Thoracic gas compression during forced expiration is greater in men than women

Intrapleural pressure during a forced vital capacity (VC) manoeuvre is often in excess of that required to generate maximal expiratory airflow. This excess pressure compresses alveolar gas (i.e., thoracic gas compression; TGC), resulting in underestimated forced expiratory flows (FEFs) at a given lung volume. It is unknown if TGC is influenced by sex; however, because men have larger lungs and stronger respiratory muscles, we hypothesized that men would have greater TGC. We examined TGC across the "effort-dependent" region of VC in healthy young men (n=11) and women (n=12). Subjects performed VC manoeuvres at varying efforts while airflow, volume, and oesophageal pressure (P_OES) were measured. Quasi-static expiratory deflation curves were used to obtain lung recoil (P_LUNG) and alveolar pressures (i.e., P_ALV=P_OES–P_LUNG). The raw maximal expiratory flow-volume (MEFV_raw) curve was obtained from the "maximum effort" VC manoeuvre. The TGC-corrected curve was obtained by constructing a "maximal perimeter" curve from all VC efforts (MEFV_corr). TGC was examined via differences between curves in FEFs (∆FEF), area under the expiratory curves (∆A_EX), and estimated compressed gas volume (∆VGC) across the VC range. Men displayed greater total ∆A_EX (5.4±2.0 vs. 2.0±1.5 L²∙s^-1; p<0.001). ∆FEF was greater in men at 25% of exhaled volume only (p<0.05), whereas ∆VGC was systematically greater in men across the entire VC (main effect; p<0.05). P_ALV was also greater in men throughout forced expiration (p<0.01). Taken together, these findings demonstrate that men display more TGC, occurring early in forced expiration, likely due to greater expiratory pressures throughout the forced VC manoeuvre.