Optimizing Cerebral Oxygenation During Cardiac Surgery: A Review of Experimental and Clinical Investigations with Near Infrared Spectrophotometry*

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Abstract
Objectives: Near-infrared spectrophotometry (NIRS) is a promising method for non-invasive monitoring of cerebral oxygenation and hemodynamics. This paper reviews studies in which we aimed to validate NIRS in an experimental model of cardiopulmonary bypass (CPB) and deep hypothermic circulatory arrest (DHCA) (validation study), use the method in experimental settings to optimize cerebral oxygenation during CPB (oxygenation study), and test its utility during routine cardiac surgery (clinical study). Methods: Validation study: Forty 8 - 10 kg piglets underwent 60 min of DHCA at 15° C, were extubated on the first postoperative day, and sacrificed on the 4th postoperative day for histologic investigations. During CPB and DHCA, the animals were investigated by NIRS (monitoring of cerebral oxygenated hemoglobin (HbO2) and oxidized cytochrome aa3 (Cytox)) and magnetic resonance spectroscopy (MRS) (monitoring of cerebral adenosine triphosphate (ATP) and phosphocreatine (PCr). Oxygenation study: A normoxic (n = 5) and a hyperoxic group (n = 5) of piglets underwent 120 min of DHCA and 6 h of reperfusion with NIRS monitoring. Neuronal damage was evaluated by histology. Clinical study: Patients (n = 41) undergoing routine cardiac surgery were investigated by NIRS and neuropsychological testings. Results: Validation study: Reductions of CytOx and HbO2 values were closely correlated with decreases in ATP, PCr, and pHi. The changes in CytOx and PCr showed the strongest correlation (r = 0.623). Maximal CytOx reduction during DHCA predicted brain damage. Oxygenation study: Histology revealed a significant increase in brain damage in the normoxic group. Cytox and HbO2 tended to be lower during DHCA (p = 0.16). Clinical study: During CPB, arterial PCO2, pH and temperature were closely related to CytOx and HbO2. Patients who suffered from neuropsychological deficits had a lower CytOx minimum compared to those without these. Conclusions: NIRS measurements, especially the CytOx signal, correlate well with high energy phosphates and have a high sensitivity to predict histologic brain damage. NIRS is an important and useful tool in studies investigating cerebral oxygenation during CPB. The CytOx signal predicted impaired neuropsychological outcome in patients. Therefore, the CytOx signal holds much interest for future studies.

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