[Perspectives on chest wall mechanics in infants and young children].

The chest wall of neonates and infants is compliant and thus the outward recoil of the chest wall and the inward recoil of the lungs theoretically balance at a low lung volume (FRC) incompatible with gas exchange. The neonates and infants, however, have a number of mechanisms that actively maintain a high FRC: (1) Laryngeal breaking during expiration. (2) Short expiratory time relative to long time expiratory time constant. (3) Sustained inspiratory muscle tone during expiratory phase. When lung recoil is increased as in many disease states, the third mechanism fails due to muscle fatigue. The increased lung recoil pulls in the chest wall and a catastrophic decrease in FRC occurs. The measurement of respiration from the outside through chest wall movement is especially important in infants and young children for these reasons. The other reason is that the breathing pattern remains unaltered, unlike measurement via the airway such as by mask and pneumotachometer.