Understanding microvascular function in preterm neonates

Abstract

Research Doctorate - Doctor of Philosophy (PhD)Appropriate regulation of microvascular blood flow in the neonate is crucial for cardiorespiratory stability and survival in the period immediately following birth. Inappropriate microvascular dilatation in the first few days of extrauterine life is associated with poor outcomes in preterm neonates. Male very preterm neonates (≤28 weeks completed gestation) have significantly higher flows than females of the same gestational age. This is of clinical importance as preterm males are twice as likely to die as females (Chapter 1). Very little is known about the mechanisms underlying microvascular tone regulation in the perinatal period. Previous studies suggest a role for the gasotransmitters nitric oxide and carbon monoxide, however, differences in levels of these molecules do not account for all the variation observed, suggesting another player (Chapter 2). In this thesis, the role of the third gasotransmitter – hydrogen sulphide – as a potential mediator of microvascular tone (dys)regulation in the preterm is explored. The aim of this thesis was to further characterise early microvascular events in the neonate associated with poor outcome and characterise the mechanisms underlying vascular tone regulation, in particular, the role of the gasotransmitter hydrogen sulphide. As studies in preterm neonates are restricted mainly to observational studies, we established an animal model of prematurity using the guinea pig that would allow us to explore the underlying mechanisms of microvascular function in the newborn (Chapter 3). Delivery of pups at 0.87 gestation resulted in significant mortality, with a greater proportion of male than female pups dying in the first 24h of life, in line with large cohort studies of preterm human infants. Additionally, physiological instability as measured by a novel scoring matrix and microvascular blood flow were significantly higher in preterm than term neonates. The mortality, morbidity and blood flow results suggest the guinea pig delivered at this time point is a suitable model for human infants born prematurely. Using a cohort of neonates recruited in the John Hunter Children’s Hospital and our established guinea pig model, we further characterised microvascular behaviour in the early extrauterine period and how this behaviour is affected by gestational age, postnatal age and sex (Chapter 4). It has been hypothesised that preterm infants may undergo a hypoperfusion-reperfusion cycle in the first 24-48h of life, with the reperfusion stage responsible for circulatory compromise and vascular damage frequently observed in preterm neonates. We observed no such cycle, with blood flow increasing following birth in male preterm neonates, both human and guinea pig. We investigated the potential of hydrogen sulphide to regulate microvascular blood flow both on its own (Chapter 5) and through its interactions with the other gasotransmitters (Chapter 6) and showed for the first time that it may play a role in physiological tone regulation during circulatory transition, with overproduction potentially associated with dysregulation in the human preterm male neonate. Interaction analysis revealed that in female neonates, carbon monoxide negated the effect of hydrogen sulphide on vascular tone, suggesting some protective role of this molecule against inappropriate vasodilatation in the female; whilst in males nitric oxide levels were associated with higher hydrogen sulphide levels and higher microvascular blood flow. We further characterised this production using our guinea pig model and investigated the effect of gestational age, postnatal age and sex (Chapter 7). Vascular hydrogen sulphide production increased during fetal-to-neonatal transition. Hydrogen sulphide produced by cystathionine γ-lyase (the key production enzyme in the vasculature) increased postnatally. Hydrogen sulphide produced via this pathway correlated with skin microvascular blood flow in males, suggesting that cystathionine γ-lyase dependent mechanisms may drive the observed increase of hydrogen sulphide production over the first 3 days of life in male preterm babies, which in turn is associated with both central and peripheral cardiovascular stability. Cystathionine γ-lyase thus represents a potential target for therapies in cases of microvascular dysfunction