ABSTRACT
The thorax has a profound effect on lung mechanics in infancy. The chest wall is highly compliant. Agostoni ( 1959) has pointed out that at birth this is a substantial advantage, as it maintains the lung close to residual volume (RV), thus reducing the volume of intrapulmonary fluid, and facilitates the compression of the thorax through the birth canal. The problem is that when the fluid is resorbed, the lung may remain close to R V. Furthermore, the chest wall is so compliant that in the preterm infant with even mild respiratory distress, it looks like a "flail chest." The extent of paradoxical inward movement of the rib cage during inspiration has, in fact, been exploited as a simple method of detecting lung disease (Allen et al., 1990; Sivan et al., 1990). Thus the diaphragm dissipates a substantial fraction of its power sucking in ribs rather than fresh air. The mechanisms capable of stabilizing the thorax appear to be switched off in rapid eye movement (REM) sleep, which is the infant's predominant behavioral state . Histochemically, the diaphragm appears poorly equipped to deal with high workloads. Despite all this, under normal conditions the thorax works well enough, but with any departure from normality, such as prematurity or quite trivial lung disease, the thorax becomes a problem.
