Quantification of the severity of hypoxic-ischemic brain injury in a neonatal preclinical model using measurements of cytochrome-c-oxidase from a miniature broadband-near-infrared spectroscopy system

Abstract

We describe the development of a miniaturized broadband near-infrared spectroscopy system (bNIRS), which measures changes in cerebral tissue oxyhemoglobin ( $\left[{\mathrm{HbO}}_2\right]$ ) and deoxyhemoglobin ([HHb]) plus tissue metabolism via changes in the oxidation state of cytochrome-c-oxidase ([oxCCO]). The system is based on a small light source and a customized mini-spectrometer. We assessed the instrument in a preclinical study in 27 newborn piglets undergoing transient cerebral hypoxia-ischemia (HI). We aimed to quantify the recovery of the HI insult and estimate the severity of the injury. The recovery in brain oxygenation ( $\Delta \left[\mathrm{HbDiff}\right]=\Delta \left[{\mathrm{HbO}}_2\right]-\Delta \left[\mathrm{HHb}\right]$ ), blood volume ( $\Delta \left[\mathrm{HbT}\right]=\Delta \left[{\mathrm{HbO}}_2\right]+\Delta \left[\mathrm{HHb}\right]$ ), and metabolism ( $\Delta \left[\mathrm{oxCCO}\right]$ ) for up to 30 min after the end of HI were quantified in percentages using the recovery fraction (RF) algorithm, which quantifies the recovery of a signal with respect to baseline. The receiver operating characteristic analysis was performed on bNIRS-RF measurements compared to proton ( ${}^{1}H$ ) magnetic resonance spectroscopic (MRS)-derived thalamic lactate/N-acetylaspartate (Lac/NAA) measured at 24-h post HI insult; Lac/NAA peak area ratio is an accurate surrogate marker of neurodevelopmental outcome in babies with neonatal HI encephalopathy. The $\Delta \left[\mathrm{oxCCO}\right]$ -RF cut-off threshold of 79% within 30 min of HI predicted injury severity based on Lac/NAA with high sensitivity (100%) and specificity (93%). A significant difference in thalamic Lac/NAA was noticed ( $p<0.0001$ ) between the two groups based on this cut-off threshold of 79% $\Delta \left[\mathrm{oxCCO}\right]$ -RF. The severe injury group ( $n=13$ ) had $\sim 30\%$ smaller recovery in $\Delta \left[\mathrm{HbDiff}\right]$ -RF ( $p=0.0001$ ) and no significant difference was observed in $\Delta \left[\mathrm{HbT}\right]$ -RF between groups. At 48 h post HI, significantly higher ${}^{31}P$ -MRS-measured inorganic phosphate/exchangeable phosphate pool (epp) ( $p=0.01$ ) and reduced phosphocreatine/epp ( $p=0.003$ ) were observed in the severe injury group indicating persistent cerebral energy depletion. Based on these results, the bNIRS measurement of the oxCCO recovery fraction offers a noninvasive real-time biomarker of brain injury severity within 30 min following HI insult.

Keywords: broadband near-infrared spectroscopy; cytochrome-c-oxidase; hypoxia-ischemia; near-infrared spectroscopy; neonatal encephalopathy.