CHRONIC OBSTRUCTIVE PULMONARY DISEASE Altered thoracic gas compression contributes to improvement in spirometry with lung volume reduction surgery

Background: Thoracic gas compression (TGC) exerts a negative effect on forced expiratory flow. Lung resistance, effort during a forced expiratory manoeuvre, and absolute lung volume influence TGC. Lung volume reduction surgery (LVRS) reduces lung resistance and absolute lung volume. LVRS may therefore reduce TGC, and such a reduction might explain in part the improvement in forced expiratory flow with the surgery. A study was conducted to determine the effect of LVRS on TGC and the extent to which reduced TGC contributed to an improvement in forced expiratory volume in 1 second (FEV1) following LVRS. Methods: The effect of LVRS on TGC was studied using prospectively collected lung mechanics data from 27 subjects with severe emphysema. Several parameters including FEV1, expiratory and inspiratory lung resistance (Rle and Rli), and lung volumes were measured at baseline and 6 months after surgery. Effort during the forced manoeuvre was measured using transpulmonary pressure. A novel method was used to estimate FEV1 corrected for the effect of TGC. Results: At baseline the FEV1 corrected for gas compression (NFEV1) was significantly higher than FEV1 (p,0.0001). FEV1 increased significantly from baseline (p,0.005) while NFEV1 did not change following surgery (p.0.15). TGC decreased significantly with LVRS (p,0.05). Rle and maximum transpulmonary pressure (TPpeak) during the forced manoeuvre significantly predicted the reduction in TGC following the surgery (Rle: p,0.01; TPpeak: p,0.0001; adjusted R 2 = 0.68). The improvement in FEV1 was associated with the reduction in TGC after surgery (p,0.0001, adjusted R = 0.58). Conclusions: LVRS decreased TGC by improving expiratory flow limitation. In turn, the reduction in TGC decreased its negative effect on expiratory flow and therefore explained, in part, the improvement in FEV1 with LVRS in this cohort.