ABSTRACT
At FRC the transmural pressure is zero. It is at this level that the opposing forces of the lung and thorax are equal and opposite. The elastic properties of the lung and thorax can be independently determined by measuring esophageal pressure (a reflection of pleural pressure, PPL) and comparing it with alveolar pressure
8. During spontaneous breathing esophageal pressure is reflective of the elastic recoil of the lungs and is used to calculate lung compliance (CL):
To calculate thoracic compliance (CTh) the effect of the ventilatory muscles must be
eliminated (using paralysis or heavy sedation) and lung volume provided by mechanical ventilation. In this setting esophageal pressure represents the elastic recoil of the thorax:
Change in bladder pressure can be used as a substitute for esophageal pressure for the calculation of thoracic compliance9. Bladder pressure has been clearly shown to be reflective of abdominal/thoracic recoil pressure in critically ill patients receiving controlled mechanical ventilation. Compliance of the total respiratory system, the lungs and the thorax are related by the following equation:
Figure 1.3 The normal pressure-volume relationship for the total respiratory system. As noted beyond FRC (to the right of the vertical volume line) the P-V relationship is linear until TLC (maximum volume) is approached.
