Respiratory mechanics in the open chest: effects of parietal pleurae

To understand how the parietal pleurae affect the mechanical behavior of the human respiratory system after the chest wall is opened by median sternotomy, we studied 18 anesthetized/paralyzed patients immediately before coronary artery bypass grafting surgery. Elastances and resistances of the total respiratory system (ETr, Rrs) were calculated from measurements of airway pressure and flow during mechanical ventilation in the frequency and tidal volume ranges of normal breathing. Elastances and resistances of the lungs (EL, RL), chest wall (Ecw, Rcw) were also estimated from measurements of esophageal pressure. Data were collected in the closed chest, after median sternotomy with the parietal pleurae intact and after the left parietal pleura was opened for internal mammary artery harvest. After sternotomy with pleurae intact (n = 14), Ers did not change but Rrs decreased (p < 0.05). Ecw (including the contribution of the pleurae) was higher than in the closed chest (p < 0.05) while EL and RL were lower (p < 0.05); Rcw did not change. Opening the left pleura (n = 10) decreased Ers (p < 0.05), but Rrs did not change. We conclude that the chest wall/pleurae compartment offers significant impedance to lung expansion after sternotomy and rib retraction, unless one pleura is opened.