Dynamic changes in cerebral oxygenation related to deep hypothermia and circulatory arrest evaluated by near-infrared spectroscopy

Background: Total circulatory arrest in deep hypothermia, which is used in corrective surgery of complex cardiovascular malformations, has been said t o cause brain injury. Near-infrared spectroscopy (NIRS) is a new non-invasi ve method that potentially monitors changes in cerebral oxygenation and tis sue oxygen utilisation. The aim of this experimental study in rabbits was t o evaluate the change in intravascular and intracellular oxygenation patter ns during cooling, deep hypothermic circulatory arrest and rewarming using a commercially available NIRS-cerebral oximeter. Methods: Ten New Zealand White male rabbits (weight, 3.1 +/-0.25 kg BW) wer e included in this study. All animals underwent cardiopulmonary bypass (CPB ), cooling to a rectal temperature below 15 degreesC, 60 min of deep hypoth ermic circulatory arrest (DHCA) followed by reperfusion and rewarming. Cere bral oxyhaemoglobin (HbO(2)), deoxyhaemoglobin (HHb) and cytochrome oxidase aa(3) (CytOxaa(3)) concentrations were continuously measured during the en tire procedure using the Cerebral RedOx (TM) Monitor 2020 (Criticon cerebra l redox monitor 2020, Johnson & Johnson Medical). Total haemoglobin concent ration (tHb) and regional cerebral oxygen saturation (rSO(2)) values were c alculated by integrated algorithm. Results: In all animals an initial increase of oxygenated haemoglobin (HbO( 2), rSO(2)) and a fall in deoxygenated haemoglobin (HHb) were found during cooling on bypass. A slight decrease in CytOxaa3 signal was observed in res ponse to initial cooling. Variation in intravascular haemoglobin oxygenatio n parameters (HbO(2), HHb) were related to haemodynamic changes associated with fluid loading, initiation and termination of CPB, bypass flow rate and cooling and rewarming. When the pump flow was stopped all NIRS parameters, except the HHb value, decreased precipitously during the DHCA-pcriod (P<0. 01). After reperfusion and rewarming, all haemoglobin oxygen saturation rea dings returned nearly to pre-CPB levels (P=0.09), but the CytO-xaa(3) was s till significantly lower than the pre-CrB levels (P<less than>0.05). Conclusion: The change in the NIRS-derived haemoglobin oxygenation paramete rs may reflect physiological changes in systemic and cerebral haemodynamics . CytOxaa(3) values may represent related effects on cellular oxygenation. Thus, continuous, real-time NIRS-monitoring may identify critical periods w ith inadequate brain tissue oxygenation, particularly during DHCA. The neur ological implications of the observed changes in NIRS oxygenation parameter s, however, require further quantitative morphological evaluation of the br ain in animals surviving a longer reperfusion and observation period.