EFFECT OF LUNG-VOLUME REDUCTION SURGERY ON NEUROMECHANICAL COUPLING OF THE DIAPHRAGM

The mechanisms for symptomatic improvement following lung volume reduc tion surgery for emphysema are poorly understood. We hypothesized that enhanced neuromechanical coupling of the diaphragm is an important fa ctor in this improvement. We studied seven patients with diffuse emphy sema before and 3 mo after surgery. Patients showed improvements in 6- min walking distance (p = 0.002) and dyspnea (p = 0.04). The pressure output of the respiratory muscles, quantified as pressure-time product per minute (PTP/min), decreased after surgery (p = 0.03), as did Pa-C O2 (p = 0.02). Maximal transdiaphragmatic pressures (Pdi(max)) increas ed from 80.3 +/- 9.5 (SE) to 110.8 +/- 9.3 cm H2O after surgery (p = 0 .03), and the twitch transdiaphragmatic pressure response to phrenic n erve stimulation (Pdi(tw)) increased from 17.2 +/- 2.4 to 25.9 +/- 3.0 cm H2O (p = 0.02); these increases were greater than could be account ed for by a decrease in lung volume. The contribution of the diaphragm to tidal breathing, assessed by relative changes in gastric and trans diaphragmatic pressures, increased after surgery (p = 0.008). Net diap hragmatic neuromechanical coupling, quantified as the quotient of tida l volume (normalized to total lung capacity) to tidal change in Pdi (n ormalized to Pdi(max)), improved after surgery (p = 0.03) and was rela ted to the increase in 6-min walking distance (r = 0.86, p = 0.03) and decrease in dyspnea (r = 0.76, p = 0.08). In conclusion, lung volume reduction surgery effects an improvement in diaphragmatic function, gr eater than can be accounted for by a decrease in operating lung volume , and enhances diaphragmatic neuromechanical coupling.