Cutaneous Microcirculatory Flow Sensing for Flap Monitoring in a Porcine Model of Arterial and Venous Occlusion

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. As an alternative means for peripheral free flap monitoring, we present a novel probe that calculates microvascular blood flow velocity by measuring tissue thermal diffusion. This device is cutaneous, wireless and not subject to the same confounding variables as NIRS. METHOD: The cutaneous perfusion sensor includes 5 thermistors, a Bluetooth chip, and small battery mounted on circuit board. A thermal actuator (4-mm diameter) delivers a small amount of thermal power to the surface of the skin. The device is cutaneously mounted with a silicone acrylate adhesive onto target tissue. The diffusion of heat is measured and analytically converted into a measurement of microcirculatory blood flow velocity. Rapid thermal diffusion occurs in situations of high tissue perfusion, while slower thermal diffusion indicates slower or absent near-surface blood 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 flow device and T.Ox were 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, 10 devices were tested on 16 flaps in 10 separate pigs over 60 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. 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 0.76 to 0.83. Two novel devices tested simultaneously on the same flap showed only minor variations in flow measurements, with a cross-correlation of 0.87. 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. Future studies will characterize longevity of the device over a period of several days.