2. John G. Webster Department of Biomedical Engineering University of Wisconsin Madison WI 53706 USA webster@engr.wisc.edu Supported by NIH grant HL56143 Electrode design for cardiac radio-frequency ablation

3. Colleagues Vicken Vorperian, MD, Electrophysiologist Supan Tungjitkusolmun, Finite element modeling Hong Cao, Temperature in vitro and in vivo Jang-Zern Tsai, Myocardial resistivity Naresh Bhavaraju, Thermal properties Young Bin Choy, Mechanical compliance Dieter Haemmerich, Liver ablation

5. Diagnosis - SVT (Accessory Pathway)

7. Beat By Beat Mapping Techniques Are Used System records location through constant interrogation of the magnetic field generated from the location pad magnetic field generated from the location pad

8. Beat by Beat Mapping

9. Superimposes point location and local activation times Superimposes point location and local activation times Connects neighboring points, creating triangles Connects neighboring points, creating triangles 10ms 50 ms 100ms

13. Goal Use Finite Element Modeling (FEM) to Improve the Efficacy of Current RF Ablation Technologies and to Design New Electrodes

14. Introduction: RF ablation & FEM Overview: Finite element modeling process 1. Effects of changes in myocardial properties 2. Needle electrode creates deep lesions 3. Uniform current density electrodes 4. Bipolar phase-shifted multielectrode catheter 5. Use FEM to predict lesion dimensions 6. FEM of hepatic ablation Outline

15. 95% success rate in curing Supraventricular tachycardias Low success rate for hepatic ablation Development for VT (Large lesions) Development for AFIB (long thin lesions) Introduction What Is Ablation? Modes of operation ~500 kHz, < 50 W Temperature-controlled Power-controlled Present Technology Heating of cardiac tissue to cure rhythm disturbances and of liver tissue to cure cancer What Is Ablation? Modes of operation

16. System for Cardiac Ablation

17. Common cardiac ablation sites AV Node Above the tricuspid valves Above and underneath the mitral valves Ventricular walls Right ventricular outflow tract Etc.

18. Tip Electrode RF generator

19. Energies Involved in RF Ablation Process

20. Bioheat Equation Heat transfer coefficient Blood temperature Density Specific heat Thermal conductivity Time Temperature Current density Electric field intensity heat loss to blood perfusion VARIABLES Heat Change MATERIAL PROPERTIES Electrical conductivity Density Specific heat Thermal conductivity Time Temperature Current density Electric field intensity heat loss to blood perfusion Heat Conduction Joule Heat

21. Finite Element Analysis Divide the regions of interest into small “elements” Partial differential equations to algebraic equations 2-D (triangular elements, quadrilateral elements, etc.) 3-D (tetrahedral elements, hexahedral elements, etc.) Nonuniform mesh is allowed Software & Hardware PATRAN 7.0 (MacNeal-Schwendler, Los Angeles ) ABAQUS 5.8 (Hibbitt, Karlsson & Sorensen, Inc., Farmington Hills, MI) HP C-180, 1152 MB of RAM, 34 GB Storage

22. Process for FEM Generation  Geometry  Material Properties  Initial Conditions  Boundary Cond.  Mesh Generation Preprocessing (PATRAN 7.0) Solution (ABAQUS/STANDARD 5.8)  Duration  Production  Adjust Loads  Check for desired parameters Postprocessing (ABAQUS/POST 5.8)  Temperature Distribution  Current Density  Determine Lesion Dimensions (from 50  C contour) Convergence test (for optimal number of elements )

23. Modes of RF Energy Applications Maintain the tip temperature at a preset value Adjust voltage applied to the electrode Temperature controlled ablation Power controlled ablation Maintain power delivered at a preset value Adjust voltage applied to the electrode

24. 1. Effects of changes in myocardial properties to lesion dimensions* *Tungjitkusolmun, S., Woo, E. J., Cao, H., Tsai, J.-Z., Vorperian, V. R.,and Webster, J. G.., Thermal-electrical finite element modeling for radio-frequency cardiac ablation: effects of changes in myocardialproperties, Med. Biol. Eng. Comput., accepted, 2000. 1.1 Electrical conductivity 1.2 Thermal conductivity 1.3 Specific heat (Density) Material Properties For each case: Temperature independent Temperature dependent Increase by 50%, or 100% Decrease by 50%

25. FEM results Lesion growth over time (Red is 50  C or higher)

26. Temperature distribution after 60 s Maximum temperature ~ 95  C Highest temperature

27. Maximum changes in Lesion Size PropertyCase% Volume Change Electrical conductivity  50%  58.6 Thermal conductivity +100%  60.7 Specific heat  50% +43.2 Power controlled

28. PropertyCase% Volume Change Electrical conductivity  50% +12.9% Thermal conductivity  50%  21.0% Specific heat+100%  29.4% Temperature controlled

29. Conclusion Temperature dependent properties are important Errors in Power-Controlled Mode are higher Better measurement techniques are needed

30. 2. Needle electrode design for VT* E. J. Woo, S. Tungjitkusolmun, H. Cao, J.-Z. Tsai, J. G. Webster, V. R. Vorperian, and J. A. Will, “A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in-vitro experiments,” IEEE Trans. Biomed. Eng., vol. 47, pp. 23  31, 2000.

31. Methods Both FEM & in vitro experiments Vary needle diameters Vary insertion depths Vary RF ablation duration Change temperature settings Compare lesion dimensions

32. FEM Results Insertion depth (mm)Lesion width (mm)Lesion depth (mm) 2.03.242.80 4.04.524.90 6.05.306.90 8.05.609.10 Needle Diameter (insertion = 8 mm) Insertion Depth (diameter = 0.5 mm) Diameter of needle (mm)Lesion width (mm)Lesion depth (mm) 0.55.609.1 0.66.069.1 0.76.249.1 0.86.509.1 0.96.779.2 1.07.049.3

33. Conclusion Lesion depths are 1  mm deeper than the insertion depth Lesion width increases with increasing diameter and duration Confirmed by in vitro experiments Good contact

34. Needle electrode designs

35. 3. Uniform current density electrodes* *Tungjitkusolmun, S., Woo, E. J., Cao, H., Tsai, J.-Z., Vorperian, V. R., and Webster, J. G., Finite element analyses of uniform current density electrodes for radio-frequency cardiac ablation, IEEE Trans. Biomed. Eng., 47, pp. 32-40, January 2000. “hot spot” at the edge of the conventional electrode Uniform current density electrode by – Recession depth – contour on the surface of the electrode (  is the parameter for the shape function). – Filled with coating material

36. FEM results Hot spot at the edge of the metal electrode

37. Current densities at the edge of the tip electrode  is the shape function

38. Cylindrical electrodes Changing conductivities Changing the curvatures  (S/m)  is for the shape function)

39. Current density distributions Cardiac tissue Catheter body Electrode Highest current density +0.00E+00 +2.50E  01 +5.00E  01 +7.50E  01 +1.00E+00 ECDM VALUE C SCALE = 144. Flat Catheter body Cardiac tissue Coating Uniform current density +0.00E + 00 +2.50E  01 +5.00E  01 +7.50E  01 +1.00E + 00 C SCALE = 582. ECDMVALUE Recessed

40. 4. Bipolar phase-shifted multielectrode catheter ablation* *S. Tungjitkusolmun, H. Cao, D. Haemmerich, J.-Z. Tsai, Y. B. Choy, V. R. Vorperian, and J. G. Webster, “Modeling bipolar phase-shifted multielectrode catheter ablation,” in preparation, IEEE Trans. Biomed. Eng., 2000 TeTe TmTm

41. Method A. 3-D Unipolar Multielectrode Catheter (MEC) B. Optimal phase-shifted for a system with fixed myocardial properties Optimal phase-shift Optimal phase-shift: T e / T m = 1 C. Effects of changes in myocardial properties on the optimal phase-shift D. Optimal phase-shift for MEC with 3 mm spacing

42. FEM results Phase = 0  Phase = 26.5  Phase = 45 

43. Phase vs. T e /T m Changes in electrical conductivity

44. Changes in thermal conductivity

45. Electrode spacing (2mm vs. 3mm)

46. Simplified Control system

47. 5. FEM predicts lesion size* Ablation over the mitral valve annulus Ablation underneath the mitral valve leaflets *S. Tungjitkusolmun, V. R. Vorperian, N. C. Bhavaraju, H. Cao, J.-Z. Tsai, and J. G. Webster, “Guidelines for predicting lesion size at common endocardial locations during radio-frequency ablation,” submitted to IEEE.Trans. Biomed. Eng., 1999.

48. Physical conditions LocationBlood velocity (cm/s) h b at blood  myocardium interface [(W/(m 2  K)] h be at blood  electrode interface [W/(m 2  K)] Position 1 11.014174191 Position 2 2.75442197 PositionContactBlood flow 1. Above the mitral valve1.3 mm embeddedHigh 2. Underneath the mitral valve3.0 mm embeddedLow

49. Temperature Controlled RF Lesion volume vs. time

50. Power controlled RF Lesion volume vs. time

51. 6. FEM for Hepatic Ablation* *S. Tungjitkusolmun, S. T. Staelin, D. Haemmerich, J.-Z. Tsai, H. Cao, V. R. Vorperian, F. T. Lee, D. M. Mahvi, and J. G. Webster, “Three-dimensional finite element analyses for radio-frequency hepatic tumor ablation,” submitted to IEEE. Trans. Biomed.Eng., 2000. Hepatic Ablation: Use RF probe to destroy tumor cancer, or cirrhosis Minimally invasive Present: -High recurrence rate -Small lesions

52. Models 4-tine RF Probe Geometry for FEM, 352,353 tetrahedral elements

53. Effect of Blood Vessel Location No Blood VesselBlood Vessel at 1 mm

54. Blood vessel at 5 mm

55. Bifurcated blood vessel

56. Summary 1. Outline a process for FEM creation for RF ablation 2. Show that needle electrode catheter design can create deep lesions by FEM & in vitro studies 3. Uniform current density electrodes reduce “hot spots” 4. Bipolar phase-shifted multielectrode catheter can create long and contiguous lesions 5. We can use FEM to predict lesion formations 6. Apply FEM for RF ablation to hepatic ablation

57. Sinus Rhythm with Surgery- Maze Procedure

58. Picture of Newer Catheters (NASPE)

60. Bipolar Hepatic Ablation BipolarUnipolar

61. Four-terminal measurement V I Four-terminal resistivity probe

62. K circuit = (V c / I c ) / (V v / V i ) Current source Tissue VeVe IeIe + V c - IcIc Function generator Differential amplifier Current-to- voltage converter Measurement circuit and equipment VvVv ViVi V ch1,V ch2 Digital oscilloscope K osc Computer (LabView virtual instrument) Resistance detector Data acquisition unit Thermistor Teflon coating Silver Epoxy K circuit  tissue = V ch1 / V ch2  K osc  K circuit  K wire  K probe K osc = (V v / V i ) / (V ch1 / V ch2 ) K probe K wire K wire = (V e / I e ) / (V c / I c ) K probe =  tissue / (V e / I e )

63. Flow Simulation System

64. Specification of Flow System Temperature 37 ± 1 °C Flow rate 0 to 6 L/min Solution 0.5% saline Ablation generator: EPT-1000XP Ablation catheter: 7Fr (2.5 mm diameter) Depth meter 0.02 mm accuracy Myocardial size 30  30  15 mm

65. Temperature Measurement Goal: Measure the temperature change inside myocardium during ablation. Previous Work Labonté: Thermographic camera Kaouk: Fluoroptic thermometer Hynynen: Impedance & power Nakagawa: Fluoroptic thermal probe

66. Temperature System Setup During ablation, we measure both the catheter tip temperature and thermocouple temperature inside the myocardium.

67. Thermocouple Probe IT-21 copper/constantan T-type thermocouple 0.08 s response time, 0.41 mm diameter Probe 0.9, 2.0, 2.9 mm from tip 1.5 mm in diameter

68. Probe Insertion Procedure Myocardium Steel needle Plastic tube Thermocouple Catheter

69. Thermocouple Circuit

70. Minimize the RF Interference Low pass filters at different stages Grounded shielding box Battery supply to avoid power interference Shielded thermocouple and cables Star network layout to avoid ground loops

71. Thermistor Circuit Constant current on thermistor from generator Measure the voltage across the catheter tip. An LP filter (fc = 95 Hz) to minimize the RF interference.

72. Calibration Calibrate between 25 and 95  C Extrapolate to 100  C Thermocouple Thermistor Polynomial curve fitting

73. Flow Effect on Lesion Formation Langberg et al.: Different electrode sizes (4, 8, 12 mm) to create lesions (convection surface) Nakagawa et al.: Saline-irrigated (cold saline ejected from catheter tip) catheter to create the lesion (temperature difference) Peterson et al. studied the lesion dimensions at different flow rates with a catheter laid down setup

74. Rationale Flow effect during temperature mode The cooling effect requires more power from catheter Current density and Joule heat generation inside the myocardium increase. More tissue exceeds 50  C threshold. The directly heated rim rises to a higher temperature and becomes larger Myocardial temperature rises faster. More time to conduct heat further.

75. Ablation Procedure Ablate in temperature mode with thermocouple probe inside the myocardium Stain myocardium with p-nitro blue tetrazolium chloride and take pictures using digital camera 6 persons measure independently and average their results Calculate dimensions (assuming ax symmetric) T: 60  C & 80  C Flow 0, 1 & 3 L/min 8 ablation /case

76. Lesion Dimension Volume Border: From dark to pink border

77. Lesion dimension vs. flow

79. Higher target temperature requires more power Higher target temperature results in larger lesion (both depth and diameter) Higher flow requires more power Higher flow rate yields larger lesion (both depth and diameter)

80. Temperature recording Two recordings of ablation. T tip rises faster and maintains at T target. Myocardial T rises gradually and may exceed T tip.

81. Table of Temperature recording 0 L/min1 L/min3 L/min 60  C t50TmTm TmTm TmTm TC12053860867 TC250~50*20551660 TC3NA46 513055 80  C t50TmTm TmTm TmTm TC1776683581 TC2773778671 TC3968772764 High flow: smaller t 50, higher T m

82. 80  C 3 L/min case Tip temperature is well below the thermocouple temperature inside myocardium. Slight charring during ablation. Impedance  Speculation: Charring covers the thermistor and prevents correct myocardial temperature reading.

83. Impedance, power and temperature of normal ablation

84. Impedance, power and temperature of 3 L/min 80 °C

85. Conclusion Setup of an in vitro system to study RF catheter ablation Study of the temperature setting on the lesion volume Study flow rate effect on the lesion volume and temperature change inside the myocardium Tissue charring under high flow rate

86. Important Parameters Affecting Lesion Dimension Tissue and blood properties Applied power during ablation. Duration of ablation. Target temperature in temperature mode. Blood flow around catheter. Contact condition such as penetration depth, contact angle.

87. Papers and links posted at: http://rf-ablation.engr.wisc.edu/ John G. Webster Department of Biomedical Engineering University of Wisconsin Madison WI 53706 USA webster@engr.wisc.edu Supported by NIH grant HL56143