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Plastic Surgery Research Council

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Intramuscular Microvascular Flow Sensing For Flap Monitoring In A Porcine Model Of Arterial And Venous Occlusion.
William R. Moritz, MD1, Di Lu, PhD2, Quansan Yang, BS2, Lauren Jacobson, MD1, Hany M. Arafa, MS2, Diana Ostojich, BS2, Wubin Bai, PhD3, Hexia Guo, MS2, Changsheng Wu, PhD2, Shuo Li, PhD2, Shupeng Li, BS4, Yonggang Huang, PhD4, Yameng Xu, MS5, Ying Yan, MD, PhD5, Amanda M. Westman, PhD1, Matthew R. MacEwan, MD, PhD5, John A. Rodgers, PhD2, Mitchell A. Pet, MD1.
1Division of Plastic and Reconstructive Surgery, Department of Surgery, School of Medicine, Washington University, St. Louis, MO, USA, 2Department of Materials Science and Engineering, Querrey Simpson Institute for Bioelectronics, Northwestern University, Chicago, IL, USA, 3Department of Applied Physical Sciences, The University of North Carolina at Chapel Hill, Chapel Hill, NC, USA, 4Department of Mechanical Engineering, Northwestern University, Evanston, IL, USA, 5Department of Neurosurgery, School of Medicine, Washington University, St. Louis, MO, USA.

PURPOSE:Cutaneous devices employing near infrared spectroscopy (NIRS) for continuous free flap tissue oxygenation (StO2) monitoring have several limitations. Confounding factors like skin pigmentation and thickness, ambient light incursion, and inconsistency at the skin-sensor interface can alter StO2 measurements. Additionally, commercially available NIRS sensors can only be used on flaps with a cutaneous component. We present a novel thermal diffusivity probe for measuring microvascular blood flow as an alternative means for peripheral free flap monitoring. This device is wireless, implantable, and is not subject to the same confounding variables as NIRS. METHODS:
The intra-tissue microvascular flow-sensing device includes a resistive heater, 4 thermistors, a small battery, and a BluetoothTM chip which allows connection to a smart device. The probe measures 2-mm x 2-cm x 1-mm and is easily inserted into target tissue (Figure 1A-C). Diffusion of heat is measured and mathematically transformed into a measurement of microcirculatory blood flow. Rapid thermal diffusion occurs in situations of high tissue blood flow, while slower thermal diffusion indicates slow or absent flow.This device was tested alongside Vioptix T.Ox in a porcine rectus abdominus myocutaneous flap model of arterial and venous pedicle occlusion. After flap elevation, the device was deployed intramuscularly, and the T.Ox device was applied to the skin. Acland clamps were alternately applied to the flap artery and veins to achieve conditions of flap ischemia and congestion, each lasting 15 minutes, with a 15 minute intervening recovery period. In total, 5 devices were tested in 3 flaps in 3 separate pigs over 16 unique vaso-occlusive events. RESULTS:Continuous monitoring was accomplished and no connection loss was observed with either T.Ox or the novel devices. Flow measurements were responsive to both ischemia and congestion, and returned to baseline during recovery periods. A representative graph of microvascular blood flow and StO2 during periods of ischemia (I), congestion (C), and recovery (R) is shown (Figure 2). Flow measurements corresponded with vascular occlusion and release, and closely followed StO2. Cross-correlation at zero lag showed agreement between these two sensing modalities, ranging from 0.739 to 0.933. Two novel devices tested simultaneously on the same flap showed only minor variations in flow measurements, with a cross-correlation of 0.953.
CONCLUSION:This novel probe is capable of detecting changes in tissue microcirculatory blood flow. This device performed well in a swine model of flap ischemia and congestion, and shows promise as a potentially useful clinical tool. Because flow measurements are not affected by the variable lighting and light absorbing properties of tissue, this monitoring strategy has the potential to provide more consistent information than NIRS. Our study demonstrated satisfactory function within muscle tissue. Future studies will investigate performance in fasciocutaneous flaps and characterize longevity of the device over a period of several days.


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