Regional Oxygen Saturation Index: A Novel Criterion for Free Flap Assessment Using Tissue Oximetry

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We read the article entitled “Regional Oxygen Saturation Index: A Novel Criterion for Free Flap Assessment Using Tissue Oximetry” by Akita et al.1 with great interest. The authors succeeded in detecting vascular compromise in postoperative free flaps with high accuracy by monitoring the regional oxygen saturation index (the ratio of regional oxygen saturation on the flap and the control nondissected portion), using the two probes of a TOS-OR (Fujita Medical Instruments Co., Ltd., Tokyo, Japan) near-infrared spectroscopy device. We agree with the authors about the usefulness of the near-infrared spectroscopy device in monitoring the free flap based on our own experience and the vast experiences of other authors in using another near-infrared spectroscopy device (Tissue Oximeter; ViOptix, Inc., Newark, Calif.).2–5 However, we would like to point out two potential problems with the authors’ method of flap monitoring.
First, the necessity of measuring the control nondissected portion is questionable. The value of tissue oxygen saturation is usually stable if the probe is set on the same position in normal tissue. Accordingly, the denominator (the control nondissected portion) of the regional oxygen saturation index is considered stable, and the change in the regional oxygen saturation index mostly depends on the numerator (the regional oxygen saturation of the flap). To improve the ease of application by co–medical staff, it would seem advantageous to use the absolute tissue oxygen saturation value on the flap or its decrease from the initial value. Moreover, because most other near-infrared spectroscopy devices have one probe, the simultaneous monitoring of two sites would require two devices.
Second, the rationale for monitoring the regional oxygen saturation of the area that has been stained earlier in indocyanine green angiography is unclear. Of course, every free flap has areas of high and low perfusion, and the ischemic or congestive changes in cases of vascular compromise usually appear first in areas of low perfusion. Thus, we wonder whether monitoring the regional oxygen saturation level in the area with delayed staining would be more appropriate for detecting vascular compromise at an earlier stage. We would be interested to hear the authors’ opinions regarding these issues.
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