P2.P309: Photoacoustic Imaging Detects an Increased Intestinal Oxygen Reactivity in a Necrotizing Enterocolitis Rat Model

Background: Necrotizing enterocolitis (NEC) is an often-lethal disease affecting premature infants’ intestinal tract. Photoacoustic imaging (PAI) is an emerging non-invasive, real-time, diagnostic modality based on ultrasonic detection of optical absorption events. Pulses of near-infrared light are absorbed by tissue chromophores, undergoing thermoelastic expansion and acoustic signal production detected by an ultrasound transducer. This ‘light in – sound out’ approach allows for more sensitive/specific detection than ultrasound with significantly increased penetration depth over optical imaging (e.g. NIRS). PAI excels in quantifying tissue oxygen saturation (sO2) as hemoglobin is a strong optical absorber with distinct spectra based on oxygenation status. Prior understanding of intestinal perfusion in NEC suggests impairment in the microvasculature, which results in lower sO2 regardless of the amount of oxygen an infant is receiving. We therefore sought to use PAI to characterize intestinal sO2 in an experimental NEC animal model under a breathing gas challenge using 21% and 100% fraction of inspired oxygen (FiO2) to assess the oxygen reactivity.

Methods: NEC was induced in neonatal rat pups for 4 days via formula feeding, hypoxic stress, and oral lipopolysaccharide. Breastfed (BF) and NEC rat pups were imaged at 2- and 4-days old (n=5 and n=4, n=5 and n=6). To measure sO2, PAI was acquired using a Vevo 2100 to obtain images at 21%and 100% FiO2. sO2 average measurements were recorded for manually defined intestinal regions-of-interest, representing average sO2 within tissue. Delta sO2 was defined as the difference between average sO2 levels at FiO2 100% and 21%.

Results: Intestinal sO2 is decreased in NEC when compared to BF rat pups when breathing FiO2 21% (44.12%7.18 vs 55.903.77%, p< 0.01 at 2-days and 38.868.33 vs 56.13 3.52, p< 0.001 at 4-days) (Figure 1). At FiO2 100%, there is no difference in intestinal sO2 between NEC and BF rat pups at either 2-days or 4-days. The delta sO2 was higher in the NEC rat pups than BF, with a more pronounced change seen in the 4-day old pups (2-days old: 21.34%9.34 vs. 6.68 3.77%; p< 0.05, 4-days old: 30.15% 12.94 vs. 12.89% 5.24; p< 0.01).

Conclusion: Paradoxically, intestinal tissue in NEC was found to be more reactive to changes in FiO2 when compared to BF, suggesting dysregulation in oxygen homeostasis, rather than simply defects in intestinal microvasculature. This has important implications for an already fragile population that often is receiving oxygen supplementation. Further studies are needed to explore the underlying mechanism as well as utilize PAI in infants with NEC to determine the significance of this oxygen reactivity.