Temperature simulator for thermocouple-based RF ablation system

Testing a thermocouple-based RF ablation system is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency power output thereof in a predefined manner in response to predefined variations in a temperature signal from the simulator. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, and while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying the communicated temperature signals, and verifying that a variation in the power output of the ablator module in response to varying the temperature signals conforms to the predefined manner.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to tissue ablation systems. More particularly, this invention relates to simulated operation of an RF generator in tissue ablation system.

2. Description of the Related Art

Cardiac arrhythmias, such as atrial fibrillation, occur when regions of cardiac tissue abnormally conduct electric signals to adjacent tissue, thereby disrupting the normal cardiac cycle and causing asynchronous rhythm.

Procedures for treating arrhythmia include surgically disrupting the origin of the signals causing the arrhythmia, as well as disrupting the conducting pathway for such signals. By selectively ablating cardiac tissue by application of energy via a catheter, it is sometimes possible to cease or modify the propagation of unwanted electrical signals from one portion of the heart to another. The ablation process destroys the unwanted electrical pathways by formation of non-conducting lesions.

U.S. Patent Application Publication No. 2009/0030411 by Werneth et al. describes an ablation catheter in which a thermocouple can be used to measure the temperature local to the thermocouple prior to, during or after the delivery of ablation energy. It is explained that when an ablation is performed, maintaining the tissue at a temperature below a threshold is required. Information recorded from the thermocouple is used to adjust energy delivery or to modify its frequency, based on temperature information analysis.

U.S. Patent Application Publication No. 2011/0218526 provides another example of a thermocouple in an ablation system, which electrodes may be electrically coupled to an output portion of an RF generator, and each thermocouple may be electrically coupled to a feedback portion of the RF generator. A processor accepts an input voltage and produces an output voltage, based on feedback signals from the thermocouples, and then adjusts a duty cycle modulator as well as an amplitude modulator according to the feedback signals.

SUMMARY OF THE INVENTION

There is provided according to embodiments of the invention an apparatus for testing a tissue ablation system, which includes emulation circuitry connectable to an ablator module being tested, the ablator module has an adjustable radiofrequency (RF) power output and a monitor display. The emulation circuitry includes a first arm of a first thermocouple metallic material linked to the power output of the ablator module, and a second arm of a second thermocouple metallic material connected to the monitor display. A return pathway extending from the first arm to the ablator module permits passage of RF current and blocks direct current (DC). An adjustable voltage source producing a DC potential is connected via an output circuit across the first arm and the second arm, the output circuit having a greater resistance to RF current than to direct current.

According to an aspect of the apparatus, the return pathway includes a DC blocking capacitor.

According to a further aspect of the apparatus, the output circuit includes a chain of resistors connected in series with an inductor.

According to one aspect of the apparatus, the inductor includes a plurality of ferrite inductors in connected in series with the adjustable voltage source.

According to yet another aspect of the apparatus, a value of the inductor is 1 mH.

There is further provided according to embodiments of the invention a method of testing a thermocouple-based RF ablation system, which is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency (RF) power output thereof in a predefined manner in response to predefined variations in a temperature signal. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, and while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying the communicated temperature signals, and verifying that a variation in the power output of the ablator module in response to varying the temperature signals conforms to the predefined manner.

There is further provided according to embodiments of the invention a method of testing a thermocouple-based RF ablation system, which is carried out by connecting a temperature simulator to an ablator module. The ablator module is operative to vary a radiofrequency (RF) power output thereof in a predefined manner in response to predefined variations in a temperature signal and has a temperature display monitor. The method is further carried out by delivering RF power from the ablator module to the temperature simulator, while delivering RF power, performing the steps of: communicating temperature signals from the temperature simulator to the ablator module, varying a potential of the communicated temperature signals in accordance with known temperature-dependent potentials of a thermocouple junction to represent respective temperatures, and calibrating the temperature display monitor to conform to the respective temperatures represented by of the communicated temperature signals.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, numerous specific details are set forth in order to provide a thorough understanding of the various principles of the present invention. It will be apparent to one skilled in the art, however, that not all these details are necessarily always needed for practicing the present invention. In this instance, well-known circuits, control logic, and the details of computer program instructions for conventional algorithms and processes have not been shown in detail in order not to obscure the general concepts unnecessarily.

Aspects of the present invention may be embodied in software programming code, which is typically maintained in permanent storage, such as a computer readable medium. In a client/server environment, such software programming code may be stored on a client or a server. The software programming code may be embodied on any of a variety of known non-transitory media for use with a data processing system, such as a diskette, hard drive, electronic media or CD-ROM. The code may be distributed on such media, or may be distributed to users from the memory or storage of one computer system over a network of some type to storage devices on other computer systems for use by users of such other systems.

The term “couple” or “coupled” is intended to mean either an indirect or direct connection. Thus, if a first device is coupled to a second device, that connection may be through a direct connection, or through an indirect connection via other devices and connections, or via inductive or capacitive coupling.

Turning now to the drawings, reference is initially made toFIG. 1, which is a pictorial illustration of a system10for performing diagnostic and therapeutic procedures on a heart12of a living subject, which is constructed and operative in accordance with a disclosed embodiment of the invention. The system comprises a catheter14, which is percutaneously inserted by an operator16through the patient's vascular system into a chamber or vascular structure of the heart12. The operator16, who is typically a physician, brings the catheter's distal tip18into contact with the heart wall at an ablation target site. Optionally, electrical activation maps may then be prepared, according to the methods disclosed in U.S. Pat. Nos. 6,226,542, and 6,301,496, and in commonly assigned U.S. Pat. No. 6,892,091, whose disclosures are herein incorporated by reference. One commercial product embodying elements of the system10is available as the CARTO® 3 System, available from Biosense Webster, Inc., 3333 Diamond Canyon Road, Diamond Bar, Calif. 91765. This system may be modified by those skilled in the art to embody the principles of the invention described herein.

Areas determined to be abnormal, for example by evaluation of the electrical activation maps, can be ablated by application of thermal energy, e.g., by passage of radiofrequency electrical current through wires in the catheter to one or more electrodes at the distal tip18, which apply the radiofrequency energy to the myocardium. The energy is absorbed in the tissue, heating it to a point (typically about 50° C.) at which it permanently loses its electrical excitability. When successful, this procedure creates non-conducting lesions in the cardiac tissue, which disrupt the abnormal electrical pathway causing the arrhythmia. The principles of the invention can be applied to different heart chambers to treat many different cardiac arrhythmias.

The catheter14typically comprises a handle20, having suitable controls on the handle to enable the operator16to steer, position and orient the distal end of the catheter as desired for the ablation. To aid the operator16, the distal portion of the catheter14contains position sensors (not shown) that provide signals to a positioning processor22, located in a console24.

Ablation energy and electrical signals can be conveyed to and from the heart12through one or more ablation electrodes32located at or near the distal tip18via cable34to the console24. Pacing signals and other control signals may be conveyed from the console24through the cable34and the electrodes32to the heart12. Sensing electrodes33, also connected to the console24, are disposed between the ablation electrodes32and have connections to the cable34.

Wire connections35link the console24with body surface electrodes30and other components of a positioning sub-system. The electrodes32and the body surface electrodes30may be used to measure tissue impedance at the ablation site as taught in U.S. Pat. No. 7,536,218, issued to Govari et al., which is herein incorporated by reference. A temperature sensor such as thermocouples31, may be mounted on or near the ablation electrode32and optionally or near the sensing electrode33. The thermocouples31are connected to the electrode circuit as described in further detail below.

The console24typically contains one or more ablation power generators25. The catheter14may be adapted to conduct ablative energy to the heart using any known ablation technique, e.g., radiofrequency energy, ultrasound energy, and laser-produced light energy. Such methods are disclosed in commonly assigned U.S. Pat. Nos. 6,814,733, 6,997,924, and 7,156,816, which are herein incorporated by reference.

The positioning processor22is an element of a positioning subsystem in the system10that measures location and orientation coordinates of the catheter14.

In one embodiment, the positioning subsystem comprises a magnetic position tracking arrangement that determines the position and orientation of the catheter14by generating magnetic fields in a predefined working volume and sensing these fields at the catheter, using field generating coils28. The positioning subsystem may employ impedance measurement, as taught, for example in U.S. Pat. No. 7,756,576, which is hereby incorporated by reference, and in the above-noted U.S. Pat. No. 7,536,218.

As noted above, the catheter14is coupled to the console24, which enables the operator16to observe and regulate the functions of the catheter14. Console24includes a processor, preferably a computer with appropriate signal processing circuits. The processor is coupled to drive a monitor29. The signal processing circuits typically receive, amplify, filter and digitize signals from the catheter14, including signals generated by the above-noted sensors and a plurality of location sensing electrodes (not shown) located distally in the catheter14. The digitized signals are received and used by the console24and the positioning system to compute the position and orientation of the catheter14and to analyze the electrical signals from the electrodes.

Typically, the system10includes other elements, which are not shown in the figures for the sake of simplicity. For example, the system10may include an electrocardiogram (ECG) monitor, coupled to receive signals from one or more body surface electrodes, to provide an ECG synchronization signal to the console24. As mentioned above, the system10typically also includes a reference position sensor, either on an externally-applied reference patch attached to the exterior of the subject's body, or on an internally-placed catheter, which is inserted into the heart12maintained in a fixed position relative to the heart12. Conventional pumps and lines for circulating liquids through the catheter14for cooling the ablation site are provided.

In order to accurately ablate tissue, for example according to known procedures in which tissue temperature is an important variable, it is desirable to understand and model the behavior of the ablation catheter in actual operation. This can be done, according to embodiments of the invention, using a jig, which behaves as a temperature simulator, and which is connected to an RF generator. The simulator operates while the RF generator is active by separating a relatively high power RF current from a low power DC current, using a capacitor to provide a preferred low-impedance path for the RF component. The DC component is a voltage of about 40 μV, approximating typical thermocouple junction voltages, and which is detected and quantitated. By externally controlling a DC source for the voltage, the detected voltage and therefore the simulated temperature is immediately affected, irrespective of the activity of the RF generator.

Embodiments of the present invention separate the two effects, i.e., the junction potential, Vj, and the RF power, P, in a simulation/calibration jig. Positive and negative terminals of the jig are connected to the μV input of the generator and to ground, respectively.

Reference is now made toFIG. 2, which is a schematic diagram of a thermocouple-based RF ablation system45, in accordance with an embodiment of the invention. A tissue ablation module, which is realized as an RF generator module47includes an RF generator43and a display49, which presents a microvolt DC reading.

The generator module47is connectable to catheter14in actual ablation operation. The generator43is adjustable, and the direct current (DC) response of thermocouples31occurs during actual operation of the catheter14. The DC output of the thermocouples31(vj) can thus be correlated with the power (P) produced by the RF generator43and measured using the display49. However, in this system, the accuracy of the monitored DC output and hence the temperature reading is affected by the presence of induced RF current in the thermocouple circuit.

Reference is now made toFIG. 3, which is a detailed electrical schematic of a jig51, which simulates operation of the ablation system45(FIG. 2), in accordance with an embodiment of the invention. In the circuitry shown inFIG. 3, the flows of direct current and RF current are indicated by solid arrows and arrows drawn as broken lines, respectively.

As noted above, in actual operation of an ablation system, the RF power produced by the generator module47heats resistive tissue of a patient, causing ablation of part of the tissue. In the jig51the resistive tissue is represented by a load resistor41, which is connected to the RF power output of a generator43. The resistor41needs to be able to dissipate power on the order of 25 W.

A positive terminal53made of copper-constantan and a constantan wire55are thermocouple metallic elements of the sort that may implement a thermocouple in an ablation catheter. In such a catheter, the thermocouple may be in physical contact with the ablation electrode or more loosely coupled to the ablation electrode without actual physical contact. In the jig51, RF current is present at the positive terminal53. In the example ofFIG. 3, the positive terminal53uses a copper conductor57(carrying the power) as one “arm” of the thermocouple—the other arm being the constantan wire55. The potential Vjgenerated at the positive terminal53, arising from the junction temperature, is typically of the order of microvolts. The potential may be fed back to the power generator via negative terminal59, which typically uses the measured potential to control the power delivered by the generator. Other thermocouple metallic elements and alloys may be substituted for copper and constantan in the positive terminal53and negative terminal59.

A generator suitable for use as the generator43is the nMARQ™ RF generator produced by Biosense Webster. This generator has a microvolt input61and the capability of displaying the power delivered by the electrode and the impedance “seen” by its output terminal. The generator also displays the temperature of the electrode, using the junction potential Vjdescribed above that is received via the microvolt input61and shown on the display49. This temperature is generally not the actual temperature of the patient tissue or of the electrode-tissue interface. As stated in the manual “The temperature displayed on the nMARQ Multi-Channel RF Generator does not represent the temperature of the tissue nor the temperature of the interface between the electrode and the tissue.” The temperature registered by the thermocouple (and displayed by the nMARQ generator), and the temperature of the tissue are different because of the heating effect of the RF power.

As shown in the schematic, known value resistors63,65are placed across the copper-constantan junction of the thermocouple. The resistors63,65are part of a resistor chain that includes resistors67,69. The resistors63,65,67,69have values of 150-200 Ohms. The chain is connected to a variable DC source71, which combines with the DC voltage Vjand appears across positive terminal53and negative terminal59. The Model NIPCI-6073 data acquisition tool, available from National Instruments Corporation, 11500 N. Mopac Expwy, Austin, Tex. 78759-3504, is suitable for the source71. RF current in a portion of the circuitry delineated by a box73is largely eliminated by the presence of 1 mH ferrite inductors75, and also by connecting a resistor77in series with a 0.15 mF DC blocking capacitor79to provide a return path having low impedance to RF, and leading from the positive terminal53to ground. As a result, DC is blocked from the return path, but RF is permitted. At the same time, RF current is effectively blocked from a second circuit, which is a path formed by source71, inductors75and the resistor chain, and which has a greater resistance to RF current than to DC current. This is largely due to the reactance of the inductors75seen by the RF source. However, the combined DC output of the source71and the voltage Vjflows readily in the second circuit, and DC voltage appears at the negative terminal59and at the microvolt input61of the generator module47.

The effect is to separate the DC potential between the positive terminal53and the negative terminal59from the RF current produced by the generator module47. The separation of the DC potential and the RF power allows the jig to be used for two purposes:

(1) Simulating different values of the thermocouple potential Vjand the RF power P independently of each other by adjusting the outputs source71and the generator43. This type of simulation allows various ablation algorithms built into generators (such as the nMARQ RF generator) to be modified or evaluated. Such algorithms typically use values of the potential Vjto control the RF power P. The jig51allows simulations of scenarios, such as rapid temperature excursions, e.g., beyond safety limits, or very stable temperatures. When such scenarios occur, the response of the generator module47can be evaluated.

(2) Calibration of the value of the potential Vjfor different values of the power P and other variables such as change of the power P with time. The electromotive force produced by copper-nickel alloys such as constantan as a function of temperature is well-known. In a calibration mode, any desired temperature can be simulated, and the readout of the generator module47may be adjusted to correct errors. This calibration can be elaborated to correct for errors that vary according to the levels of RF power being produced. Such calibrations are typically performed at the factory, but may be repeated by maintenance personnel, or even by an operator if desired.

While the generator43and the display49may be integral, as inFIG. 3, this is not essential, and they may be provided separately. The simulations and calibrations described above may be performed in any case.

The following procedures are explained for convenience with respect to the circuitry shown inFIG. 3, but they are not limited to the particular configuration shown therein.

Reference is now made toFIG. 4, which is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system, in accordance with an embodiment of the invention. At initial step81the jig51is connected to the generator module47, to the display49and the direct current source71.

Next, at step83, the power output of the generator module47and the source71are independently adjusted so as to simulate a sequence of events, which the generator module47is expected to recognize and to respond in accordance with its internal programming.

For example, the simulator may be adjusted such that the display49initially registers 38° C. and progresses to 44° C. while the power varies up to 25 W.

In an alternative testing sequence, the simulator may be adjusted such that the display49initially registers 38° C. and progresses to an upper temperature limit of 47° C., with oscillations of +/−2° C., during which the power may reach a target of 25 W and then drop, so as to maintain the temperature readings below 47° C.

In yet another alternative testing sequence, designed for testing safety of the ablator, the display49may initially be set to register 47° C. and progress to 80° C. It is expected that the generator module47will issue an alert indicating an abnormally high temperature and will produce control signals intended to reduce or discontinue power output in order to stop the ablation.

Next, at decision step85, it is determined if the generator module47has responded to the testing sequence as programmed. If the determination is affirmative, then control proceeds to final step87where a successful result is reported.

If the determination at decision step85is negative, then control proceeds to final step89where failure is reported.

Reference is now made toFIG. 5, which is a flow chart of a method of operating a temperature simulator for thermocouple-based RF ablation system to calibrate an RF generator having a microvolt input, in accordance with an embodiment of the invention.

At initial step91, the jig51is connected to the generator module47, to the display49and the direct current source71.

Next, at step93, the source71is adjusted to simulate a first temperature, e.g., 25° C. A bias control in the generator module47is adjusted such that the display49reads 25° C. The generator module47may be activated to produce power at an operational level to assure that the display49continues to read 25° C.

Next at step95, the source71is adjusted to simulate a second temperature, e.g., 75° C. A sensitivity control in the generator module47is adjusted such that the display49reads 48° C. The generator module47may be activated to produce power at an operational level to assure that the display49continues to read 75° C.

Steps93,95may be iterated, varying the bias and sensitivity controls as necessary to improve the quality of the readings of the display49.

Next, at decision step97, it is determined if the readings of the display49are accurate within a defined tolerance limit. If the determination is affirmative, then control proceeds to final step99where a successful result is reported.

If the determination at decision step97is negative, then control proceeds to final step101where failure is reported.