Diminished vagal tone is a predictive biomarker of necrotizing enterocolitis-risk in preterm infants.

Necrotizing enterocolitis (NEC) is the most prevalent and devastating bowel disease in the neonatal intensive care unit (NICU) affecting 6-10% of preterm infants, with a mortality rate of 15-25%, increasing to 50% in surgically-treated cases (1). NEC primarily affects infants less than 32 weeks postmenstrual age (PMA), and is diagnosed only after clinical observations such as delayed gastric emptying, lethargy, hypotension, abdominal distention and blood in stools become apparent. Often, these symptoms develop suddenly in an otherwise well infant; diagnosis of NEC is then confirmed by laboratory tests and abdominal radiographics, during which time NEC may progress rapidly to pneumatosis intestinalis, intestinal perforation or death (2). Given multifactorial influences associated with NEC such as, for example, immature GI motility, hypoxia-ischemia, inappropriate bacterial colonization, several, diverse approaches have been proposed to diagnose pre-clinical NEC (3). Many of these approaches are cumbersome with several practical and theoretical flaws, and an authoritative agreement on their use is still lacking; to date there are no strategies for identifying which infants are most likely to develop NEC. Heart rate variability (HRV) is a non-invasive measure of autonomic nervous system regulation that has become the conventionally accepted term to describe variations of both instantaneous heart rate and R-R interval. Previous studies have identified HRV as an indicator of fetal and neonatal well-being (4) and HRV is altered by several physiological and pathophysiological factors including stress and inflammation. Indeed, there are several studies hypothesizing that HRV may be used as a predictor of morbidity (5-7) and may, potentially, be used as a window into stress and inflammation in preterm infants in the NICU. Frequency domain analysis of HRV, separates spectral frequencies reflective of the influence and integrity of sympathetic and parasympathetic activity on the cardiovascular system (8). In particular, the high frequency (HF) spectrum provides a reliable reflection of parasympathetic modulation, i.e. vagal tone (9), which is associated with the maintenance of physiological homeostasis. Many studies have shown that HRV and vagal tone decrease in anxiety, stress and inflammation/sepsis(10-16); indeed, the vagus also plays a critical role in the cholinergic anti-inflammatory reflex (17, 18), which has been shown to confer protection against tissue damage in many GI-related inflammatory diseases, including acute pancreatitis, colitis and inflammatory bowel disease (19-21). While HF-HRV provides a direct, non-invasive measure of vagal efferent activity (8); additional potentially useful indirect measures of vagal activity on the cholinergic anti-inflammatory reflex include blood sampling for C-reactive protein and cytokine analysis (18). It has long been known that vagal efferent outflow regulates motility and secretion of the upper GI tract; and it is well accepted that their main modulator is a tonic cholinergic vagal tone (22, 23). In the preterm infant, immature vagal innervation results in low gastric motility, reduction of anti-inflammatory response and down-regulation of intestinal immune defenses required for cell adhesion and chemotaxis (13, 18). It is well accepted that the pathogenesis of NEC includes an exaggerated inflammatory response resulting in high levels of pro-inflammatory cytokines (2). We surmised that in preterm infants vagal dysregulation with impaired neuroimmune modulation of inflammation, detectable through low HF-HRV power may be a predisposing condition for later development of NEC. The aim of this study was to test the hypothesis that alterations of the HF component of HRV may be used as a predictive biomarker for NEC-risk before the onset of clinical disease.