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Comparison Of Intradermal And Subcutaneous Tissue Oxygen Tension Monitor To Detect Flap Compromise
Mohamed M. Ibrahim, MD, Hysem Eldik, BS, Mahmoud M. Mohammed, B.Eng, Zi Jun Wu, MD, Bruce Klitzman, PhD.
The Division of Plastic, Maxillofacial, and Oral Surgery, Department of Surgery, Duke University School of Medicine, Durham, NC, USA.

PURPOSE:
The ideal monitoring tool to evaluate free flap success should be minimally invasive, continuous, cost-effective and reliable. Our group has previously introduced implantable oxygen sensors as a mean to monitor flaps in the immediate post-operative period and detect acute vascular compromise. The purpose of the current study was to compare and contrast intradermal vs. subcutaneous implantation of the sensors in their ability to detect flap compromise.
METHODS:
Experimental sensors were made by incorporating benzo-porphyrin dye into a matrix of biocompatible hydrogel. These sensors were approximately 3mm-long, 1.5mm-wide, and 0.5mm-thick. Two groups of male Sprague-Dawley rats had the skin flap site outlined and three sensors were intradermally (ID) implanted at tip, middle and base of the impending flap of one group, while subcutaneously (SQ) implanted in the second group. Corresponding control sensors were implanted laterally at least 1 cm away from the proposed flap in both groups. One day later, the outlined, caudally-based, full thickness flap was elevated on dorsum of rats. Gross flap viability was assessed with computer planimetric analysis. Inspired oxygen was modulated between 100% and 12%. Real-time tissue oxygen tension (TOT) readings were obtained from the sensors on days 0, 3 and 7.
RESULTS:
Oxygen readings by sensors modulated as expected when inspired oxygen was changed, indicating that the sensors are responsive and sensitive within a physiologic range. Gross planimetric analysis of both groups showed that 16% of the flap was necrotic at the tip of the flap as measured on d3 and was more pronounced on d7. Readings from the ID and the SQ sensors have demonstrated statistically significant decreases in oxygenation in all regions of the flap at all time points compared to the control sensors. Overall, SQ implanted sensors showed faster response times than ID implanted sensors. However, ID implantation was less invasive, and makes it easier to localize the sensor for measurement and also avoid migration of the sensor in the SQ plane.
CONCLUSION:
Our analysis revealed that even though both methods are efficacious and accurate in determining changes of oxygenation, SQ sensors responded faster that ID sensors, however ID implantation is easier, less invasive and keep the sensor localized in the specific spot where is it implanted.


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