Multi-electrode catheter, system and method

• US 6,546,270 B1
• Filed: 07/07/2000
• Issued: 04/08/2003
• Est. Priority Date: 07/07/2000
• Status: Expired due to Term

• Current Assignee: Biosense, Inc. (Cardinal Health, Inc.)
• Original Assignee: Biosense, Inc. (Cardinal Health, Inc.)
• Inventors: Goldin, Alexander, Levin, Michael, Holzer, Asher, Matcovitch, Avraham
• Primary Examiner(s): Schaetzle, Kennedy
• Assistant Examiner(s): DROESCH, KRISTEN L
• Application Number:
  US09/612,487
• Time in Patent Office: 1,005 Days
• Field of Search: 128/897-898, 128/901, 600/372-374, 600/381, 600/547, 607/1-2, 607/115-116, 607/119, 607/122, 607/96, 607/98-99, 607/101, 607/105, 606/32-35, 606/41
• US Class Current: 600/374
• CPC Class Codes: A61B 18/12 by passing a current through the tissue to be heated, e.g. high-frequency current
• CPC Class Codes: A61B 18/1206 Generators therefor
• CPC Class Codes: A61B 18/1492 having a flexible, catheter-like structure, e.g. for heart ablation A61B18/1477 takes precedence
• CPC Class Codes: A61B 2017/003 Steerable
• CPC Class Codes: A61B 2018/00065 porous
• CPC Class Codes: A61B 2018/00351 Heart
• CPC Class Codes: A61B 2018/00654 with individual control of each of a plurality of energy emitting elements
• CPC Class Codes: A61B 2018/0066 without feedback, i.e. open loop control
• CPC Class Codes: A61B 2018/00875 Resistance or impedance
• CPC Class Codes: A61B 2018/1253 monopolar
• CPC Class Codes: A61B 2018/126 bipolar
• CPC Class Codes: A61B 2018/1467 using more than two electrodes on a single probe
• CPC Class Codes: A61B 2018/1497 Electrodes covering only part of the probe circumference
• CPC Class Codes: A61B 2018/162 located on the probe body
• CPC Class Codes: A61B 2034/2051 Electromagnetic tracking systems

Abstract

A system for detecting electrode-tissue contact comprises a multi-electrode catheter having a location sensor and a plurality of contact electrodes. The catheter preferably further comprises a reference electrode that is preferably protected from making contact with tissue. The system further comprises a signal generator to transmit test signals to each of the contact electrodes and to the reference electrode. Tissue contact is detected by comparing the signals across the tip electrode to a return electrode versus the signal across the reference electrode to a return electrode. Ablation energy may be delivered to the contact electrodes if contact of the electrode with tissue is detected.

Claims

1. A system for detecting contact of an electrode with tissue, said system comprising:

a) a catheter comprising;

i) a body having a proximal end and a distal end, said distal end having a distal tip;

ii) a plurality of contact electrodes adapted for contact with tissue at a plurality of points for receiving electrical signals therefrom and transmitting electrical signals thereto, said electrical signals defining electrical information; and

iii) a location sensor which generates signals responsive to a location of said distal end, said signals responsive to a location of said distal end defining location information;

b) a reference electrode for measuring an electrical characteristic when said reference electrode is in contact with a fluid and is not in contact with tissue;

c) a contact detection circuit comprising;

i) a signal generator for sending test signals to said contact electrodes and to said reference electrode; and

ii) a circuit to measure a differential electrical response to said test signals, said differential electrical response being indicative of contact of said contact electrodes with tissue; and

d) a map of said tissue generated from said electrical information and location information, said electrical information and location information at each of said points being weighted in said map in accordance with contact being detected between said contact electrodes and said tissue at said points.

2. The system of claim 1 wherein said plurality of contact electrodes is positioned at said catheter distal tip.

3. The system of claim 1 wherein said plurality of contact electrodes is positioned longitudinally along said catheter body.

4. The system of claim 1 wherein said plurality of contact electrodes is positioned circumferentially around said catheter body.

5. The system of claim 1 wherein said location sensor is an electromagnetic location sensor.

6. The system of claim 1 wherein said reference electrode is positioned on said catheter.

7. The system of claim 1 wherein said reference electrode is protected from making contact with tissue.

8. The system of claim 7 wherein said reference electrode is protected by a membrane covering said electrode, said membrane permitting contact of said reference electrode with blood but not permitting contact of said reference electrode with tissue.

9. The system of claim 7 wherein said reference electrode is recessed relative to said catheter body.

10. The system of claim 1 which further comprises a return electrode, said return electrode functioning as a sink for said test signals to said contact electrodes and to said reference electrode.

11. The system of claim 10 wherein said return electrode is adapted for positioning internal to a human body.

12. The system of claim 11 wherein said return electrode is positioned on said catheter.

13. The system of claim 10 wherein said return electrode is adapted for contact with skin external to a human body.

14. The system of claim 10 wherein said return electrode is dedicated for measuring differential signals with said contact electrodes and with said reference electrode.

15. The system of claim 10 wherein said return electrode is connected to isolated ground.

16. The system of claim 10 wherein said return electrode is connected to an electrocardiogram device isolated ground.

17. The system of claim 10 wherein said circuit to measure a differential electrical response to said test signals comprises:

i) a first differential amplifier to measure a first electrical difference signal between said contact electrodes and said return electrode; and

ii) a second differential amplifier to measure a second electrical difference signal between said reference electrode and said return electrode.

18. The system of claim 17 wherein said contact electrodes and said reference electrode are supplied with a first constant current and a second constant current, respectively, said first constant current being equal to said second constant current.

19. The system of claim 18 wherein said return electrode is driven with a third constant current, said third current being opposite in phase with said first constant current and said second constant current.

20. The system of claim 17 which further comprises a third differential amplifier to measure an electrical difference signal between said first difference signal and said second difference signal.

21. The system of claim 20 wherein said first differential amplifier measures a first voltage difference between said contact electrodes and said return electrode, said second differential amplifier measures a second voltage difference between said reference electrode and said return electrode, and said third differential amplifier measures a voltage difference between said first voltage difference and said second voltage difference.

22. The system of claim 21 wherein said electrical difference signal measured by said third differential amplifier is rectified by a synchronous detector.

23. The system of claim 17 wherein each of said contact electrodes and said reference electrode have a first area and a second area, respectively, and said first amplifier and said second amplifier have a first gain and a second gain, respectively, said first gain to said second gain being proportional to said first area to said second area.

24. The system of claim 17 wherein said location sensor is an electromagnetic location sensor.

25. The system of claim 1 wherein said circuit to measure a differential electrical response to said test signals comprises a bridge circuit, said bridge circuit comprising a first resistive element and a second resistive element, said resistive elements each having a first side and a second side, said first side of said first resistive element connected with said first side of said second resistive element, said second side of said first resistive element connected with said reference electrode, said second side of said second resistive element connected with said contact electrodes, said bridge having a first input between said first resistive element and said second resistive element and a second input connected to said return electrode, and a first output between said first resistive element and said reference electrode and a second output between said second resistive element and said contact electrodes.

26. The system of claim 25 wherein said first resistive element is a first resistor and said second resistive element is a second resistor.

27. The system of claim 26 wherein said first resistor has a first resistance and said second resistor has a second resistance, and wherein each of said contact electrodes has a tip electrode area and said reference electrode has a reference electrode area, said first resistance to said second resistance being proportional to said contact electrode area to said reference electrode area.

28. The system of claim 25 wherein said first resistive element is a first high output impedance buffer and said second resistive element is a second high output impedance buffer.

29. The system of claim 28 wherein said first high output impedance buffer has a first output current, said second high output buffer has a second output current, and wherein each of said contact electrodes has a contact electrode area and said reference electrode has a reference electrode area, said first output current to said second output current being proportional to said contact electrode area to said reference electrode area.

30. The system of claim 25 wherein said bridge outputs are connected to a differential amplifier, said differential amplifier measuring a bridge output voltage indicative of contact of said contact electrodes with tissue.

31. The system of claim 30 wherein said differential amplifier has an output that is rectified by a synchronous detector.

32. The system of claim 25 wherein said location sensor is an electromagnetic location sensor.

33. The system of claim 1 wherein said circuit to measure a differential electrical response to said test signals comprises a first current sensor for measuring current to said reference electrode and a second current sensor for measuring current to said contact electrodes.

34. The system of claim 33 wherein said current sensors are selected from current transformers and Hall effect sensors.

35. The system of claim 33 wherein said first current sensor has a first gain and said second current sensor has a second gain, each of said contact electrodes has a contact electrode area and said reference electrode has a reference electrode area, said first gain to said second gain being proportional to said contact electrode area to said reference electrode area.

36. The system of claim 33 wherein said first current sensor and said second current sensor have outputs connected to a differential amplifier, said amplifier measuring a voltage indicative of contact of said contact electrodes with tissue.

37. The system of claim 36 wherein said differential amplifier has an output rectified by a synchronous detector.

38. The system of claim 33 wherein said location sensor is an electromagnetic location sensor.

39. The system of claim 1 comprising circuitry for measuring local electrograms from said contact electrodes.

40. The system of claim 1 comprising circuitry to measure body surface electrograms.

41. The system of claim 1 comprising a plurality of channels, said number of channels corresponding to the number of electrodes in said plurality of contact electrodes.

42. The system of claim 1 further comprising a multiplexer to switch each of said contact electrodes into communication with said contact detection circuit.

43. The system of claim 1 further comprising an ablation power source.

44. The system of claim 43 further comprising a multiplexer to switch each of said contact electrodes determined by said contact detection circuit to be in contact with tissue into communication with said ablation power source.

45. A method for detecting contact of an electrode with tissue, said method comprising the steps of:

a) providing a catheter comprising;

i) a body having a proximal end and a distal end, said distal end having a distal tip;

ii) a plurality of contact electrodes adapted for contact with tissue for receiving electrical signals therefrom and for transmitting electrical signals thereto; and

iii) a location sensor which generates a signal responsive to a location of said distal end;

b) providing a reference electrode for measuring an electrical characteristic when said reference electrode is in contact with a fluid and is not in contact with tissue;

c) providing test signals to said reference electrode and to said contact electrodes;

d) measuring a differential electrical response to said test signals, said differential electrical response being indicative of contact of said contact electrodes with tissue;

e) collecting electrical information with said contact electrodes and location information with said location sensor at a plurality of points on said tissue; and

f) generating a map of said tissue from said electrical information and location information, said electrical information and location information at each of said points being weighted in said map in accordance with contact being detected between said contact electrodes and said tissue at said points.

46. The method of claim 45 wherein said plurality of contact electrodes is positioned at said catheter distal tip.

47. The method of claim 45 wherein said plurality of contact electrodes is positioned longitudinally along said catheter body.

48. The method of claim 45 wherein said plurality of contact electrodes is positioned circumferentially around said catheter body.

49. The method of claim 45 wherein said location sensor is an electromagnetic location sensor.

50. The method of claim 45 wherein said reference electrode is positioned on said catheter.

51. The method of claim 45 wherein said reference electrode is protected from making contact with tissue.

52. The method of claim 51 wherein said reference electrode is protected by a membrane covering said electrode, said membrane permitting contact of said reference electrode with blood but not permitting contact of said reference electrode with tissue.

53. The method of claim 51 wherein said reference electrode is recessed relative to said catheter body.

54. The method of claim 45 wherein measuring a differential electrical response to said test signals comprises:

a) measuring a first electrical difference signal between each of said contact electrodes and a return electrode and a second electrical difference signal between said reference electrode and said return electrode; and

b) comparing said first electrical difference signal with said second electrical difference signal to detect contact of said contact electrodes with tissue.

55. The method of claim 54 wherein said test signals provided to said contact electrodes and to said reference electrode are constant current signals.

56. The method of claim 54 wherein comparing said first electrical difference signal with said second electrical difference signal comprises feeding said first electrical difference signal and said second electrical difference signal to a differential amplifier to produce a third electrical difference signal indicative of contact of said contact electrodes with tissue.

57. The method of claim 56 wherein said first and said second electrical difference signals are adjusted to provide a null difference signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

58. The method of claim 54 wherein said location sensor is an electromagnetic location sensor.

59. The method of claim 45 wherein measuring a differential electrical response to said test signals comprises:

a) providing a bridge circuit comprising a first resistive element and a second resistive element, said resistive elements each having a first side and a second side, said first side of said first resistive element connected with said first side of said second resistive element, said second side of said first resistive element connected with said reference electrode, said second side of said second resistive element connected with said contact electrodes, said bridge having a first input between said first resistive element and said second resistive element and a second input connected to said return electrode, and a first output between said first resistive element and said reference electrode and a second output between said second resistive element and said contact electrodes; and

b) measuring a signal across said bridge outputs to detect contact of said contact electrodes with tissue.

60. The method of claim 59 wherein said first resistive element comprises a first resistor and said second resistive element comprises a second resistor.

61. The method of claim 59 wherein said first resistive element comprises a first high output impedance buffer and said second resistive element comprises a second high output impedance buffer.

62. The method of claim 59 wherein said signal across said bridge outputs is measured with a differential amplifier.

63. The method of claim 62 wherein said signal across said bridge outputs is adjusted to provide a null signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

64. The method of claim 59 wherein said location sensor is an electromagnetic sensor.

65. The method of claim 45 wherein measuring a differential electrical response to said test signals comprises measuring current to said reference electrode with a first current sensor and measuring current to said contact electrodes with a second current sensor, said first current sensor and said second current sensor having outputs connected to a differential amplifier, said amplifier measuring a voltage indicative of contact of said contact electrodes with tissue.

66. The method of claim 65 wherein said current sensors are selected from current transformers and Hall effect sensors.

67. The method of claim 65 wherein said current sensors have output signals, said signals being adjusted to provide a null signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

68. The method of claim 65 wherein said location sensor is an electromagnetic location sensor.

69. The method of claim 45 wherein measuring a differential electrical response to said test signals comprises:

a) measuring a first impedance between said contact electrodes and a return electrode and a second impedance between said reference electrode and said return electrode; and

b) comparing said first and said second impedances to detect contact of said contact electrodes with tissue.

70. The method of claim 69 wherein said location sensor is an electromagnetic location sensor.

71. The method of claim 45 which further comprises delivering ablation energy to said contact electrodes in accordance with said contact electrodes being in contact with tissue.

72. A system for detecting contact of an electrode with tissue, said system comprising:

a) a catheter comprising;

i) a body having a proximal end and a distal end, said distal end having a distal tip;

ii) a plurality of contact electrodes adapted for contact with tissue at a plurality of points for receiving electrical signals therefrom and transmitting electrical signals thereto, said electrical signals defining electrical information; and

iii) a location sensor which generates signals responsive to a location of said distal end, said signals responsive to a location of said distal end also defining mechanical information;

b) a reference electrode for measuring an electrical characteristic when said reference electrode is in contact with a fluid and is not in contact with tissue;

c) a contact detection circuit comprising;

i) a signal generator for sending test signals to said contact electrodes and to said reference electrode;

ii) a circuit to measure a differential electrical response to said test signals, said differential electrical response being indicative of contact of said contact electrodes with tissue generated, and

d) an electromechanical map of said tissue from said electrical information and mechanical information, said electrical information and mechanical information at each of said points being weighted in said map in accordance with contact being detected between said contact electrodes and said tissue at said points.

73. The system of claim 72 wherein said plurality of contact electrodes is positioned at said catheter distal tip.

74. The system of claim 72 wherein said plurality of contact electrodes is positioned longitudinally along said catheter body.

75. The system of claim 72 wherein said plurality of contact electrodes is positioned circumferentially around said catheter body.

76. The system of claim 72 wherein said location sensor is an electromagnetic location sensor.

77. The system of claim 72 wherein said reference electrode is positioned on said catheter.

78. The system of claim 72 wherein said reference electrode is protected from making contact with tissue.

79. The system of claim 78 wherein said reference electrode is protected by a membrane covering said electrode, said membrane permitting contact of said reference electrode with blood but not permitting contact of said reference electrode with tissue.

80. The system of claim 78 wherein said reference electrode is recessed relative to said catheter body.

81. The system of claim 72 which further comprises a return electrode, said return electrode functioning as a sink for said test signals to said contact electrodes and to said reference electrode.

82. The system of claim 81 wherein said return electrode is adapted for positioning internal to a human body.

83. The system of claim 82 wherein said return electrode is positioned on said catheter.

84. The system of claim 81 wherein said return electrode is adapted for contact with skin external to a human body.

85. The system of claim 81 wherein said return electrode is dedicated for measuring differential signals with said contact electrodes and with said reference electrode.

86. The system of claim 81 wherein said return electrode is connected to isolated ground.

87. The system of claim 81 wherein said return electrode is connected to an electrocardiogram device isolated ground.

88. The system of claim 81 wherein said circuit to measure a differential electrical response to said test signals comprises:

i) a first differential amplifier to measure a first electrical difference signal between said contact electrodes and said return electrode; and

ii) a second differential amplifier to measure a second electrical difference signal between said reference electrode and said return electrode.

89. The system of claim 88 wherein said contact electrodes and said reference electrode are supplied with a first constant current and a second constant current, respectively, said first constant current being equal to said second constant current.

90. The system of claim 89 wherein said return electrode is driven with a third constant current, said third current being opposite in phase with said first constant current and said second constant current.

91. The system of claim 88 which further comprises a third differential amplifier to measure an electrical difference signal between said first difference signal and said second difference signal.

92. The system of claim 91 wherein said first differential amplifier measures a first voltage difference between said contact electrodes and said return electrode, said second differential amplifier measures a second voltage difference between said reference electrode and said return electrode, and said third differential amplifier measures a voltage difference between said first voltage and said second voltage difference.

93. The system of claim 92 wherein said electrical difference signal measured by said third differential amplifier is rectified by a synchronous detector.

94. The system of claim 88 wherein each of said contact electrodes and said reference electrode have a first area and a second area, respectively, and said first amplifier and said second amplifier have a first gain and a second gain, respectively, said first gain to said second gain being proportional to said first area to said second area.

95. The system of claim 88 wherein said location sensor is an electromagnetic location sensor.

96. The system of claim 72 wherein said circuit to measure a differential electrical response to said test signals comprises a bridge circuit, said bridge circuit comprising a first resistive element and a second resistive element, said resistive elements each having a first side and a second side, said first side of said first resistive element connected with said first side of said second resistive element, said second side of said first resistive element connected with said reference electrode, said second side of said second resistive element connected with said contact electrodes, said bridge having a first input between said first resistive element and said second resistive element and a second input connected to said return electrode, and a first output between said first resistive element and said reference electrode and a second output between said second resistive element and said contact electrodes.

97. The system of claim 96 wherein said first resistive element is a first resistor and said second resistive element is a second resistor.

98. The system of claim 97 wherein said first resistor has a first resistance and said second resistor has a second resistance, and wherein each of said contact electrodes has a tip electrode area and said reference electrode has a reference electrode area, said first resistance to said second resistance being proportional to said contact electrode area to said reference electrode area.

99. The system of claim 96 wherein said first resistive element is a first high output impedance buffer and said second resistive element is a second high output impedance buffer.

100. The system of claim 99 wherein said first high output impedance buffer has a first output current, said second high output buffer has a second output current, and wherein each of said contact electrodes has a contact electrode area and said reference electrode has a reference electrode area, said first output current to said second output current being proportional to said contact electrode area to said reference electrode area.

101. The system of claim 96 wherein said bridge outputs are connected to a differential amplifier, said differential amplifier measuring a bridge output voltage indicative of contact of said contact electrodes with tissue.

102. The system of claim 101 wherein said differential amplifier has an output that is rectified by a synchronous detector.

103. The system of claim 96 wherein said location sensor is an electromagnetic location sensor.

104. The system of claim 72 wherein said circuit to measure a differential electrical response to said test signals comprises a first current sensor for measuring current to said reference electrode and a second current sensor for measuring current to said contact electrodes.

105. The system of claim 104 wherein said current sensors are selected from current transformers and Hall effect sensors.

106. The system of claim 104 wherein said first current sensor has a first gain and said second current sensor has a second gain, each of said contact electrodes has a contact electrode area and said reference electrode has a reference electrode area, said first gain to said second gain being proportional to said contact electrode area to said reference electrode area.

107. The system of claim 104 wherein said first current sensor and said second current sensor have outputs connected to a differential amplifier, said amplifier measuring a voltage indicative of contact of said contact electrodes with tissue.

108. The system of claim 107 wherein said differential amplifier has an output rectified by a synchronous detector.

109. The system of claim 104 wherein said location sensor is an electromagnetic location sensor.

110. The system of claim 72 comprising circuitry for measuring local electrograms from said contact electrodes.

111. The system of claim 72 comprising circuitry to measure body surface electrograms.

112. The system of claim 72 comprising a plurality of channels, said number of channels corresponding to the number of electrodes in said plurality of contact electrodes.

113. The system of claim 72 further comprising a multiplexer to switch each of said contact electrodes into communication with said contact detection circuit.

114. The system of claim 72 further comprising an ablation power source.

115. The system of claim 114 further comprising a multiplexer to switch each of said contact electrodes determined by said contact detection circuit to be in contact with tissue into communication with said ablation power source.

116. A method for detecting contact of an electrode with tissue, said method comprising the steps of:

a) providing a catheter comprising;

i) a body having a proximal end and a distal end, said distal end having a distal tip;

ii) a plurality of contact electrodes adapted for contact with tissue for receiving electrical signals therefrom and for transmitting electrical signals thereto; and

iii) a location sensor which generates a signal responsive to a location of said distal end;

b) providing a reference electrode for measuring an electrical characteristic when said reference electrode is in contact with a fluid and is not in contact with tissue;

c) providing test signals to said reference electrode and to said contact electrodes;

d) measuring a differential electrical response to said test signals, said differential electrical response being indicative of contact of said contact electrodes with tissue;

e) collecting electrical information from said contact electrodes and mechanical information from said location sensor at a plurality of points on said tissue; and

f) generating an electromechanical map of said tissue from said electrical information and mechanical information, said electrical information and mechanical information at each of said points being weighted in said map in accordance with contact being detected between said contact electrodes and said tissue at said points.

117. The method of claim 116 wherein said plurality of contact electrodes is positioned at said catheter distal tip.

118. The method of claim 116 wherein said plurality of contact electrodes is positioned longitudinally along said catheter body.

119. The method of claim 116 wherein said plurality of contact electrodes is positioned circumferentially around said catheter body.

120. The method of claim 116 wherein said location sensor is an electromagnetic location sensor.

121. The method of claim 116 wherein said reference electrode is positioned on said catheter.

122. The method of claim 116 wherein said reference electrode is protected from making contact with tissue.

123. The method of claim 122 wherein said reference electrode is protected by a membrane covering said electrode, said membrane permitting contact of said reference electrode with blood but not permitting contact of said reference electrode with tissue.

124. The method of claim 122 wherein said reference electrode is recessed relative to said catheter body.

125. The method of claim 116 wherein measuring a differential electrical response to said test signals comprises:

a) measuring a first electrical difference signal between each of said contact electrodes and a return electrode and a second electrical difference signal between said reference electrode and said return electrode; and

b) comparing said first electrical difference signal with said second electrical difference signal to detect contact of said contact electrodes with tissue.

126. The method of claim 125 wherein said test signals provided to said contact electrodes and to said reference electrode are constant current signals.

127. The method of claim 125 wherein comparing said first electrical difference signal with said second electrical difference signal comprises feeding said first electrical difference signal and said second electrical difference signal to a differential amplifier to produce a third electrical difference signal indicative of contact of said contact electrodes with tissue.

128. The method of claim 127 wherein said first and said second electrical difference signals are adjusted to provide a null difference signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

129. The method of claim 125 wherein said location sensor is an electromagnetic location sensor.

130. The method of claim 116 wherein measuring a differential electrical response to said test signals comprises:

a) providing a bridge circuit comprising a first resistive element and a second resistive element, said resistive elements each having a first side and a second side, said first side of said first resistive element connected with said first side of said second resistive element, said second side of said first resistive element connected with said reference electrode, said second side of said second resistive element connected with said contact electrodes, said bridge having a first input between said first resistive element and said second resistive element and a second input connected to said return electrode, and a first output between said first resistive element and said reference electrode and a second output between said second resistive element and said contact electrodes; and

b) measuring a signal across said bridge outputs to detect contact of said contact electrodes with tissue.

131. The method of claim 130 wherein said first resistive element comprises a first resistor and said second resistive element comprises a second resistor.

132. The method of claim 130 wherein said first resistive element comprises a first high output impedance buffer and said second resistive element comprises a second high output impedance buffer.

133. The method of claim 130 wherein said signal across said bridge outputs is measured with a differential amplifier.

134. The method of claim 133 wherein said signal across said bridge outputs is adjusted to provide a null signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

135. The method of claim 130 wherein said location sensor is an electromagnetic sensor.

136. The method of claim 116 wherein measuring a differential electrical response to said test signals comprises measuring current to said reference electrode with a first current sensor and measuring current to said contact electrodes with a second current sensor, said first current sensor and said second current sensor having outputs connected to a differential amplifier, said amplifier measuring a voltage indicative of contact of said contact electrodes with tissue.

137. The method of claim 136 wherein said current sensors are selected from current transformers and Hall effect sensors.

138. The method of claim 136 wherein said current sensors have output signals, said signals being adjusted to provide a null signal from said differential amplifier when said contact electrodes and said reference electrode are in blood and not in contact with tissue.

139. The method of claim 136 wherein said location sensor is an electromagnetic location sensor.

140. The method of claim 116 wherein measuring a differential electrical response to said test signals comprises:

a) measuring a first impedance between said contact electrodes and a return electrode and a second impedance between said reference electrode and said return electrode; and

b) comparing said first and said second impedances to detect contact of said contact electrodes with tissue.

141. The method of claim 140 wherein said location sensor is an electromagnetic location sensor.

142. The method of claim 116 which further comprises delivering ablation energy to said contact electrodes in accordance with said contact electrodes being in contact with tissue.