Components of effective elastance and their maturational changes in human newborns.

The newborn's ability to compensate for a sudden increase in respiratory load remains unclear. The mouth occlusion pressure (Pmo) and the effective elastance (E'rs) were determined in 58 neonates ranging in gestational age from 28 to 41 wk. These results were compared with the esophageal pressure change (delta Pes) and with lung elastance (EL) obtained during regular breathing. Since E'rs = Pmo/delta Pes X EL, E'rs is proportional to an active component, Pmo/delta Pes and a passive component, EL. Although EL is 1.10 +/- 0.31 (mean +/- SD) cmH2O/ml at 28 wk gestation, this value decreases to 0.20 +/- 0.04 cmH2O/ml at full-term. The active component Pmo/delta Pes changes in the opposite direction. It is 0.92 +/- 0.25 at 28 wk gestation, indicating that there is no load compensation in the very premature infant. It increases to 2.06 +/- 0.50 in the full-term newborn, reflecting a linear increase in the ability for load compensation with advancing gestational age. The E'rs is largely determined by EL and is, therefore, not a good measure to compare load compensating ability. The quotient Pmo/delta Pes, being independent of tidal volume, lung maturation, and growth, reflects load compensating activity better than E'rs.