DIFFERING PATTERNS OF MECHANICAL RESPONSE TO DIRECT FETAL HORMONE-TREATMENT

A single combined intramuscular dose of betamethasone and 1-thyroxine (T4) or placebo was injected into the shoulder of fetal lambs 48 hours prior to delivery at days 121 (n = 14), 128 (n = 25) or 135 (n = 20) of gestation. Respiratory mechanics were calculated using multiple lin ear regression analysis. Both respiratory system resistance (R(RS)) an d elastance (E(RS)) decreased approximately 4 fold between gestational days 121 (D121) and 135 (D135). Both variables were also reduced by h ormone treatment. Reduction in E(RS) was due to a reduction in both lu ng (E(L)) and chest wall (E(W)) components. In absolute terms E(W) dec reased with gestational age; however, E(W) as a proportion of total el astance (% E(W)) increased. Inclusion of a volume-dependent elastance term in the multiple linear regression model enabled us to separate to tal elastance into volume-independent (El) and volume-dependent (E2V) components. E1 decreased almost 8-fold compared with only a 2.5-fold f all in E2V between D121 and D135. %E2, the proportion of E(RS) which i s volume-dependent and which provides an index of overventilation, dou bled over this time period. Hormone treatment affected E1 and E2V comp onents equally hence %E2 was not altered. Both excised lung volume and end expiratory alveolar volume increased with gestational age and wit h treatment, The response to treatment was qualitatively similar at ea ch of the gestational ages examined, however, for all mechanics variab les, except resistance and E1, the magnitude of response to treatment was significantly smaller in D135 animals compared with other age grou ps.