Fundamentals of respiratory mechanics

The respiratory system includes the lung and the chest wall, in series, and the overall mechanical behavior depends on the mechanical characteristics of its components and their interactions]. The common increase in the elastance (decrease in compliance) of the whole respiratory system in acute lung injury (ALI) and in acute respiratory distress syndrome (ARDS) has traditionally been attributed to the lung component. It has long been reported, however, that the chest wall elastance was also altered in many cases. Recently, mainly due to the increased concern for the abdominal pressure, more attention has been paid to the chest wall mechanics in critically ill patients .The problems of the mechanical impairment of the chest wall and its consequences are now widely recognized. Therefore chest wall compliance may be deeply altered and the driving pressure (Paw) is spent to move the stiffer chest wall and less distending pressure is available for the lung. Therefore, for the same driving pressure, the transpulmonary pressure may be too high (VILI) or too low (Insufficient total ventilation, PCO2 rise, pH decrease, Progressive atelectasis, Worsening hypoxemia, Increased pulmonary hypertension, Right ventricular failure). Pleural pressure is of paramount importance when: *Setting mechanical ventilation *Estimating risks of VILI *Monitoring Spontaneous breathing *Monitoring non-invasive ventilation *Extensive application has to be made to completely define its role in ICU practice .