Importance of inspiratory muscle tone in maintenance of FRC in the newborn.

The importance of inspiratory muscle tone in the maintenance of functional residual capacity (FRC) in newborns was studied in eight premature infants with birth weights of 1,166 +/- 217 g and gestational age 29 +/- 1.9 wk (mean +/- SD). Rib cage and abdominal anteroposterior diameters were monitored with magnetometers, and electromyograms of the diaphragm and intercostal muscles were recorded with surface electrodes. Sleep state was monitored using electrooculogram and behavioral criteria. We assessed the decrease in tonic activity of the inspiratory muscles and the fall in end-expiratory lung volume during apnea compared with the period just preceding apnea. A total of 98 apneas were analyzed. In all instances a decrease in diaphragmatic and intercostal tone was associated with a decrease in the anteroposterior diameter of both rib cage and abdomen, indicating a fall in FRC. These changes were more marked during quiet sleep than during rapid-eye-movement sleep (P less than 0.01). Our results suggest that inspiratory muscle tone is a major determinant of FRC in the newborn.