Fiber Optic Monitoring for Spinal Cord Ischemia

1. Fiber Optic Monitoring for Spinal Cord Ischemia Thomas V. Bilfinger, MD1, ScD; Robert M. Galler, DO2; Rickson Mesquita, PhD3; Angela D’Souza, PhD4; Asher Emanuel, MD5; Arjun Yodh, PhD3*; Thomas F. Floyd, MD4,5,6*. Stony Brook University, Department of Surgery1, Neurosurgery2, Biomedical Engineering4, Anesthesiology5 University of Pennsylvania, Department of Physics and Astronomy3, Surgery6 Principal Investigators*

2. Aortic Surgery & Spinal Cord Ischemia • Frequency of spinal ischemia after aortic procedures varies between 0.2% in elective AAA to 40% in acute dissections requiring repair with Type 2 aneurysm-repairs having an up to 21% incidence. • Hypotension, hypoxia, vasospasm, inflammation and edema, and hemorrhage all propagate spinal ischemia after injuryas well as the number of interrupted segmental arteries. • Prevention of secondary injury from ischemia may assist in limiting disability.

3. The Technology • Diffuse Correlation Spectroscopy: Quantifies the fluctuations of light scattered in tissues. Fluctuations in light scattering occur due to the motion of scatterers in tissue, mainly red blood cells. The greater the scattering, the greater the blood flow. • Boas, D.A., L.E. Campbell, and A.G. Yodh, Scattering and Imaging with Diffusing Temporal Field Correlations. Phys Rev Lett, 1995. 75(9): p. 1855-1858. • Durduran, T., R. Choe, W.B. Baker, and A.G. Yodh, Diffuse optics for tissue monitoring and tomography. Rep. Prog. Phys., 2010. 73: 076701. • Diffuse Optical Spectroscopy: Quantifies the reflectance or absorbance of light by tissue chromophores and fluctuations in these measurements. Oxy- and deoxy-hemoglobin concentrations (e.g. cHbO2, cHb respectively) are the most significant tissue absorbers in the NIR.

4. Experimental Design • Adult Dahl sheep underwent general anesthesia with controlled ventilation. • Arterial catheters placed in carotid and femoral arteries. • Endo-aortic balloon placed via femoral artery and advanced to proximal descending thoracic aorta. • DCS/DOC probe was tested after placement in the mid-thoracic region, after both open (laminectomy) and percutaneous approaches, and in intrathecal or epidural locations. • Spinal blood flow and oxygenation, along with mean arterial pressure (femoral & carotid) were measured during blood pressure manipulation via sodium nitroprusside, neosynephrine or vasopressin administration, and after intra-aortic balloon inflation in the superior descending thoracic aorta.

5. Spinal Cord Blood Supply Nicholas Theodore, M.D., Division of Neurological Surgery, Barrow Neurological Institute, St. Joseph’s Hospital and Medical Center, Phoenix, Arizona

6. DCS/DOS Sensor Source wavelength may be switched between 685nm, 785nm, and 830 nm to effect DCS or DOS measurements S D1 D2 DCS/DOS Sensor: The current probe in has one source and two detectors. The source fiber is shared for both DCS and DOS through an optical switch. Each detector location has a pair of single mode and multimode fibers. The single mode fibers are used to detect the blood flow, as monitored by DCS, while the multimode fibers detect the blood oxygenation measured using DOS. The probe is very flexible and thin (less than 1 mm thick), and is about 3 m long, which permits flexibility in accommodating the instrument in the operating room. The source-detector separations are 1.0 cm and 2.0 cm, which allows the technique to interrogate to depths of approximately 0.0-1.0 cm.

7. Open Subdural Probe Placement

8. Percutaneous Subdural Placement

9. Blood Flow-DCS A bolus of 400 µg neosynephrine results in a parallel increase in Mean Arterial Pressure (MAP) and blood flow. A bolus of 4 units vasopressin also resulted in a parallel increase in MAP and blood flow. For vasopression, blood flow returns to baseline levels in spite of persistent elevation of MAP, likely the result of autoregulation.

10. Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow. Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow. Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow. Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow. Blood Flow-DCS Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow.

11. Repetitive boluses of sodium nitroprusside (400ug) cause parallel decreases in MAP and blood flow. Blood Flow-DCS Percutaneous-Proximal Descending Aortic Balloon Inflation Balloon inflation in the proximal descending thoracic aorta results in a marked increase in MAP above the balloon, as measured by the carotid artery catheter, while pressure below the balloon, as measured by the femoral artery catheter, falls to near zero. Blood flow in the thoracic aorta dramatically increases in parallel with the carotid MAP, but falls back to baseline likely due to either autoregulation and or due to developing ischemia.

12. Tissue Oxygenation-DOS Percutaneous-Proximal Descending Aortic Balloon Inflation Following balloon inflation, tissue oxygenation remains steady or increases slightly.

13. Results Summary • Blood Flow data correlates well with changes in MAP • Fiber-optic probe effectively measuring changes in blood flow in the spinal cord in both subdural and epidural positions. • Probe can be placed via open and percutaneous approaches