Increases in lung volume achieved by continuous distending airway pressure (CDAP) are known to alter the patterns of breathing in infants. The proposed studies are planned to learn more about the response of the respiratory muscles to increases in lung volume, which in turn may affect input to the respiratory center. The configuration of the chest wall and diaphragm will be examined with magnetometers in spontaneously breathing premature infants before and after application of CDAP. The electrical activity of the diaphragm, intercostal and abdominal muscles will be assessed with surface electromyograms before and after lung volume is increased, and inspiratory pressure during airway occlusion will also be measured. If an increase in lung volume alters the configuration of the chest wall and diaphragm, changes will occur in the length-tension relationship and consequently the mechanical advantage of the respiratory muscles. Chest wall and diaphragm dynamics will be altered by neuromuscular blockade in infants mechanically ventilated for respiratory failure. Preliminary evidence suggests that muscle relaxation improves oxygenation in mechanically ventilated infants, in contrast to the usual response in normal adults. Lung volume changes, compliance, resistance, chest wall and diaphragm configuration, muscle electrical activity, and right-to-left shunt will be assessed before and after neuromuscular blockade in order to elucidate the mechanism of changes in gas exchange. A better understanding of the effects of lung volume increases on chest wall and diaphragm dynamics may lead to improved techniques of assisted ventilation in infants.