In their recent article, Piquilloud et al.1 explain the value of knowing pleural pressure (Ppl) for the individual calculation of compliance/elastance of thorax wall and lungs. In the interpretation of Ppl values some important issues must be included: Ppl in the healthy subject is sub-atmospheric, and secondly it shows a gradient of about 7 cmH2O to 8 cmH2O in the pressure going up from non-dependent to dependent parts of the lung. Third, airway closure interferes with the assumption that airway pressure equals alveolar pressure in an equilibrium situation.
At functional residual capacity (FRC) level, the alveoli tend to become smaller, while the thorax wall tends to take larger volume. This results in a sub-atmospheric average Ppl, which is necessary to keep the lung open. Due to its flexibility, the lung acts somewhat like a liquid in the propagation of pressure and Ppl increases from top to bottom by the weight of the tissue above.
The excised lung still contains air due to airway closure, though much less than FRC2. Airways close at a transmural pressure of about 2 cmH2O or 2.5 cmH2O to prevent full collapse of the alveolus. From the experiments of Dollfuss3, it follows that in the upright position the Ppl at the bottom of the lung is still lower than this threshold, because peripheral airway closure starts below the FRC level. Dollfuss evoked this airway closure by exhalation below FRC level. From measurements during positive pressure ventilation in the obese patient4, we know that very high levels of Ppl force all peripheral airways to close which leads to the phenomenon of airway opening pressure (AOP) during inspiration: the rising airway pressure evokes no flow into the lung until the AOP is reached and the first alveoli are opening.
The above facts lead to the conclusion that defining transpulmonary pressure as airway pressure minus Ppl is only valid in the absence of airway closure. Hedenstierna5 showed that 50% of the standard surgery patients suffer from airway closure above the FRC level. In the obese patients, (full) airway closure is commonly observed leading to high values of positive end-expiratory pressure (PEEP) to keep the lungs open. If in the healthy subject pleural pressure is -8 cmH2O, then we must be convinced that transpulmonary pressure during mechanical ventilation will be higher than 8 cmH2O because of increased volume and/or reduced lung compliance due to illness. Therefore, every transpulmonary pressure value (defined as airway pressure minus pleural pressure) lower than 8 cmH2O must lead to the conclusion that airway closure is present. This implies, because of the high pleural pressures in patients with acute respiratory distress syndrome during mechanical ventilation, that almost all ARDS patients suffer from full airway closure. For instance, if Ppl equals 15 cmH2O, alveolar pressure must be at least 23 cmH2O. When alveolar pressure is higher than PEEP, this implies (full) airway closure. The presence of an AOP much higher than the actual PEEP renders the use of esophageal pressure meaningless in calculating lung compliance.