Brain oxygenation decreases significantly during circulatory arrest and remains impaired after rewarming and after termination of CPB despite normalization of oxygen availability . In contrast to experimental low-flow CPB, cerebral hyperaemia was found during rewarming followed by a rise in cerebrovascular resistance lasting for more than 8 h after hypothermic cardiac arrest . In children undergoing repair of congenital heart defects, electroencephalogram (EEG) alterations but also clinical seizures have been shown to be more likely to occur in patients after hypothermic cardiac arrest than with low-flow CPB. Furthermore, release of creatine kinase brain isoenzyme as a marker of cell damage was higher in hypothermic cardiac arrest children than with low-flow CPB. Only in children with a hypothermic cardiac arrest duration <35 min at 18°C were neither EEG alterations nor clinical seizures found . In addition, hypothermic cardiac arrest has been associated with a higher risk of delayed motor development and neurological abnormalities at the age of 1 yr when compared to surgery with low-flow bypass . Clancy and colleagues reported a 19% incidence of adverse neurological events with genetic conditions (dysmorphy, chromosomal aberrations), aortic arch obstruction and deep hypothermic cardiac arrest time >60 min as the major contributing factors . The use of allopurinol, a scavenger and inhibitor of oxygen free radicals, resulted in a better outcome in children with hypoplastic left heart syndrome .
To try and prevent the above, selective ACP has been established in adult aortic arch surgery and with adequate cerebral perfusion ACP significantly reduced neurological dysfunction and in-hospital mortality [10,11]. For paediatric cardiac surgery with reconstruction of the aortic arch various methods of cerebral protection have been discussed including pH-strategy before hypothermic cardiac arrest to provide a more uniform cooling of the brain [2,12]. However, because of continuing concerns regarding the adverse effects of deep hypothermic circulatory arrest on the neonatal brain, a technique of regional low-flow perfusion that provides cerebral circulation has been introduced by Pigula and colleagues .
As interventions based on neurophysiological monitoring may decrease the incidence of postoperative neurological sequelae, monitoring of cerebral perfusion and oxygenation during ACP by using a combination of different techniques is mandatory and must be primarily directed to assure that oxygen delivery to the brain is adequate. This is of special importance in neonates where accidental interruption of cerebral perfusion may occur simply due to technical problems .
In paediatric cardiac surgery routine neurophysiological monitoring mainly consists of EEG, transcranial Doppler and near infrared spectroscopy, and has been shown to improve neurological outcome if an intervention is made following a deviation from defined target values . As EEG monitoring for children during deep hypothermia is not feasible, transcranial Doppler and near infrared spectroscopy are methods of choice for monitoring ACP .
Changes in cerebral blood flow velocity correlate well with changes in cerebral blood flow [16-18]. However, comparable to our results, in the presence of low-pump flow cerebral perfusion may not be detectable any more because of limited threshold resolution of the Doppler device [19,20]. In neonates undergoing Switch procedures a minimum bypass flow rate of 30 mL kg−1 min−1 was needed to detect cerebral perfusion .
Under the condition of sufficient arterial oxygen supply and stable anaesthetic level jugular bulb saturation is a function of cerebral metabolic rate and cerebral blood flow during CPB . As there is a strong correlation between cerebral oxygen extraction and jugular venous saturation, the latter provides an online monitor for cerebral cooling . However, it is important to recognize that the oxygen content and saturation of the jugular venous bulb is the average of many regions of the brain and may not reflect areas of regional hypoperfusion.
Due to the reduced oxygen demand cerebrovascular saturation increases during progressive cooling, but decreases during cardiac arrest. The critical level of oxygenation is unknown, but it is recommended to keep residual saturation well above 30% during hypothermic cardiac arrest at 18°C to prolong neuronal saturation [28,29].
The limitations of this study are the absence of blinding, the relatively small sample size and the absence of a control group. For ethical reasons a control group of children with deep hypothermic circulatory arrest was not feasible. The consistency among the patients suggests that the results of the study reflect the benefit of near infrared spectroscopy as part of multimodal cerebral monitoring equipment. However with the device used only cerebral oxygen saturation and delivery can be measured. For the determination of the actual tissue uptake of oxygen, cytochrome aa3 should be used. Nevertheless the present analysis also confirms at least partially previous studies and extends the evidence of the benefit of near infrared monitoring in children. On the other hand, the device is not certified for very small children because of the adhesive which may irritate the neonate's skin. Only small areas of the brain underneath the sensors are detected and some deeper areas are excluded from monitoring, though the near infrared light penetration in small children is much better than in adults. As there is no critical cut-off known, only changes of oxygen saturation can be used to detect improper cannulation or compromised cerebral perfusion.
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