Effect of maternal steroid on developing diaphragm integrity

Diaphragmatic muscle function in term and preterm infants

We aimed to assess the determinants of diaphragmatic function in term and preterm infants. 149 infants (56 term; 93 preterm, of whom 14 were diagnosed with bronchopulmonary dysplasia-BPD) were studied before discharge. Diaphragmatic function was assessed by measurement of the maximum transdiaphragmatic pressure (Pdimax)-a measure of diaphragmatic strength, and the pressure-time index of the diaphragm (PTIdi)-a measure of the load-to-capacity ratio of the diaphragm. The Pdimax was higher in term than preterm infants without BPD (90.1 ± 16.3 vs 81.1 ± 11.8 cmH2O; P = 0.001). Term-born infants also had lower PTIdi compared to preterms without BPD (0.052 ± 0.014 vs 0.060 ± 0.017; P = 0.006). In term and preterm infants without BPD, GA was the most significant predictor of Pdimax and PTIdi, independently of the duration of mechanical ventilation and oxygen support. In infants with GA < 32 weeks (n = 30), the Pdimax was higher in infants without BPD compared to those with BPD (76.1 ± 11.1 vs 65.2 ± 11.9 cmH2O; P = 0.015). Preterms without BPD also had lower PTIdi compared to those with BPD (0.069 ± 0.016 vs 0.109 ± 0.017; P < 0.001). In this subgroup, GA was the only significant independent determinant of Pdimax, while BPD and the GA were significant determinants of the PTIdi.  Conclusions: Preterm infants present lower diaphragmatic strength and impaired ability to sustain the generated force over time, which renders them prone to diaphragmatic fatigue. In very preterm infants, BPD may further aggravate diaphragmatic function. What is Known: • The diaphragm of preterm infants has limited capacity to undertake the work of breathing effectively. • The maximum transdiaphragmatic pressure (a measure of diaphragmatic strength) and the pressure-time index of the diaphragm (a measure of the load-to-capacity ratio of the muscle) have not been extensively assessed in small infants. What is New: • Preterm infants have lower diaphragmatic strength and impaired ability to sustain the generated force over time, which renders them prone to diaphragmatic fatigue. • In very preterm infants, bronchopulmonary dysplasia may further impair diaphragmatic function.

Respiratory muscle function in the newborn: a narrative review

Our aim was to summarise the current evidence and methods used to assess respiratory muscle function in the newborn, focusing on current and future potential clinical applications. The respiratory muscles undertake the work of breathing and consist mainly of the diaphragm, which in the newborn is prone to dysfunction due to lower muscle mass, flattened shape and decreased content of fatigue-resistant muscle fibres. Premature infants are prone to diaphragmatic dysfunction due to limited reserves and limited capacity to generate force and avoid fatigue. Methods to assess the respiratory muscles in the newborn include electromyography, maximal respiratory pressures, assessment for thoraco-abdominal asynchrony and composite indices, such as the pressure-time product and the tension time index. Recently, there has been significant interest and a growing body of research in assessing respiratory muscle function using bedside ultrasonography. Neurally adjusted ventilator assist is a novel ventilation mode, where the level of the respiratory support is determined by the diaphragmatic electrical activity. Prolonged mechanical ventilation, hypercapnia and hypoxia, congenital anomalies and systemic or respiratory infection can negatively impact respiratory muscle function in the newborn, while caffeine and synchronised or volume-targeted ventilation have a positive effect on respiratory muscle function compared to conventional, non-triggered or pressure-limited ventilation, respectively. IMPACT: Respiratory muscle function is impaired in prematurely born neonates and infants with congenital anomalies, such as congenital diaphragmatic hernia. Respiratory muscle function is negatively affected by prolonged ventilation and infection and positively affected by caffeine and synchronised compared to non-synchronised ventilation modes. Point-of-care diaphragmatic ultrasound and neurally adjusted ventilator assist are recent diagnostic and therapeutic technological developments with significant clinical applicability.

Intrauterine growth restriction affects diaphragm function in adult female and male mice

BACKGROUND

In utero diaphragm development is critically important for postnatal respiratory function and any disturbance to fetal development may lead to diaphragm dysfunction and respiratory complications in the postnatal period. Intrauterine growth restriction (IUGR) has been shown to affect respiratory function in a sex-dependent manner; however, the effect of IUGR on diaphragm function is unknown.

AIM

This study used a maternal hypoxia-induced mouse model of IUGR to investigate the impact of IUGR on diaphragm function and structure in male and female adult offspring.

MATERIALS AND METHODS

Pregnant BALB/c mice were housed under hypoxic conditions (10.5% O₂ ) from gestational days 11 to 17.5 and then returned to normoxic conditions. Control mice were housed under normoxic conditions throughout pregnancy. At 8 weeks of age, offspring were euthanized and diaphragms isolated for functional assessment in organ bath experiments and for histological analysis.

RESULTS

IUGR offspring were lighter at birth and remained lighter at 8 weeks of age compared to Controls. While diaphragm force (maximal or twitch) was not affected by treatment or sex, the IUGR group exhibited a longer half-relaxation time after twitch contractions compared to Control. Female offspring had a lower maximum rate of force development and higher fatigue resistance compared to males, independent of IUGR. There was no difference in the diaphragm myofibre cross-sectional area between groups or sexes.

CONCLUSION

Sex and IUGR independently affect diaphragm contraction in adult mice without changes in structure. This study demonstrates that IUGR affects diaphragm contractile function in later life and could impair respiratory function if exacerbated under conditions of increased respiratory load.

Influence of antenatal glucocorticoid on preterm lamb diaphragm

BackgroundPregnant women at a high risk of preterm delivery receive glucocorticoids to accelerate fetal lung maturation and surfactant synthesis. However, the effect of antenatal steroids on the developing diaphragm remains unclear. We hypothesized that maternal betamethasone impairs the fetal diaphragm, and the magnitude of the detrimental effect increases with longer duration of exposure. We aimed to determine how different durations of fetal exposure to maternal betamethasone treatment influence the fetal diaphragm at the functional and molecular levels.MethodsDate-mated merino ewes received intramuscular injections of saline (control) or two doses of betamethasone (5.7 mg) at an interval of 24 h commencing either 2 or 14 days before delivery. Preterm lambs were killed after cesarean delivery at 121-day gestational age. In vitro contractile measurements were performed on the right hemidiaphragm, whereas molecular/cellular analyses used the left costal diaphragm.ResultsDifferent durations of fetal exposure to maternal betamethasone had no consistent effect on the protein metabolic pathway, expression of glucocorticoid receptor and its target genes, cellular oxidative status, or contractile properties of the fetal lamb diaphragm.ConclusionThese data suggest that the potential benefits of betamethasone exposure on preterm respiratory function are not compromised by impaired diaphragm function after low-dose maternal intramuscular glucocorticoid exposure.

Optical coherence tomography-based contact indentation for diaphragm mechanics in a mouse model of transforming growth factor alpha induced lung disease

This study tested the utility of optical coherence tomography (OCT)-based indentation to assess mechanical properties of respiratory tissues in disease. Using OCT-based indentation, the elastic modulus of mouse diaphragm was measured from changes in diaphragm thickness in response to an applied force provided by an indenter. We used a transgenic mouse model of chronic lung disease induced by the overexpression of transforming growth factor-alpha (TGF-α), established by the presence of pleural and peribronchial fibrosis and impaired lung mechanics determined by the forced oscillation technique and plethysmography. Diaphragm elastic modulus assessed by OCT-based indentation was reduced by TGF-α at both left and right lateral locations (p < 0.05). Diaphragm elastic modulus at left and right lateral locations were correlated within mice (r = 0.67, p < 0.01) suggesting that measurements were representative of tissue beyond the indenter field. Co-localised images of diaphragm after TGF-α overexpression revealed a layered fibrotic appearance. Maximum diaphragm force in conventional organ bath studies was also reduced by TGF-α overexpression (p < 0.01). Results show that OCT-based indentation provided clear delineation of diseased diaphragm, and together with organ bath assessment, provides new evidence suggesting that TGF-α overexpression produces impairment in diaphragm function and, therefore, an increase in the work of breathing in chronic lung disease.

Precocious glucocorticoid exposure reduces skeletal muscle satellite cells in the fetal rat

Perinatal skeletal muscle growth rates are a function of protein and myonuclear accretion. Precocious exposure of the fetus to glucocorticoids (GLC) in utero impairs muscle growth. Reduced muscle protein synthesis rates contribute to this response, but the consequences for myonuclear hyperplasia are unknown. To test the hypothesis that blunting of Pax7+ muscle progenitor cell proliferative activity by GLC in vivo also contributes to reduced fetal muscle growth, pregnant rats were administered dexamethasone (DEX: 1 mg/L drinking water) from embryonic day (ED) 13 to ED21. Their responses were compared to pair-fed (PF) and ad libitum-fed controls (CON). Bromodeoxyuridine (BrdU) was administered before delivery to measure myonuclear accretion. Fetal hind limb and diaphragm muscles were collected at term and analyzed for myofiber cross-sectional area (CSA), total and BrdU+ myonuclei, Pax7+ nuclei, MyoD and myogenin protein and mRNA abundance and myosin heavy chain (MyHC) isoform composition. Mean fiber CSA, myonuclei/myofiber and Pax7+ nuclei/myofiber ratios were reduced in DEX compared to those in CON and PF muscles; CSA/myonucleus, BrdU+/total myonuclei and BrdU+ myonuclei/Pax7+ nuclei were similar among groups. Myogenin abundance was reduced and MyHC-slow was increased in DEX fetuses. The data are consistent with GLC inhibition of muscle progenitor cell proliferation limiting satellite cell and myonuclear accretion. The response of PF-fed compared to CON muscles indicated that decreased food consumption by DEX dams contributed to the smaller myofiber CSA but did not affect Pax7+ nuclear accretion. Thus, the effect on satellite cell reserve and myonuclear number also contributes to the blunting of fetal muscle growth by GLC.

Understanding the Full Spectrum of Organ Injury Following Intrapartum Asphyxia

Birth asphyxia is a significant global health problem, responsible for ~1.2 million neonatal deaths each year worldwide. Those who survive often suffer from a range of health issues including brain damage-manifesting as cerebral palsy (CP)-respiratory insufficiency, cardiovascular collapse, and renal dysfunction, to name a few. Although the majority of research is directed toward reducing the brain injury that results from intrapartum birth asphyxia, the multi-organ injury observed in surviving neonates is of equal importance. Despite the advent of hypothermia therapy for the treatment of hypoxic-ischemic encephalopathy (HIE), treatment options following asphyxia at birth remain limited, particularly in low-resource settings where the incidence of birth asphyxia is highest. Furthermore, although cooling of the neonate results in improved neurological outcomes for a small proportion of treated infants, it does not provide any benefit to the other organ systems affected by asphyxia at birth. The aim of this review is to summarize the current knowledge of the multi-organ effects of intrapartum asphyxia, with particular reference to the findings from our laboratory using the precocial spiny mouse to model birth asphyxia. Furthermore, we reviewed the current treatments available for neonates who have undergone intrapartum asphyxia, and highlight the emergence of maternal dietary creatine supplementation as a preventative therapy, which has been shown to provide multi-organ protection from birth asphyxia-induced injury in our preclinical studies. This cheap and effective nutritional supplement may be the key to reducing birth asphyxia-induced death and disability, particularly in low-resource settings where current treatments are unavailable.

Maternal Creatine Supplementation during Pregnancy Prevents Long-Term Changes in Diaphragm Muscle Structure and Function after Birth Asphyxia

Using a model of birth asphyxia, we previously reported significant structural and functional deficits in the diaphragm muscle in spiny mice, deficits that are prevented by supplementing the maternal diet with 5% creatine from mid-pregnancy. The long-term effects of this exposure are unknown. Pregnant spiny mice were fed control or 5% creatine-supplemented diet for the second half of pregnancy, and fetuses were delivered by caesarean section with or without 7.5 min of in-utero asphyxia. Surviving pups were raised by a cross-foster dam until 33±2 days of age when they were euthanized to obtain the diaphragm muscle for ex-vivo study of twitch tension and muscle fatigue, and for structural and enzymatic analyses. Functional analysis of the diaphragm revealed no differences in single twitch contractile parameters between any groups. However, muscle fatigue, induced by stimulation of diaphragm strips with a train of pulses (330 ms train/sec, 40 Hz) for 300 sec, was significantly greater for asphyxia pups compared with controls (p<0.05), and this did not occur in diaphragms of creatine + asphyxia pups. Birth asphyxia resulted in a significant increase in the proportion of glycolytic, fast-twitch fibres and a reduction in oxidative capacity of Type I and IIb fibres in male offspring, as well as reduced cross-sectional area of all muscle fibre types (Type I, IIa, IIb/d) in both males and females at 33 days of age. None of these changes were observed in creatine + asphyxia animals. Thus, the changes in diaphragm fatigue and structure induced by birth asphyxia persist long-term but are prevented by maternal creatine supplementation.