Fig.2.

Airway opening, esophageal (P_eso) and transpulmonary pressures (P_L) measurements. P_L is defined as the difference between airway opening pressure (blue lines) and pleural pressure. Pleural pressure is frequently estimated from esophageal balloon pressure measurements (P_eso). Using a specific protocol, the esophageal balloon is placed in the lower third of the esophagus (2A). Cardiac oscillations in P_eso (2B, green lines) indicate accurate placement of the balloon, which can be confirmed by observation of similar airway pressure and P_eso measurements as gentle chest compressions are performed during expiratory pause or with occluded airway opening (2A). P_L can be estimated as the difference between airway and esophageal pressures (red and orange lines). Interventions such as pneumoperitoneum (2B, mid panel) produce a marked change in driving pressures (ΔP = plateau pressure, P_Plat, minus positive end-expiratory pressure, PEEP). In this example, ΔP increased by 7 cmH₂O. Yet, delta P_L (end-inspiratory P_L, P_L EI, minus end-expiratory P_L, P_L EE) does not increase to the same degree as ΔP and P_Plat. The change in delta P_L in this example was 4 cm H₂O. This demonstrates that part of the increase in ΔP and P_Plat are due to the chest wall component and not to pressures applied to the lung parenchyma. This contribution of the chest wall is evidenced by the increased end-inspiratory (EI) to end-expiratory (EE) oscillation in P_eso after as compared to before pneumoperitoneum. In addition, the esophageal pressure at end-expiration (P_eso EE, at ~4 seconds on time scale) is positive before pneumoperitoneum while it is negative after pneumoperitoneum. This implies mechanical conditions consistent with lung collapse after pneumoperitoneum. Indeed, while P_L did not increase by the same magnitude as ΔP, it also increased, indicating loss of lung compliance. Such conditions could prompt use of higher PEEP to prevent lung derecruitment.