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Response to Letter to the Editor Regarding “Noninvasive Tissue Oximetry for Flap Monitoring: An Initial Study (J Reconstr Microsurg 2007;23:189–197)”
01. Oktober 2008 (online)
The comments by Dr. Knobloch et al are appreciated. He and his colleagues are gracious to share their experience on this matter, and they clearly understand the relationship of oxygen levels and flap health.
Indeed, as they detail, the length of time that a flap has been compromised determines to a large extent if that flap can be salvaged. Monitoring techniques vary from institution to institution, but earlier diagnosis of a compromised flap leads to a higher salvage rate, and, importantly, a more “normal” flap (i.e., less fat necrosis, fibrosis, and overall firmness). Although all of the flaps in the study were evaluated by trained nursing and resident staff, it can be difficult even for a trained microsurgical attending physician to make an early diagnosis of vascular compromise to a flap.
The probe used in the study takes its readings throughout a minimum depth range of 0 to 5 mm. The depth of readings is determined by the distance between the emitters and sensors on the probe head and the sensitivity of the light detectors. A maximum depth range of 0 to 10 mm can be achieved in less pigmented individuals, due to the corresponding reduction in light absorption. Because the flaps were all cutaneous flaps and the readings were not taken from buried portions of the flap, the reading depth was appropriate. We have no reason to suspect that the readings would be different at a deeper level within the flap because both shallow and deep regions share a common blood supply. Work is currently being done to measure the tissue oxygen at deeper depths to provide a method to monitor a buried flap.
Dr. Knobloch correctly notes that ambient light and the operating room lights could interfere with the measurements of tissue oxygen. This problem has been addressed in the ODISseyTissue Oximeter (ViOptix Inc., Fremont, CA), through the incorporation of a foil layer in the fixation device that holds the probe in place. This adequately blocks ambient light.
To facilitate continuous monitoring, the probe is fixed in place by means of a surrounding adhesive pad in a manner similar to an adhesive electrocardiogram lead. The sensor head rests flush with no significant positive pressure exerted on the skin. The gentle contact of the probe is constant and has not interfered with readings.
Although the numeric value reflecting Sto 2 by itself provides earlier warning than existing flap monitoring methods, achieving an even earlier diagnosis is possible by viewing the numeric value in the context of the tissue oxygen trend graph, with special attention to the rate of drop of the Sto 2 level. The addition of these criteria can allow for the diagnosis of flap compromise to be made in these cases even before a highly trained observer can make the same determination. Notably, this can be done without the need for additional types of monitoring equipment.
Looking to the future, it should be possible to shorten the time to diagnosis of a vascular compromise even further by monitoring data that should be readily available from the monitor, including a rate of drop alert driven by the tissue oxygen trend graph and the tissue hemoglobin level.
Attending Surgeon, North Shore Long Island Jewish Health System, Assistant Clinical Professor, New York University
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