The immediate ventilatory response to added inspiratory elastic and resistive loads in preterm infants

We measured the changes in tidal volume, duration of the various phases of the respiratory cycle, and peak nasal pressure during elastic and resistive loading in preterm infants. Values were calculated during the first loaded breath, when chemical drive was unchanged. Tidal volume decreased by equivalent percentages with resistive loads of 400, 900, and 2,400 cm H2O/liter/sec, and elastic loads of 330, 1,000, and 3,000 cm H2O/liter. Infinite load was also applied (nasal occlusion). Inspiratory duration (ti) was prolonged during resistive loading, as compared with elastic loading (P less than 0.05). Changes in expiratory duration (Te) were not different with both loads (P greater than 0.05). Total duration of the respiratory cycle (T), however, tended to increase in relation to control, more so with resistive loads. Peak nasal pressure was greater with resistive than with elastic loads ( less than 0.025). We suggest that (1) preterm infants, like adult subjects and other animal species, increase inspiratory duration with resistive loads as compared with elastic loads; (2) T of the first loaded breath tends to increase with progressively larger loads and, consequently, instantaneous frequency tends to decrease; and (3) if peak nasal pressure reflects tension developed by the respiratory muscles, then the latter does not offer the inhibitory information needed to terminate inspiration.