Respiratory Mechanics

Respiratory mechanics describes the physical work required to move air into and out of the lungs. Effective breathing depends on an open airway, a chest wall and diaphragm that can generate pressure, lungs that expand appropriately, and enough time for exhalation. Gas exchange fails when one or more of these components cannot keep pace with the patient’s metabolic demand. The lungs do not actively expand. During a spontaneous breath, diaphragmatic contraction makes intrathoracic pressure more negative, drawing air inward. Exhalation is usually passive: elastic recoil of the lungs and chest wall pushes air out. This arrangement is efficient until airway resistance rises, lung compliance falls, respiratory muscles weaken, or the patient develops an excessive ventilatory load. Compliance is the ease with which the lung and chest wall expand. A compliant lung inflates with relatively little pressure; a stiff lung requires far more effort. Pulmonary oedema, pneumonia, atelectasis, and acute respiratory distress syndrome (ARDS) reduce compliance. The patient often develops rapid, shallow breathing because a large breath is mechanically expensive. Airway resistance is the opposition to airflow within conducting airways. Bronchospasm,...