BACKGROUND Spinal cord ischemia occurs frequently during thoracic aneurysm repair. Current methods to detect ischemia, based upon electrophysiology techniques, are indirect, non-specific, and temporally slow. Here we report the testing of a spinal cord blood flow and oxygenation monitor, based on Diffuse Correlation and Optical Spectroscopies, during aortic occlusion in a sheep model. METHODS Testing was carried out in sixteen Dorset sheep. Sensitivity in detecting spinal cord blood flow and oxygenation changes during aortic occlusion, pharmacologically induced hypotension and hypertension, and physiologically induced hypoxia/hypercarbia were assessed. Accuracy of the Diffuse Correlation Spectroscopy measurements was determined via comparison to microsphere blood flow measurements. Precision was assessed through repeated measurements in response to pharmacologic interventions. RESULTS The fiber optic probe can be placed percutaneously, and is capable of continuously measuring spinal cord blood flow and oxygenation preoperatively, intraoperatively, and postoperatively. The device is sensitive to spinal cord blood flow and oxygenation changes associated with aortic occlusion, immediately detecting a fall in blood flow (−65 ± 32%, n=32) and blood oxygenation (−17 ± 13%, n=11) in 100% of trials. Comparison of spinal cord blood flow measurements by the device with microsphere measurements led to a correlation of R2=0.49, p<0.01 and the within-sheep coefficient of variation was 9.69%. Finally, Diffuse Correlation Spectroscopy is temporally more sensitive to ischemic interventions than motor evoked potentials. CONCLUSIONS The first generation spinal fiber optic monitoring device offers a novel and potentially important step forward in the monitoring of spinal cord ischemia.
The prevention and treatment of spinal cord injury are focused upon the maintenance of spinal cord blood flow, yet no technology exists to monitor spinal cord ischemia. We recently demonstrated continuous monitoring of spinal cord ischemia with diffuse correlation and optical spectroscopies using an optical probe. Prior to clinical translation of this technology, it is critically important to demonstrate the safety profile of spinal cord exposure to the required light. To our knowledge, this is the first report of in situ safety testing of such a monitor. We expose the spinal cord to laser light utilizing a custom fiber-optic epidural probe in a survival surgery model (11 adult Dorset sheep). We compare the tissue illumination from our instrument with the American National Standards Institute maximum permissible exposures. We experimentally evaluate neurological and pathological outcomes of the irradiated sheep associated with prolonged exposure to the laser source and evaluate heating in ex vivo spinal cord samples. Spinal cord tissue was exposed to light levels at ∼18 × the maximum permissible exposure for the eye and ∼ ( 1 / 3 ) × for the skin. Multidisciplinary testing revealed no functional neurological sequelae, histopathologic evidence of laser-related injury to the spinal cord, or significant temperature changes in ex vivo samples. Low tissue irradiance and the lack of neurological, pathological, and temperature changes upon prolonged exposure to the laser source offer evidence that spinal cord tissues can be monitored safely with near-infrared optical probes placed within the epidural space.
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