To the Editor:

We are very thankful for Dr. Day's insightful comments and interest in our manuscript.

Initially, cytochrome aa3 (CytO2) was believed to be a very reliable indicator for reflecting the energy status of tissue. As Dr. Day pointed out, we believed that the measuring of cytochrome was effective when discussing oxygen delivery at the level of brain neurons. We conducted CytO2 measurements on all the infants in our study, but did not perform any follow-up examinations related to CytO2 because we were unable to produce any stable results, and recent research has not shown a reliability of CytO2. The manuscript did not include CytO2 data that could be used for estimating tissue oxygenation. As there have been various viewpoints concerning data on CytO2 from the past, the new model (NIRO-200) that has been provided by NIRS has been removed as a measurement item. At the moment, we believe that the tissue oxygenation index (TOI) is the most effective indicator for measuring the oxygenation of tissue.

We anticipated that oxygen delivery to tissue would be maintained because systemic and cerebral blood flow would increase, offsetting the decrease in oxygen saturation through hypoxic gas management. However, through this study, we found that an increase in blood flow did not compensate for tissue oxygenation, although systemic and cerebral blood flow increased with the increase in cHb and low oxygenation.

As Dr. Day indicated, we agree that there is no need to maintain very low oxygen saturation in actual preoperative management for every infant with a large left to right shunt, such as CoA complex. However, we do feel that hypoxic gas management in large left to right shunt diseases is very useful in maintaining systemic circulation, which is usually manifested by stable urination and an acid base balance. As a result, oxygen saturation could be lower than 90% in some cases.

We also fully understand the usefulness of hypoxic gas management in congenital heart disease with increased pulmonary blood flow. This maintenance with over 80% in oxygen saturation that we have shown this time is an indicator taken into consideration from the results of present study. As we have mentioned in our manuscript, while we require further studies with larger sample sizes, we also require further studies with long-term follow-up.