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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>[</mml:mo> <mml:msub><mml:mrow><mml:mi>HbO</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> <mml:mo>]</mml:mo></mml:mrow> </mml:math> ) 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HbDiff</mml:mi> <mml:mo>]</mml:mo> <mml:mo>=</mml:mo> <mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:msub><mml:mrow><mml:mi>HbO</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> <mml:mo>]</mml:mo> <mml:mo>-</mml:mo> <mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HHb</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> ), blood volume ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HbT</mml:mi> <mml:mo>]</mml:mo> <mml:mo>=</mml:mo> <mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:msub><mml:mrow><mml:mi>HbO</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> <mml:mo>]</mml:mo> <mml:mo>+</mml:mo> <mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HHb</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> ), and metabolism ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>oxCCO</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> ) 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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts><mml:mrow><mml:mi>H</mml:mi></mml:mrow> <mml:mprescripts></mml:mprescripts> <mml:none></mml:none> <mml:mrow><mml:mn>1</mml:mn></mml:mrow> </mml:mmultiscripts> </mml:mrow> </mml:math> ) 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>oxCCO</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> -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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi> <mml:mo><</mml:mo> <mml:mn>0.0001</mml:mn></mml:mrow> </mml:math> ) between the two groups based on this cut-off threshold of 79% <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>oxCCO</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> -RF. The severe injury group ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>n</mml:mi> <mml:mo>=</mml:mo> <mml:mn>13</mml:mn></mml:mrow> </mml:math> ) had <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mo>∼</mml:mo> <mml:mn>30</mml:mn> <mml:mo>%</mml:mo></mml:mrow> </mml:math> smaller recovery in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HbDiff</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> -RF ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.0001</mml:mn></mml:mrow> </mml:math> ) and no significant difference was observed in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Δ</mml:mi> <mml:mo>[</mml:mo> <mml:mi>HbT</mml:mi> <mml:mo>]</mml:mo></mml:mrow> </mml:math> -RF between groups. At 48 h post HI, significantly higher <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mrow> <mml:mmultiscripts><mml:mrow><mml:mi>P</mml:mi></mml:mrow> <mml:mprescripts></mml:mprescripts> <mml:none></mml:none> <mml:mrow><mml:mn>31</mml:mn></mml:mrow> </mml:mmultiscripts> </mml:mrow> </mml:math> -MRS-measured inorganic phosphate/exchangeable phosphate pool (epp) ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.01</mml:mn></mml:mrow> </mml:math> ) and reduced phosphocreatine/epp ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>p</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.003</mml:mn></mml:mrow> </mml:math> ) 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.