## 4 Discussion  
Near-infrared spectroscopy (NIRS) has demonstrated the potential to detect cardiac arrest (CA) and monitor tissue oxygenation even in the absence of a pulse waveform. This suggests its application in identifying CA outside hospital settings and continuously monitoring patients until circulation is restored [32]. In this study, CA and cardiopulmonary resuscitation (CPR) in pigs using hyperspectral NIRS combined with laser Doppler flowmetry (LDF) were investigated. Broadband NIRS captures the full NIR spectrum at each time point, providing (1) the ability to detect chromophores beyond [_HbO_2] and [_HHb_], such as cytochrome c oxidase (_CCO_) and water; (2) improved accuracy in determining the chromophore concentration by resolving full spectral features; and (3) the capability to determine the path length and absolute concentrations using second differential spectral methods [33]. Our study aimed to analyze the relationship between redox cytochrome c oxidase (_rCCO_), the cerebral metabolic rate of oxygen (_CMRO_2), and the cerebral blood flow (_CBF_) during CA, CPR, and the return of spontaneous circulation (ROSC). Our results, obtained invasively directly from the brain, reveal distinct patterns of interaction between these variables, shedding light on the non-linear dynamics governing cerebral metabolism under extreme physiological conditions. The findings of this study emphasize the potential for advanced neuro-monitoring techniques to guide resuscitation strategies, particularly for critical care physicians managing post-cardiac arrest patients. Future clinical practice could integrate the continuous real-time monitoring of cerebral oxygen metabolism to optimize CPR's effectiveness and improve long-term neurological recovery. The strong linear relationship observed between _CBF_ and _CMRO_2, with \(R^{2}\) values approaching 1.0, aligns with expectations based on [PERSON]'s principle, indicating a robust coupling between oxygen delivery and consumption. This consistency underscores the fundamental role of _CBF_ in sustaining cerebral oxygen metabolism. In contrast, the relationships involving _rCCO_ deviated from linearity, particularly during CA. The power law fits for _CMRO_2 vs. _rCCO_ and _CBF_ vs. _rCCO_ exhibited lower \(R^{2}\) values and \(\gamma\) exponents of around 0.5, suggesting that mitochondrial function and oxygen utilization become less predictable as the blood flow diminishes. During cardiac arrest, the scaling factor \(k\) and power law exponent \(\gamma\) were consistently positive and significant (\(p<0.05\)), reflecting strong coupling between _CMRO_2, _CBF_, and _rCCO_, while the intercept \(a\) was not significant for _CBF_ vs. _CMRO_2, suggesting proportional flow-metabolism coupling without a baseline offset. In contrast, during CPR, \(a\) and \(k\) were not significant for _rCCO_-related relationships, indicating disrupted mitochondrial coupling, likely due to mechanical chest compressions. However, \(k\) remained significant for _CBF_ vs. _CMRO_2, highlighting the partial preservation of a linear flow-metabolism interaction even under unstable conditions. This suggests that while cerebral blood flow and oxygen metabolism coupling persists during CPR, mitochondrial dynamics are more sensitive to resuscitative efforts, emphasizing the need to protect mitochondrial function for better neurological outcomes. Interestingly, during CPR, the relationships between _rCCO_ and _CMRO_2 or _CBF_ weakened, likely due to the physical disturbances and hemodynamic fluctuations induced by mechanical chest compressions. The low \(R^{2}\) values during CPR reflect the challenges of accurately capturing cerebral metabolism under such turbulent conditions. This finding highlights the need for refined measurement techniques to mitigate motion artifacts and enhance data reliability. The \(p\)-values for \(\gamma\
eq 1\) across the entire cardiac event reveal distinct patterns in the relationships between _CMRO_2, _CBF_, and _rCCO_ during CA, CPR, and ROSC.