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Altered thoracic gas compression contributes to improvement in spirometry with lung volume reduction surgery

Michael E DeBakey Veterans Affairs Medical Center, Baylor College of Medicine, Houston, TX 77030, USA

Correspondence to:
Correspondence to:
Dr A Sharafkhaneh
Assistant Professor of Medicine, Baylor College of Medicine, MED VAMC, 2002 Holcombe Blvd, Houston, Texas 77030, USA; amirs{at}bcm.tmc.edu

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 (TP_peak) during the forced manoeuvre significantly predicted the reduction in TGC following the surgery (Rle: p<0.01; TP_peak: p<0.0001; adjusted R² = 0.68). The improvement in FEV₁ was associated with the reduction in TGC after surgery (p<0.0001, adjusted R² = 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.

Abbreviations: FEV₁, forced expiratory volume in 1 second; FVC, forced vital capacity; LVRS, lung volume reduction surgery; NFEV₁, FEV₁ obtained using no compression method; absolute DFEV₁, NFEV₁ – FEV₁; percentage DFEV₁, absolute DFEV₁/FEV₁; PEF, peak expiratory flow; P_el, elastic recoil pressure at TLC/TLC; Rle, expiratory lung resistance; Rli, inspiratory lung resistance; RV, residual volume; TGC, thoracic gas compression; TLC, total lung capacity; TP_peak, peak transpulmonary pressure during a forced expiratory manoeuvre

Keywords: lung volume reduction surgery; thoracic gas compression; chronic obstructive pulmonary disease; lung mechanics

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