Differences in brain oxygenation between two sequential cardiac arrests during axillobifemoral bypass in a stanford type A dissecting aneurysm

Abstract I SCHEMIC PRECONDITIONING is a protective mechanism against long-lasting ischemic events after a short period of ischemia in many organ systems. 1,2 There has been only one study addressing this phenomenon in the nervous system in humans. 3 Several mechanisms of ischemic preconditioning, such as adenosine triphosphate (ATP)—sensitive potassium channel (K ATP ), reactive oxygen species, and protein kinase C, have been proposed, 1 and an animal study revealed that mild cerebral ischemic stress can induce improvement in brain oxygen metabolism detected by near-infrared spectroscopy (NIR) during subsequent ischemia. 4 Although there have been several reports using NIR to monitor cerebral oxygenation during cardiac arrest, only hemoglobin saturation was measured. 5,6 The dynamic changes of saturation can be useful, and the changes of this value correlated well with brain hypoxia within individuals. 7 The readings obtained from the instrument need to be carefully interpreted in clinical medicine, however. 7–9 Monitoring of hemoglobin saturation alone may not be adequate for the assessment of cerebral oxygenation because during hypothermic cardiopulmonary bypass, a decreased in tissue oxygenation occurred despite an increase in cerebral oxyhemoglobin (HbO 2 ) concentration. 10 There are two types of commercially available NIR instruments that can measure brain oxygenation: tissue saturation instruments, as previously mentioned, and concentration-measuring instruments. Concentration-measuring instruments show the changes of HbO 2 , deoxyhemoglobin (Hb), sum of HbO 2 and Hb (total Hb), and cytochrome aa 3 (Cytaa 3 ). 11 During brain ischemia, the concentration-measuring instrument shows a decrease of HbO 2 accompanied by an increase of Hb. 11,12 In addition to the changes of HbO 2 and Hb, it has been reported that the changes in total Hb are useful indicators of alterations in cerebral blood volume and that the alterations in total Hb are linearly related to those in cerebral blood flow, 13,14 if the concentration of hemoglobin in blood is not changed. A report also indicated that a significant correlation between changes in cerebral blood flow and changes in total Hb was found in human subjects using NIR and positron-emission tomography. 15 Cytaa 3 is the terminal enzyme of the mitochondrial electron transport chain, the site of electron transfer to molecular oxygen. It has been shown that decreases in phosphocreatine and nucleotide triphosphate (mostly ATP) measured by magnetic resonance spectroscopy correlated closely with decreased Cyaa 3 detected by NIR and arterial blood pressure in piglets. 12 Monitoring the oxidative status of Cytaa 3 may provide a true look at the status of tissue oxygenation inside the neuronal cells, 16 if the chromophore path length is not changed. The authors report intraoperative brain oxygenation detected by NIR during an axillobifemoral bypass in a patient with a Stanford type A dissecting aneurysm who experienced two episodes of intraoperative ventricular fibrillation (VF).