Abstract 1902 Neonatal Pulmonology I: Mechanical Influences on Lung Development Platform, Tuesday, 5/4

Pre-term infants have difficulty maintaining an adequate FRC without PEEP. The FRC is a function of the elastic recoil pressure. We quantitated the parenchymal elastic tissue in 63 control infants (22-42 wks gestation) who died within the first 2 days of life and another 39 infants (23-30 wks gestation), who died after 18 ± 11 days (mean ± SD) at 28.4 ± 2.3 wks post-conceptual age. The causes of death were both pulmonary and non-pulmonary. All were intubated with variable ventilator assistance. The Respiratory Score (FiO2 × MAP) was averaged daily and the area under the curve was calculated for the life of the infant. The infants were separated into the following three groups, chosen arbitrarily, according to their score: <20 (n=12); 21-69 (n=14); and 70-200 (n=13). The lungs were standardly inflated with 10% formalin, fixed, and measured for displacement volume. Five-micron paraffin sections from five left lung tissue blocks were stained with Miller's elastic stain. Using point counting, the volume density (fraction of parenchyma that is elastic tissue) and absolute volume of elastic tissue in the parenchyma (septal tissue, alveolar air space, ductal walls, and ductal air space) were determined. The control lung elastic tissue volume density rose smoothly from 0.5 at 22 wks to 1.94 at 42 wks, and the absolute elastic tissue volume rose from 0.03 to 2.26 ml. At death, postconceptual age elastic measurements in the three groups of infants with evolving CLD were expressed as percent of the control lungs at the same gestational age. The volume densities were: <20 group: 107 ± 20%; 21-70 group: 154 ± 35%; and the 71-200 group: 241 ± 49%. The <20 group was not different from controls, but the 21-70 and 71-200 groups were, and the volume density increased significantly with greater scores (p<0.05). The absolute volume of the elastic tissue followed the same pattern and significance. The <20 group: 126 ± 41%; 21-70 group: 229 ± 60%; and the 71-200 group: 284 ± 80%. We conclude that: 1) maturation of elastic tissue is tightly controlled during fetal development; 2) with increasing Respiratory Score the parenchymal elastic tissue significantly increases; and 3) the elastic recoil pressure could, potentially, more than double during evolution of CLD, necessitating, ironically, higher ventilator pressures to achieve an adequate FRC.