Study objectives: We previously reported that patients with emphysema show an increase in diaphragmatic neuromechanical coupling at 3 months after lung volume reduction surgery. Diaphragmatic neuromechanical coupling was quantified as the quotient of tidal volume (normalized to total lung capacity) to tidal change in transdiaphragmatic pressure (normalized to maximal transdiaphragmatic pressure). As such, neuromechanical coupling estimates the fraction of diaphragmatic capacity used to generate tidal breathing. The present investigation was conducted to determine whether benefit is maintained at 2 years.
Subjects: Fifteen patients with severe COPD, 8 of whom completed the 2-year study.
Methods: Lung volumes, exercise capacity (6-min walking distance), diaphragmatic function (maximal transdiaphragmatic pressure and twitch transdiaphragmatic pressure elicited by phrenic nerve stimulation), and diaphragmatic neuromechanical coupling were recorded before surgery, and at 3 months and 2 years after surgery.
Results: Two years after surgery, lung volumes deteriorated to preoperative values, but patients showed persistent improvements in 6-min walking distance (p < 0.05). Three months after surgery, maximal transdiaphragmatic pressure (p < 0.05), twitch transdiaphragmatic pressure (p < 0.01), and diaphragmatic neuromechanical coupling (p < 0.01) had increased over preoperative values. The improvements in neuromechanical coupling resulted from improvements in diaphragmatic strength and, to a lesser extent, from a decrease in transdiaphragmatic pressure required to maintain tidal breathing. The change in respiratory muscle function at 2 years varied among patients: diaphragmatic contractility was > 10% of preoperative value in half of the patients who concluded our study, and neuromechanical coupling was > 10% of preoperative value in three fourths of the patients who concluded our study. Patients who maintained their gains in neuromechanical coupling also maintained their gains in 6-min walking distance.
Conclusion: Patients undergoing lung volume reduction surgery can maintain early gains in neuromechanical coupling and exercise capacity 2 years later.