Bilateral cerebral rSO2 measurements in a patient who had undergone aortic arch reconstruction were useful in determining the SCP flow rate during DHCA. Bilateral cerebral rSO2 measurements are necessary because the left side of the brain may exhibit lower rSO2 compared to the right side, presumably due to blood flow to the left side during SCP from the brachiocephalic artery occurring via the circle of Willis [2]. Moreover, various anatomic variations and obstructions to left-sided venous drainage may affect the blood flow to the left side of the brain [3]. As mentioned above, a prospective study demonstrated that out of 19 neonatal patients who had undergone Norwood surgery, 9 exhibited sustained differences in rSO2 values exceeding 10%, with the left-side values being lower than the right. In some cases, rSO2 differences were as large as 30% [3]. Hence, without monitoring the left side of the brain, undetected desaturation in the left cerebral hemisphere could occur.

In the present case, cerebral rSO2 values rose in conjunction with the escalation of the SCP flow rate. This rate varies widely in the literature, and the optimal flow rate for effective brain protection during development remains unclear. Although the SCP flow rate was defined as 50 mL/kg/min in the initial description of SCP with the DHCA technique [4], the flow rates varied widely, ranging from 20 to 94 mL/kg/min [1]. To overcome the uncertainties around the SCP flow rate, a suggested approach involved adjusting the flow rate to maintain cerebral rSO2 and Doppler flow velocity to within 10% of the baseline values recorded during full-flow CPB [2]. However, thresholds of rSO2 associated with central nervous system injury in pediatric cardiac surgery are under investigation and also there was no clear evidence for avoiding the injury during SCP. Some basic and clinical studies revealed that thresholds of near-infrared spectroscopy for cerebral injury were oxygen saturations in the range of 33 to 55% during perioperative period [5,6,7,8]. In a piglet model to determine thresholds for neurologic injury, brain tissue lactate accumulation began when cerebral rSO2 values decreased to less than 45% [7]. A clinical study also revealed that the development of new or worsened ischemia on postoperative magnetic resonance imaging following the Norwood procedure was associated prolonged low postoperative cerebral rSO2 values (rSO2 < 45% for > 180 min) [9]. These results suggest that cerebral rSO2 values at least 45% should be maintained during SCP. On the contrary, with 14 out of 34 patients exhibiting cerebral rSO2 values of 95% [2], there is a potential risk of cerebral hyperperfusion, and brain injury can occur at high cerebral rSO2 levels if the guidance for SCP flow relies solely on rSO2 measurements. Further studies are needed to elucidate the targeted cerebral rSO2 levels during SCP in pediatric patients.

In addition to the evaluation of local tissue ischemia, measurement of rSO2 in patients with congenital heart disease has some advantages. The baseline values of rSO2 during the induction of anesthesia are clinically important to assess the global cardiopulmonary function of congenital heart disease patients. Previous studies showed that preoperative rSO2 values in awake state could be used to predict poor outcomes in patients undergoing congenital heart surgery [10]. Additionally, we have previously reported that baseline rSO2 values after induction of anesthesia were associated with several adverse postoperative outcomes [11] and Modestini and colleagues supported our findings; lower baseline rSO2 values were associated with a longer ICU and hospital stay, as well as with a longer duration of mechanical ventilation [12]. As shown previously in adult patients undergoing cardiac surgery, low preoperative rSO2 values were associated not only with neurologic adverse outcome but also with increased mortality rates [13]. These results suggested that the brain might be an “index organ” reflecting the severity of cardiopulmonary compromise in patients with cardiac disease [14].

This case showed that bilateral cerebral rSO2 measurements in a pediatric patient undergoing aortic arch reconstruction were useful in establishing the SCP flow rate during DHCA. Although the limited frontal area of a pediatric patient makes bilateral rSO2 measurements difficult, such measurements are indispensable, especially in the context of a pediatric patient undergoing SCP.