EFFECTS OF INSPIRATORY RESISTIVE LOADING ON CHEST-WALL MOTION AND VENTILATION - DIFFERENCES BETWEEN PRETERM AND FULL-TERM INFANTS

The ability to maintain effective tidal volume and minute ventilation during resistive loaded breathing depends on both adequate central neural respiratory output response and respiratory system mechanical properties such as respiratory muscle strength and chest wall stability. We hypothesized that chest wall instability limits the ability of the preterm (PT) infant to respond to inspiratory resistive loading (IRL) compared with full-term (FT) infants. To test this hypothesis, we subjected eight FT and 10 PT infants to IRL with loads of 1.3, 2, and 6 times intrinsic lung resistance and measured steady state tidal volume (V(T)), minute ventilation (V(E)), and chest wall motion. Thoracoabdominal asynchrony was measured by respiratory inductive plethysmography and quantitated by measuring the phase angle, theta, between rib cage and abdominal motion (0-degrees = synchronous motion, 180-degrees = paradoxic motion). At baseline, V(T)/kg (mL/kg, mean +/- SEM) was similar between PT (7.0 +/- 0.7) and FT (7.5 +/- 0.5) infants. V(E)/kg (mL/min/kg) was greater in PT (545 +/- 50) than in FT (385 +/- 33) infants (p < 0.05) as a result of increased respiratory frequency in the former. PT infants demonstrated significantly greater chest wall asynchrony (0 = 38 +/- 9-degrees) than FT infants (theta = 9 +/- 3-degrees) (p < 0.01). With the highest resistive loads, V(T) decreased significantly in the PT but not the FT infants. Furthermore, during IRL, V(E) decreased to 417 +/- 50 mL/min/kg (p < 0.05) and theta increased to 56 +/- 7 (p < 0.05) in the PT infants, whereas no significant change in either value was observed in the FT group. We conclude that IRL breathing significantly decreases V(T) and V(E) in PT infants. Chest wall instability in the PT group, as reflected by the increased asynchrony between rib cage and abdomen, contributes to the relative inability of PT infants to maintain ventilation during inspiratory resistive loaded breathing.