Abstract:
Medical apparatus includes a pacing generator, which has first active and indifferent outputs and is configured to generate electrical pacing pulses between the first active and indifferent outputs for pacing a heart of a subject. A radio frequency (RF) generator has second active and indifferent outputs and is configured to generate RF electrical energy of a predetermined frequency between the second active and indifferent outputs for application to the heart of the subject simultaneously with the pacing pulses. A filter includes a first branch connected between the first and second active outputs and a second branch connected between the first and second indifferent outputs, each of the first and second branches including one or more notch filters having a high impedance in a vicinity of the frequency of the RF electrical energy.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates generally to invasive cardiac therapies, and specifically to validating and monitoring percutaneous cardiac ablation procedures. 
       BACKGROUND OF THE INVENTION 
       [0002]    Invasive cardiac ablation techniques for the treatment of arrhythmias are well known in the art. For example, U.S. Pat. Nos. 5,443,489 and 5,480,422, whose disclosures are incorporated herein by reference, describe systems for ablating cardiac tissue by application of radio-frequency (RF) energy to the tissue through a catheter. 
         [0003]    In a cardiac ablation procedure, it is important to apply sufficient energy to create a lesion that will block undesired conduction, while minimizing collateral damage to surrounding tissues. Various methods have been proposed for monitoring ablation procedures for this purpose. For example, U.S. Pat. No. 6,743,225, whose disclosure is incorporated herein by reference, proposes to measure electrical activity of the cardiac tissue proximate a lesion site during an ablation treatment, and then to compare the measurements in order to determine whether the lesion is clinically efficacious so as to be able to block myocardial propagation. The electrical activity can include electrical signals corresponding to the local electrogram signal, pacing threshold value, and the like. 
         [0004]    As another example, U.S. Patent Application Publication 2007/0198007, whose disclosure is incorporated herein by reference, describes methods and devices for monitoring intracardiac ablation progress in near real time, by evaluating capture of a pacing signal while ablation energy is concurrently directed to a target site. Sufficiency of ablation is indicated by failure of signal capture at a maximum predetermined pacing voltage. An electrode in a cardiac catheter is simultaneously used to test pacing capture and to deliver ablation energy. 
       SUMMARY OF THE INVENTION 
       [0005]    Notwithstanding the above-mentioned references, safety concerns have led practitioners to avoid applying pacing and RF ablation energy to the heart simultaneously, mainly due to the possibility of leakage of substantial RF power into the pacing circuit. Embodiments of the present invention that are described hereinbelow overcome this problem by using a novel array of notch filters to keep RF energy from penetrating through the pacing circuit. The array comprises two branches of notch filters with high impedance in the frequency range that is used for ablation: one branch protecting the signal path between the pacing circuit and the catheter tip, and the other protecting the return path. The filters have low impedance at the pacing frequency, thus permitting pacing to proceed simultaneously with ablation. 
         [0006]    There is therefore provided, in accordance with an embodiment of the present invention, medical apparatus, including: 
         [0007]    a pacing generator, which has first active and indifferent outputs and is configured to generate electrical pacing pulses between the first active and indifferent outputs for pacing a heart of a subject; 
         [0008]    a radio frequency (RF) generator, which has second active and indifferent outputs and is configured to generate RF electrical energy of a predetermined frequency between the second active and indifferent outputs for application to the heart of the subject simultaneously with the pacing pulses; and 
         [0009]    a filter including a first branch connected between the first and second active outputs and a second branch connected between the first and second indifferent outputs, each of the first and second branches including one or more notch filters having a high impedance in a vicinity of the frequency of the RF electrical energy. 
         [0010]    In some embodiments, the apparatus includes a catheter, which includes a distal tip that is configured to be inserted into a chamber of the heart and an electrode at the distal tip, wherein the first and second active outputs are coupled together to deliver the pacing pulses and the RF electrical energy to the heart via the electrode. Additionally or alternatively, the apparatus includes a monitor, which is configured to detect capture of the pacing pulses by the heart during application of the RF electrical energy. 
         [0011]    In some embodiments, each of the first and second branches includes a plurality of notch filters having respective notch frequencies in the vicinity of the frequency of the RF electrical energy. In a disclosed embodiment, each of the notch filters includes an inductor and a capacitor arranged in parallel. 
         [0012]    Additionally or alternatively, the filter includes a third branch, which is connected between the first and second branches and the first active and indifferent outputs and which includes a further one or more notch filters that have a low impedance in the vicinity of the frequency of the RF electrical energy. The further one or more notch filters may include a plurality of notch filters including an inductor and a capacitor arranged in series and having respective notch frequencies in the vicinity of the frequency of the RF electrical energy. Further additionally or alternatively, the filter includes a common mode choke connected between the third branch and the first active and indifferent outputs. 
         [0013]    There is also provided, in accordance with an embodiment of the present invention, a method for treating a heart of a subject, the method including: 
         [0014]    operating a pacing generator, which has first active and indifferent outputs, to generate electrical pacing pulses between the first active and indifferent outputs so as to pace the heart; 
         [0015]    actuating a radio frequency (RF) generator, which has second active and indifferent outputs, to generate RF electrical energy of a predetermined frequency between the second active and indifferent outputs for application to the heart simultaneously with the pacing pulses; 
         [0016]    inhibiting penetration of the RF electrical energy into the pacing generator by connecting a first branch of a filter between the first and second active outputs and a second branch of the filter between the first and second indifferent outputs, each of the first and second branches including one or more notch filters having a high impedance in a vicinity of the frequency of the RF electrical energy; and 
         [0017]    simultaneously applying the pacing pulses and the RF electrical energy to the heart. 
         [0018]    The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which: 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is a schematic, pictorial illustration showing a system for percutaneous ablation therapy in the heart of a subject, in accordance with an embodiment of the present invention; 
           [0020]      FIG. 2  is a block diagram that schematically shows circuitry for delivering RF ablation and pacing signals to a catheter electrode, in accordance with an embodiment of the present invention; 
           [0021]      FIG. 3  is a block diagram that schematically shows a filter circuit for use in simultaneous RF ablation and pacing of the heart, in accordance with an embodiment of the present invention; and 
           [0022]      FIG. 4  is a schematic circuit diagram showing details of a filter circuit for use in simultaneous RF ablation and pacing of the heart, in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0023]      FIG. 1  is a schematic, pictorial illustration of a system  10  for performing ablative procedures on a heart  12  of a living subject, in accordance with a disclosed embodiment of the invention. The system comprises a probe, typically a catheter  14 , which is percutaneously inserted by an operator  16 , who is typically a physician, through the patient&#39;s vascular system into a chamber or vascular structure of the heart. Operator  16  brings a distal tip  18  of the catheter into contact with the heart wall at a target site that is to be ablated. RF electrical current is then conducted through wires in the catheter to one or more electrodes at distal tip  18 , which apply the RF 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 can disrupt abnormal electrical pathways that cause arrhythmias. 
         [0024]    Catheter  14  typically comprises a handle  20 , having suitable controls to enable operator  16  to steer, position and orient distal tip  18  of the catheter as desired during the ablation. To aid operator  16  in positioning the catheter, the distal portion of the catheter may contains position sensors (not shown) that provide signals to a positioning processor located in a console  24 . Catheter  14 , may be adapted, mutatis mutandis, from ablation catheters that are known in the art, such as the catheters described in U.S. Pat. No. 6,669,692, whose disclosure is incorporated herein by reference. ECG electrodes (not shown) on the patient&#39;s body surface conduct electrical signals via a cable  26  to an ECG monitor  28  (which may also be integrated into console  24 ). A user interface  34  provides feedback to the operator and permits the operator to adjust system functions as appropriate. 
         [0025]    Embodiments of the present invention combine simultaneous ablation and pacing so that an ablation lesion can be assessed in real time, without interrupting the ablation procedure. Console  24  includes a RF power source  36  that generates an ablation power signal, which is conveyed to catheter  14  via a cable  32 . The RF power may be generated at any suitable frequency, but around 500 kHz is typical. A ground cable  33  provides a return path (typically via a back pad or other skin-surface electrode). Alternatively, the RF power may be delivered in a bipolar mode, whereby catheter  14  provides the return path, as well. 
         [0026]    Console  24  also comprises a low-frequency pacing generator  38  that produces a cardiac pacing signal. Pacing generator  38  typically comprises circuitry for varying its output voltage under control of the operator  16 , for example, from 3 to 6 volts, while maintaining a constant current output. Alternatively, pacing generator  38  may maintain a constant voltage, while varying its current output or may permit both the voltage and the current to be adjusted. RF power source  36  and pacing generator  38  are both coupled to catheter  14  via cable  32  and to the return path provided by cable  33 . 
         [0027]      FIG. 2  is a block diagram that schematically shows distal tip  18  of catheter  14  and associated circuitry in console  24 , in accordance with an embodiment of the present invention. The output of pacing generator  38 , including both the active and return connections, is connected to a filter  40 , which is shown in detail in the figures that follow. The purpose of this filter is to prevent leakage of RF energy from RF power source  36  into the pacing circuits, as well as blocking direct current return from the pacing generator to the catheter during ablation. (These features of the filter are very important for patient safety and address concerns that have contraindicated the use of simultaneous ablation and pacing in the past.) The output of RF power source  36  is mixed with the pacing signal following filter  40  in a mixer  42 , which may comprise any suitable type of high-frequency electrical junction that is known in the art. The combined RF and pacing waveform is conducted by cable  32  through catheter  14  and is applied to a common electrode  44  at distal tip  18  of the catheter. The combined waveform simultaneously paces the patient&#39;s heart and delivers ablation energy to the target. 
         [0028]    An “indifferent” electrode  46  is connected to cable  33  as the return path for both the RF and pacing currents. Electrode  46  may typically comprise a back pad or other skin-surface electrode, as noted above. Alternatively, the RF power source and pacing generator may have separate indifferent electrodes and return paths (not shown). 
         [0029]    Although electrode  44  is shown in  FIG. 2  as a single unit, catheter  14  may alternatively comprise any number of electrodes in any form. For example, the catheter may comprise two or more ring electrodes, a plurality or array of point electrodes, or any combination of these types of electrodes for performing the therapeutic functions described herein. Although it is advantageous for pacing generator  38  and RF power source  36  to be connected to the same electrode  44  via mixer  42 , as shown in the figures, catheter  14  may alternatively comprise separate pacing and RF electrodes (typically in close proximity to one another), which are driven separately by the pacing generator and RF power source. Even in this latter configuration, the isolation provided by filter  40  is important when ablation and pacing go on simultaneously. 
         [0030]    During the ablation procedure, ECG monitor  28  ( FIG. 1 ) indicates whether the heart has actually captured the pacing signal, i.e., whether the heartbeat synchronizes with the pacing signal applied through electrode  44 . As long as the pacing signal is captured, lesion formation is considered to be incomplete. The pacing amplitude may be increased gradually during the ablation procedure, as the pacing threshold increases due to lesion formation. When the pacing signal can no longer be captured even at high amplitude, lesion formation is considered to be complete, and the procedure at the current ablation site is terminated. In general, the pacing threshold increases with the size of the lesion, and this technique may thus be used to control the size of the lesion that is created by ablation. Additional lesions may then be ablated at other locations, depending on the therapeutic plan. 
         [0031]    This sort of pacing-based ablation technique is described in greater detail in the above-mentioned U.S. Patent Application Publication 2007/0198007, which also describes variations and additions to the technique that may be combined with the embodiments of the present invention that are described herein. 
         [0032]    Reference is now made to  FIGS. 3 and 4 , which schematically show details of filter  40 , in accordance with an embodiment of the present invention.  FIG. 3  is a block diagram that shows the overall architecture of the filter, while  FIG. 4  is a circuit diagram showing details of a particular implementation of this architecture. 
         [0000]    Filter  40  has the following features:
       Two branches  54  and  56  of parallel notch filter blocks  60 . Each block is a passive unit comprising an inductor and capacitor connected in parallel. These blocks have high impedance in the frequency range close to the specified resonant frequency of RF ablation. In the example shown in the figures, the component values of the inductors and capacitors are chosen so as to create multiple, overlapping notches, spaced 25 kHz apart over a range of ±50 kHz around the 500 kHz center frequency.   A branch  52  of serial notch filter blocks  58 . Each block is a passive unit with a serially-connected inductor and capacitor. These blocks have low impedance in the frequency range close to the specified resonant frequency and thus shunt to ground any RF energy that penetrated through branches  54  and  56 . Here, too, blocks  58  create multiple, overlapping notches, spaced 25 kHz apart over a range of ±50 kHz around the 500 kHz center frequency.   A common mode choke  50  attenuates any small common-mode noise that would otherwise pass to the pacer side of filter  40 .       
 
         [0036]    Of the two branches of parallel blocks  60 , branch  54  connects via mixer  42  to electrode  44  at the tip of catheter  14 , while branch  56  connects to indifferent electrode  46 . Thus, these blocks separate not only the energetic catheter tip electrode from pacing generator  38 , but also the return path. On the other hand, for the low-frequency pacing pulses from the pacing generator, parallel blocks  60  and common mode choke  50  have low impedance, so pacing is enabled. Combining the parallel and serial blocks as shown in the figures creates strong energy attenuation (and isolation) in the frequency range of 500 kHz. Alternatively, the notch filters may be designed for other frequency ranges, and larger or smaller numbers of the filter blocks may be used, depending on the ablation parameters and the required degree of attenuation. 
         [0037]    RF power source  36  typically includes an impedance measurement circuit (not shown), which checks the impedance of the RF circuit through the patient&#39;s body to ensure that there is good electrical contact before the high-power RF ablation current is actuated. Because branches  54  and  56  connect to both the active and indifferent electrodes, filter  40  will not affect the impedance measurement. 
         [0038]    Although the embodiment described above relates specifically to combination of pacing with RF ablation therapy, the principles of the present invention may likewise be applied in any sort of diagnostic or therapeutic environment in which RF energy is applied to the body simultaneously with pacing of the heart. It will thus be appreciated that the embodiments described above are cited by way of example, and that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.