ABSTRACT
Records of chest wall movements and spirometric measurements in infants during spontaneous resting breathing reveal several differences from the adult, including two aspects that will be addressed in this review. During inspiration, the infant's thorax does not expand as clearly as in the adult. In fact, during active (rapid-eyemovement; REM) sleep the newborn's rib cage can actually retract during inspiration, and expand during expiration (Fig. 1), following a pattern that is opposite that occurring in the adult. The spirometric record indicates that the end-expiratory level (FRC) is not as stable as in the adult; long expiratory times, or short apneas, which are not rare in the early postnatal period, are often accompanied by an obvious decrease in FRC, whereas rapid breathing increases FRC (Olinsky et al., 1974). [The terms end-expiratory level and functional residual capacity (FRC) are here used interchangeably to indicate the lung volume at end-expiration. Relaxation volume (Vr) indicates the lung volume in relaxed (or passive) conditions.]
618 Mortola
ing of these two phenomena of infants' breathing; they include the passive structure and function of the chest wall, its dynamic behavior, and some functional properties of the respiratory muscles. Generalities of chest wall mechanics and respiratory muscles are reviewed elsewhere in this volume. Reference to other species will be sought only to complete information not available from human studies; comparative reviews of respiratory mechanics in newborn mammals, including chest wall function, have been recently published (Mortola, 1987, 1991).
