Functional adaptation of the preterm lamb diaphragm in the first week of life is not influenced by in utero lipopolysaccharide exposure.

In utero inflammation exacerbates preterm diaphragm weakness at birth (Song et al. 2013). Persistent diaphragm dysfunction, resulting from immaturity and in utero inflammation, may predispose preterm infants to postnatal respiratory failure. We determined the combined effects of preterm birth and in utero inflammation on postnatal diaphragm function in lambs. Lambs were exposed to intra-amniotic saline (n=8) or 4mg lipopolysaccharide (LPS; n=9) 48h before caesarean delivery at 129d gestational age (GA; term=150d). Before delivery, ewes were pre-medicated with Acepromazine (0.05mg/kg, IM) and Buprenorphine (0.01mg/kg, IM) prior to anesthetic induction with Thiopentone (15mg/kg, IV) and maintenance with isofluorane (1.5-2.5%). After deliver, lambs were resuscitated, commenced mechanical ventilation immediately and weaned as early as possible. Lambs were killed (pentobarbitone, 150mg/kg IV) at 7d postnatal age (PNA). Fetal control lambs were delivered at 136d GA and killed immediately (n=7). The diaphragm was excised for in vitro assessment of contractile function and snap-frozen in liquid nitrogen for biochemical analysis. Data are presented as mean ± S.D. and compared by ANOVA. There were no significant differences in diaphragm contractile function, myosin heavy chain fibre expression, markers of inflammation, protein metabolism or oxidative status, between saline and LPS groups. However, contractile properties of the saline and LPS groups were significantly different to the fetal control group, indicating postnatal adaptations to extra-uterine life. Maximum specific force (N/cm2) was significantly higher in the saline group (18.77±1.99) compared to the fetal control group (14.25±2.08; p=0.007). Time to peak twitch force (ms) was significantly shorter in the saline (117.0±15.0) and LPS groups (110.3±9.9) compared with the fetal control group (163.8±22.2; p<0.001). Similarly, half-relaxation time (ms) was significantly shorter in the saline group (172.9±41.2) and LPS group (151.6±21.9) compared with the fetal control group (230.3±29.0; p<0.01). Importantly, LPS lambs had a greater reliance on mechanical ventilation than saline lambs (range: LPS=4-143h, saline= 3-54h). Furthermore, duration of mechanical ventilation exhibited a significant negative correlation with maximum specific force (R2=0.528; p<0.01; Pearson correlation) explaining 51% of the variation observed in maximum diaphragm force at 7d PNA, independent of LPS exposure. Although LPS-induced diaphragm weakness at birth did not persist at 7d PNA, the increased reliance on mechanical ventilation after in utero inflammation highlights potential interactions between clinical complications associated with preterm birth that may affect diaphragm function and contribute to the development of respiratory failure during early postnatal life.