Respiratory Mechanics: Principles, Utility and Advances

Pulmonary gas exchange requires a continuous flow of fresh gas and blood through the alveoli and alveolar capillaries. Air flows from a region of higher pressure to one of lower pressure. At end expiration, the alveolar pressure is equal to atmospheric pressure, and during inspiration, the alveolar pressure must be less than atmospheric pressure. As the movements of the lungs are entirely passive, forces must be applied in order to expand the lungs, and as a consequence, alveolar pressure is decreased from its resting pressure at the end of expiration. In the case of spontaneous breathing, the respiratory muscles provide the external forces, whereas artificial ventilation moves the relaxed respiratory system [1]. During inspiration, the external forces must overcome the impedance of the lung and chest wall, the two components of the respiratory system. This impedance stems mainly from the force to overcome elastic recoil, the frictional resistance during the movement of the tissues of the lungs and thorax, and the force to overcome the frictional resistance to airflow through the tracheobronchial tree. The inertial component of gas and tissue is usually negligible during conventional ventilation [2].