength only MIP fully reflects the pathophysiological continuum of hypercapnic failure and predicts isolated nocturnal hypercapnia.