Microvascular tone in the preterm neonate: gasotransmitter interactions may be the key

Abstract of an oral presentation that was presented at the 5th Congress of the European Academy of Paediatric Societies EAPS 17-21 October 2014, Barcelona, Spain

A role for H₂S in the microcirculation of newborns: the major metabolite of H₂S (Thiosulphate) is increased in preterm infants.

Excessive vasodilatation during the perinatal period is associated with cardiorespiratory instability in preterm neonates. Little evidence of the mechanisms controlling microvascular tone during circulatory transition exists. We hypothesised that hydrogen sulphide (H₂S), an important regulator of microvascular reactivity and central cardiac function in adults and animal models, may contribute to the vasodilatation observed in preterm newborns. Term and preterm neonates (24–43 weeks gestational age) were studied. Peripheral microvascular blood flow was assessed by laser Doppler. Thiosulphate, a urinary metabolite of H₂S, was determined by high performance liquid chromatography as a measure of 24 hr total body H₂S turnover for the first 3 days of postnatal life. H₂S turnover was greatest in very preterm infants and decreased with increasing gestational age (p = 0.0001). H₂S turnover was stable across the first 72 hrs of life in older neonates. In very preterm neonates, H₂S turnover increased significantly from day 1 to 3 (p = 0.0001); and males had higher H₂S turnover than females (p = 0.04). A significant relationship between microvascular blood flow and H₂S turnover was observed on day 2 of postnatal life (p = 0.0004). H₂S may play a role in maintaining microvascular tone in the perinatal period. Neonates at the greatest risk of microvascular dysfunction characterised by inappropriate peripheral vasodilatation - very preterm male neonates - are also the neonates with highest levels of total body H₂S turnover suggesting that overproduction of this gasotransmitter may contribute to microvascular dysfunction in preterms. Potentially, H₂S is a target to selectively control microvascular tone in the circulation of newborns

Microvascular circulatory dysregulation driven in part by cystathionine gamma-lyase: a new paradigm for cardiovascular compromise in the preterm newborn

Objective: H 2 S may explain the dysregulation of microvascular tone associated with poor outcome following preterm birth. In adult vasculature, H 2 S is predominantly produced by CSE. We hypothesized that vascular CSE activity contributes to microvascular tone regulation during circulatory transition. Methods: Preterm (GA62) and full-term (GA69) guinea pig fetuses and neonates were studied. Microvascular blood flow was assessed by laser Doppler flowmetry. Thiosulfate, primary urinary metabolite of H 2 S, was determined by high-performance liquid chromatography. Real-time H 2 S production was assessed using a microrespiration system in fetal and postnatal (10, 24 hours) skin and heart samples. CSE contribution was investigated by inhibition via propargylglycine. Results: In preterm animals, postnatal H 2 S production capacity in peripheral vasculature increased significantly and was significantly reduced by the inhibition of CSE. Urinary thiosulfate correlated with both microvascular blood flow and capacity of the vasculature to produce H 2 S. H 2 S produced via CSE did not correlate directly with microvascular blood flow. Conclusions: In preterm neonates, H 2 S production increases during fetal-to-neonatal transition and CSE contribution to total H 2 S increases postnatally. CSE-dependent mechanisms may therefore underpin the increase in H 2 S production over the first 72 hours of life in preterm human neonates, associated with both central and peripheral cardiovascular instability

Sex differences in thiosulphate levels in very preterm neonates in early postnatal life (median±IQR).

<p>H₂S turnover, measured as urinary thiosulphate excreted per day per kg body weight, was significantly higher in males than females on both day 1 (*p = 0.01) and day 2 (**p = 0.04) of postnatal life (Friedman repeated measures ANOVA for non-parametric data).</p

Relationship between baseline microvascular blood flow at 24 hr and H₂S turnover on day 2 of postnatal life.

<p>H₂S turnover (measured as urinary thiosulphate) was significantly correlated with baseline microvascular blood flow in preterm male neonates 29–36 wk GA (Pearson correlation; p = 0.04, <i>r</i> = 0.43). No relationship was observed for females of the same gestational age, very preterm neonates (24–28 wk GA) or term neonates (37+wk GA).</p

Urinary thiosulphate levels over the first three days of life.

<p>H₂S turnover was stable across the first three days of life in term and preterm neonates. In very preterm neonates, levels rose significantly over the first 72 hours of life (median±IQR). ^a-b-cp<0.0001 significant difference across days in very preterm gestational age group (Friedman repeated measures ANOVA for non-parametric data).</p

Clinical Characteristics of Neonates.

<p>Data presented as median (minimum-maximum) or number (%). APGAR Score – scores 7 and above are generally regarded as normal, 4 to 6 fairly low and 3 and below critically low; CRIB II Score – Clinical Risk Index for Babies II, higher scores reflect poorer physiological stability; CPAP – Continuous Positive Air Pressure respiratory support; Patent Ductus Arteriosus refers to a hemodynamically significant duct diagnosed in first 72 hrs; IVH – intraventricular hemorrhage greater than grade II (significant IVH); Mean Blood Pressure reported is that at 24 h postnatal age and was not assessed in term controls; Death is those infants that survived to 72 h postnatal age but died prior to discharge.</p><p>*significantly different from females of the same gestational age group p<0.05;</p>†<p>significantly different from preterm neonates, within sex.</p

Structural equation model of predicted interactions of the gasotransmitters and their contribution to the regulation of microvascular blood flow at 24h postnatal age in the preterm human.

<p>The overall model (males and females combined) is presented and has a Goodness of Fit of χ² = 1.02 and RMSEA value of 0.017 (CI 0.00–0.28). Structural equation modelling examines linear causal relationships among variables, while simultaneously accounting for measurement error. The measurement error, or variance, determined in the model is 0.66 for microvascular blood flow, 0.77 for hydrogen sulphide, 0.24 for nitric oxide and 0.07 for carbon monoxide. NO was positively correlated with H₂S (p = 0.002, z = 3.05). There was an inverse correlation between CO and H₂S (p = 0.18, z = -1.34). There was a significant relationship between H₂S and microvascular blood flow (p = 0.012, z = 2.52) when the input of NO and CO to H₂S was included in the model.</p

Published Interactions of the Gasotransmitters.

<p>HO heme oxygenase; CSE cystathionine-γ-lyase; CBS cystathionine-β-synthase; NOS nitric oxide synthase (eNOS endothelial isoform, iNOS inducible isoform, nNOS neuronal isoform). ¹leukaemic monocyte macrophage cell line.</p><p>Published Interactions of the Gasotransmitters.</p

Physical Characteristics of Neonates.

<p>Data presented as median (range) or number (percentage) as appropriate. PU laser Doppler perfusion units</p><p>Physical Characteristics of Neonates.</p