Reply: Regional Oxygen Saturation Index

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We thank Dr. Kagaya and Dr. Miyamoto for giving us the opportunity to discuss the potential problems regarding our proposed method of flap monitoring.1 Our responses to the two issues that they mentioned are as follows.
Our study aimed to offer a new, more rapid method for detecting perfusion problems with free perforator flaps using tissue oximetry while maintaining high diagnostic accuracy. The value of regional oxygen saturation is not always stable during the postoperative course. In our series, even when both systematic oxygen saturation and absolute value of regional oxygen saturation dropped because of extubation or discontinuation of oxygen administration, the regional oxygen saturation index was stable until the transferred flap remained healthy. This contributed to a reduction in the number of false-positive cases of vascular occlusion. However, in compromised flaps, the regional oxygen saturation index dropped faster than the absolute value of regional oxygen saturation, which dropped below the existing criterion values reported.
Using two probes is advantageous in terms of detection speed and diagnostic accuracy. In this study, we emphasized the usefulness of the TOS-OR (Fujita Medical Instruments Co., Ltd., Tokyo, Japan), which has two sensors to separately measure regional oxygen saturation in a device.
We are opposed to the proposal that a delayed-stained area may be more appropriate for detecting vascular compromise at an earlier stage, because the margins of the perforator flap beyond the angiosome are at risk of venous congestion even when these areas are stained using indocyanine green angiography. In such cases, the value of regional oxygen saturation may gradually decline without the occurrence of problems at the anastomosis site. The early-stained area is recommended as the first choice because false-positive outcomes should be excluded to the greatest extent possible. In contrast, in some situations, such as those with the thoracodorsal artery perforator flap described in our study, when the delayed-stained area should be used, the risk of false-positive outcomes should be considered. Future studies for identifying another criterion value for the delayed-stained area are necessary.
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