Abstract:
A method of treating cardiac arrhythmia, including guiding a distal end portion of a catheter, the distal end portion having a distal tip and accommodating an elongated configuration of ablation electrodes, from the inferior vena cava into the right atrium of a human heart, guiding the distal end portion from the right atrium into the right ventricle of the heart, deflecting the distal tip into a hook configuration, pulling the catheter towards the inferior vena cava until the hook configuration engages the tricuspid annulus of the heart and the configuration of electrodes engages the isthmus of tissue between the tricuspid annulus and the inferior vena cava of the heart, and activating the configuration of electrodes to produce a substantially continuous lesion on the isthmus of tissue.

Description:
CROSS-REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation of U.S. Ser. No. 09/981,543, filed on Oct. 17, 2001, which is a continuation of U.S. Ser. No. 09/197,812, filed on Nov. 23, 1998, now U.S. Pat. No. 6,319,250, each of which is hereby incorporated by reference in its entirety. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a steerable medical catheter and, more particularly, to a flexible, electrode-bearing catheter of the type used in electrophysiological studies for intracardiac electrocardiographic recording, mapping, stimulation and ablation.  
         BACKGROUND OF THE INVENTION  
         [0003]    Catheters are often used in medical procedures to provide physical access to remote locations within a patient via a relatively small passageway, reducing the need for traditional invasive surgery. The catheter tube can be inserted into an artery or other passageway through a relatively small incision in the patient&#39;s body, and threaded through the patient&#39;s system of blood vessels to reach the desired target.  
           [0004]    Various types of catheters are used in various procedures, both diagnostic and therapeutic. One general type of catheter used for both diagnostic and therapeutic applications is a cardiac electrode catheter. The diagnostic uses for a cardiac electrode catheter include recording and mapping of the electrical signals generated in the course of normal (or abnormal) heart function. Therapeutic applications include pacing, or generating and placing the appropriate electrical signals to stimulate the patient&#39;s heart to beat in a specified manner, and ablation. In an ablation procedure, electrical or radio-frequency energy is applied through an electrode catheter to form lesions in a desired portion of the patient&#39;s heart, for example the right atrium. When properly made, such lesions will alter the conductive characteristics of portions of the patient&#39;s heart, thereby controlling the symptoms of supra-ventricular tachycardia, ventricular tachycardia, atrial flutter, atrial fibrillation, and other arrhythmias.  
           [0005]    Such a catheter is typically placed within a desired portion of the patient&#39;s heart or arterial system by making a small incision in the patient&#39;s body at a location where a suitable artery or vein is relatively close to the patient&#39;s skin. The catheter is inserted through the incision into the artery and manipulated into position by threading it through a sequence of arteries, which may include branches, turns, and other obstructions.  
           [0006]    Once the cardiac electrode catheter has been maneuvered into the region of interest, the electrodes at the distal end of the catheter are placed against the anatomical feature or area sought to be diagnosed or treated. This can be a difficult procedure. The electrophysiologist manipulating the catheter typically can only do so by operating a system of controls at the proximal end of the catheter shaft. The catheter can be advanced and withdrawn longitudinally by pushing and pulling on the catheter shaft, and can be rotated about its axis by rotating a control at the proximal end. Both of these operations are rendered even more difficult by the likelihood that the catheter must be threaded through an extremely tortuous path to reach the target area. Finally, once the tip of the catheter has reached the target area, the electrodes at the distal end of the catheter are placed in proximity to the anatomical feature, and diagnosis or treatment can begin.  
           [0007]    In the past, the difficulties experienced by electrophysiologists in the use of a cardiac electrode catheter have been addressed in a number of different ways.  
           [0008]    To facilitate maneuvering a catheter through a tight and sinuous sequence of arterial or venous passageways, catheters having a pre-shaped curve at their distal end have been developed. To negotiate the twists and branches common in a patient&#39;s arterial or venous system, the catheter typically is rotatable to orient the pre-shaped curve in a desired direction. Although the tip of the catheter may be somewhat flexible, the curve is fixed into the catheter at the time of manufacture. The radius and extent of the curvature generally cannot be altered. Therefore, extensive pre-surgical planning is frequently necessary to determine what curvature of catheter is necessary. If the predicted curvature turns out to be incorrect, the entire catheter may need to be removed and replaced with one having the proper curvature. This is an expensive and time-consuming ordeal, as catheters are generally designed to be used only once and discarded. Moreover, the additional delay may place the patient at some additional risk.  
           [0009]    A variation of the pre-shaped catheter uses a deflectable curve structure in the tip. This type of catheter has a tip that is ordinarily substantially straight, but is deflectable to assume a curved configuration upon application of force to the tip. However, the tip deflection is not remotely controllable. In a certain patient&#39;s arterial system, a point may be reached at which the proper force cannot be applied to the catheter tip. In such cases, the catheter must be withdrawn and reinserted through a more appropriate passage, or another catheter with a different tip configuration must be used.  
           [0010]    Another attempt to facilitate the placement of catheters takes the form of a unidirectional steering catheter. A typical unidirectional steering catheter has a steering mechanism, such as a wire, that extends the length of the catheter to the distal tip. The steering mechanism is coupled to the tip in such a way that manipulation of the proximal end of the mechanism (e.g., by pulling the steering wire) results in deflection of the catheter tip in a single direction. This type of catheter is illustrated, for example, in U.S. Pat. No. 5,125,896 issued to Hojeibane. The direction of deflection can be controlled by embedding a ribbon of wire in the tip; the ribbon is flexible along one dimension but not in others. This type of catheter can further be controlled by rotating the entire shaft of the catheter; in this manner, the direction of bend within the patient can be controlled. The shaft of such a catheter must be strong enough to transmit torque for the latter form of control to be possible.  
           [0011]    U.S. Pat. No. 5,383,852 to Stevens-Wright describes a steerable electrocardial catheter including a flexible tip assembly having a proximal and a distal section. In this catheter, two steering mechanisms are used to separately control bending of either or both the proximal and distal sections. The steering mechanisms for the proximal and distal sections include separate steering wires, as described above, which are coupled to the proximal and distal sections, respectively.  
           [0012]    Bidirectional steering catheters also exist. The distal end of a bidirectional steering catheter can be maneuvered in two planes, allowing the tip to be positioned with greater accuracy. However, bidirectional steering catheters are complex mechanically and are often difficult to manipulate.  
           [0013]    Although the foregoing types of catheters address the issue of maneuverability in different ways, none of them is ideally configured to maintain contact with and apply a desired amount of pressure to a desired anatomical feature, such as an atrial wall.  
           [0014]    One device used for the latter purpose is known as a basket catheter. See, for example, the HIGH DENSITY MAPPING BASKET CATHETER manufactured by Cardiac Pathways Corporation. A basket catheter has several spring-biased arms near the distal tip. When these arms are unconstrained, they bow outward to define a basket-like shape. The arms of the basket are constrained for implantation in a sheath structure. When the tip of the catheter has reached the desired location, the sheath is retracted, or the arms are advanced out of the sheath.  
           [0015]    However, because the tip of the catheter is sheathed, it is not easily steerable into location, and is not as flexible as one might desire. Moreover, the sheath adds bulk to the device, which might significantly limit the range of applications in which the basket catheter can be used. The basket has only one shape and size. Once the arms are deployed from the sheath, the basket assumes a single configuration defined upon manufacture. If the predefined configuration of the basket is not suitable, then substantially no correction is possible. Also, known basket catheters are not indicated for use in high-energy therapeutic applications, such as ablation.  
           [0016]    A variable-geometry sheathed electrode catheter is also known in the art. This device has a single electrode-bearing tip portion that is initially disposed within a relatively inflexible sheath. When the tip portion is advanced with respect to the sheath, the tip portion bows out of a slot-shaped aperture in the sheath. The shape of the tip portion can be controlled to apply a desired amount of pressure to an anatomical feature. However, as a sheath is used around the catheter, the device is not easily steerable into location. Moreover, as discussed above, the sheath structure adds undesirable bulk to the device.  
           [0017]    Radio frequency ablation (RFA) has become the treatment of choice for specific rhythm disturbances. To eliminate the precise location in the heart from which an arrhythmia originates, high frequency radio waves are generated onto the target tissue, whereby heat induced in the tissue bums the tissue to eliminate the source of arrhythmia.  
           [0018]    For successful ablation treatment, e.g., to produce a lesion at a given anatomical site, it is generally required that the catheter be accurately positioned at the ablation site and that continuous contact be maintained between the electrode and the ablation site for the duration of the ablation treatment.  
           [0019]    U.S. Pat. No. 5,617,854 to Munsif describes, inter alia, a pre-shaped catheter particularly useful for ablating in the vicinity of the sinoatrial node, the left atrium, and up to the mitral valve. The tip of the catheter is formed of a temperature-sensitive shape-memory material, e.g., Nitinol, or is otherwise invoked to assume a segmented configuration upon reaching a desired position. The segmented configuration includes a distal segment which bears an ablation electrode. In operation, the segmented shape produces tension which urges the ablation electrode on the distal segment into contact with a wall of the left atrium, while other segments are urged against other tissue. Since the shape of the catheter tip is fixed, the tip is not easily manipulated. Further, the tension produced between the segments of the catheter tip is dependent on the shape and dimensions of the ablation site, e.g., the left atrium.  
           [0020]    Atrial fibrillation and atrial flutter are the most common type of arrhythmia found in clinical practice. Although the potential adverse consequences of these types of arrhythmia is well known, the basic electrophysiological mechanisms and certain management strategies to control these types of arrhythmia have been understood only recently.  
           [0021]    Reference is made to FIG. 1 which schematically illustrates a cross-section of a human heart  100  showing typical atrial flutter circuits. Such circuits includes macro-entrant, counter-clockwise, pathways  120  from the right atrium  102 , through the inter-atrial septum  114 , down the free wall  116 , and across the isthmus of tissue  108  between the inferior vena cava  112  and the tricuspid annulus  106  of the tricuspid valve  110 .  
           [0022]    Most electrophysiologists recommend treating atrial flutter by producing a linear contiguous lesion  118  at the isthmus of tissue  108 , between vena cava  112  and the tricuspid annulus  106 . Linear lesion  118  can be produced by RF ablation electrodes which are placed in contact with tissue  108 . It is contemplated that isthmus tissue  108  is a critical link of the atrial flutter circuit and, thus, linear lesion  118  is expected to terminate this source of arrhythmia and prevent the recurrence of such arrhythmia.  
           [0023]    Existing ablation treatment for atrial flutter includes the use of a catheter bearing at least one single or bi-polar ablation electrode. Unfortunately, an undue amount of time is spent in correctly positioning the ablation electrode of the catheter against the site to be treated. Further, in existing electrode catheter configurations, the catheter must generally be repeatedly repositioned until an acceptable lesion  118  is produced. Thus, lesion  118  is often non-continuous, i.e., there may be gaps in the lesion line which may require further repositioning of the ablation catheter. Such repeated repositioning of the catheter is time consuming and may result in prolonged, potentially harmful, exposure of patients to X-ray radiation.  
           [0024]    Accordingly, there is a need for a cardiac electrode catheter that can be conveniently and quickly steered into secured, operative, engagement with a preselected portion of the isthmus of tissue between the inferior vena cava and the tricuspid annulus, to produce a predefined, substantially continuous, lesion on this isthmus of tissue.  
         SUMMARY OF THE INVENTION  
         [0025]    The present invention seeks to provide a steerable electrode catheter having a relatively flexible distal end portion accommodating an elongated configuration of at least one ablation electrode, that can be conveniently guided to the vicinity of the tricuspid valve, and that can be steered into a shape which enables convenient positioning of at least one ablation electrode in secure operative engagement with an ablation site along the isthmus of tissue between the inferior vena cava and the tricuspid annulus, thereby to produce a predefined, elongated, substantially continuous, lesion at the ablation site.  
           [0026]    According to an embodiment of the present invention, the catheter includes a flexible distal end portion which may be controlled by one or two steering mechanisms, namely, a distal steering mechanism and, optionally, a proximal steering mechanism. The distal steering mechanism is adapted to deflect only the tip of the distal end portion into a hook-shaped configuration. The proximal steering mechanism is adapted to deflect the entire distal end portion.  
           [0027]    According to an embodiment of the present invention, the at least one ablation electrode is brought into secured engagement with the isthmus of tissue between the inferior vena cava and the tricuspid annulus, as follows. First, the distal end of the catheter is guided into the right atrium. As the distal end of the catheter advances in the right atrium, the proximal steering mechanism may be activated to deflect the entire distal end portion, such that the distal end portion may be conveniently inserted into the right ventricle. Once the distal end portion is inside the right ventricle, the distal steering mechanism is activated to produce the hook-shape configuration at the tip of the distal end portion. Then, the catheter is pulled back, i.e., in the direction of the right atrium, until the hook-shaped tip of the distal end is anchored at the tricuspid annulus. The catheter may then be pulled further back and the curvature of the distal end portion may be adjusted, e.g., using the proximal steering mechanism, until the at least one ablation electrode securely engages an ablation site along the isthmus of tissue between the tricuspid annulus and the inferior vena cava. Once such secured engagement is obtained, the at least one ablation electrode may be activated to produce a substantially continuous, linear, lesion at the ablation site. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    The present invention will be understood and appreciated more fully from the following detailed description of the preferred embodiment taken in conjunction with the accompanying drawings in which:  
         [0029]    [0029]FIG. 1 is a schematic, cross-sectional, illustration of a human heart showing an atrial flutter circuit including an isthmus of tissue between the inferior vena cava and the tricuspid annulus;  
         [0030]    [0030]FIG. 2 is a schematic, cross-sectional, illustration of an electrode catheter in accordance with an embodiment of the present invention;  
         [0031]    [0031]FIG. 3 is a schematic, side view, cross-sectional, illustration of a distal end portion of the electrode catheter of FIG. 2;  
         [0032]    FIGS.  4 A- 4 C are schematic, front view, cross-sections of the distal end portion of FIG. 3, taken along section lines A-A, B-B and C-C, respectively;  
         [0033]    [0033]FIG. 5 is a schematic, cross-sectional, illustration of the human heart, showing the electrode catheter of FIG. 2 being introduced into the right atrium;  
         [0034]    [0034]FIG. 6 is a schematic, cross-sectional, illustration of the human heart, showing the electrode catheter of FIG. 2 being steered from the right atrium into the right ventricle;  
         [0035]    [0035]FIG. 7 is a schematic, cross-sectional, illustration of the human heart, showing the tip of the electrode catheter of FIG. 2 being deflected into a “hook” shape;  
         [0036]    [0036]FIG. 8 is a schematic, cross-sectional, illustration of the human heart, showing the electrode catheter of FIG. 2 being pulled back to engage the isthmus of tissue between the inferior vena cava and the tricuspid annulus with the tip of the catheter anchored at the tricuspid annulus; and  
         [0037]    [0037]FIG. 9 is a schematic illustration of an end portion of an electrode catheter in accordance with another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0038]    Reference is made to FIG. 2 which schematically illustrates a perspective view of an ablation catheter  10  in accordance with an embodiment of the present invention.  
         [0039]    Catheter  10  includes a handle portion  22 , electric connectors  24 , a tubular catheter shaft  11  and a distal end portion  12  including an end shaft  13 . Distal end portion  12  includes a distal tip  16 , a distal tip deflection region  60  and a proximal deflection region  62 . According to the present invention, distal end portion  12  can be steered from a generally straight configuration, indicated by the solid lines in FIG. 1, to a deflected configuration, indicated by the broken lines in FIG. 1. The broken line configuration in FIG. 1 also illustrates how distal tip  16  can be deflected into a hook-shaped configuration, as described in detail below.  
         [0040]    In an embodiment of the present invention, tip  16  may include a sensor or mapping electrode, as is known in the art, for monitoring the electric potential of tissue in contact therewith. This may be helpful in guiding and positioning distal end portion  12 , as described below.  
         [0041]    Reference is now also made to FIG. 3 which schematically illustrates a side-view, cross-section, of distal end portion  12 . End shaft  13 , which is preferably hollow as shown in FIG. 3, accommodates an elongated configuration  40  of ablation electrodes  14 . Elongated configuration  40  may include any number of electrodes  14 , with a predetermined spacing therebetween, or a single elongated electrode, as known in the art, adapted to produce a substantially continuous, substantially linear, lesion when brought into operative engagement with a target tissue. Electrodes  14  are preferably all ring-electrodes covering the entire circumference of shaft  13 .  
         [0042]    Reference is now made also to FIGS.  4 A- 4 C which schematically illustrate front-view cross-sections of distal end portion  12  along section lines A-A, B-B, and C-C, respectively, in FIG. 3. In accordance with the present invention, catheter  10  includes a distal steering mechanism which is used to deflect tip  16  of distal end portion  12 , as mentioned above, by producing a small radius of curvature at region  60 . Catheter  10  further includes a proximal steering mechanism which controls the curvature of region  62 , between shaft  11  and  13 , thereby to control the deflection of the entire distal end portion  12 .  
         [0043]    The distal and proximal steering mechanisms may include any suitable steering mechanisms known in the art, for example, the control mechanisms described in U.S. Pat. No. 5,383,852 to Stevens-Wright, the disclosure of which is incorporated herein by reference. As shown in FIGS.  3 - 4 C, the distal and proximal control mechanisms may include control wires  55  and  64 , respectively, which extend along the interior of shaft  11  from handle portion  22  to regions  60  and  62 , respectively, of distal end portion  12 . Wire  55  is attached to tip  16  and may extend through middle guiding loops along most of the length of shaft  13 , as shown in FIGS. 4B and 4C, and then through off-center guiding loops at region  60 , as shown in FIG. 4A, whereby only a small segment adjacent to tip  16  is deflected by wire  55 . Wire  64  may extends through off-center guiding loops in shaft  13 , as shown in FIG. 4C, and is attached to end shaft  13  at region  62 .  
         [0044]    The deflection of distal end portion  12  into a desired configuration is preferably controlled by an electrophysiologist using control members  26  and/or  27  on handle portion  22 . In the embodiment shown in FIG. 2, control member  26  may include a rotatable control member attached to wire  55 , such that forward or backward rotation of control member  26  results in corresponding movement of wire  55 , thereby controlling the deflection of end portion  12  at region  60 . Control member  27  may include a slidable control member attached to wire  64 , such that forward or backward sliding of control member  27  results in corresponding movement of wire  64 , thereby controlling the deflection of end portion  12  at region  62 . As known in the art, the electrophysiologist may also rotate distal end portion  12  about the longitudinal axis of catheter  10 . Any suitable rotation mechanism, as is known in the art, can be used to control the rotation of distal end portion  12 . For example, catheter shaft can be made of a rotationally rigid material that transmits the rotation of handle portion  22  to distal end  12 . Alternatively, the rotation of handle  22  may be transmitted by a rotationally stiff member (not shown) extending longitudinally through the interior of catheter shaft  11 .  
         [0045]    In an embodiment of the present invention, electrodes  14  are addressed, together or separately, via connectors  24 , which are connected to electrodes  14  by conductors  66 . Conductors  66  may extend along the interior of catheter shaft  11  and end shaft  13 , for example, through middle guiding loops, as shown in FIGS.  4 A- 4 C.  
         [0046]    Using connectors  24 , electrodes  14  are connected to an ablation energizing circuit, which may be activated by user controls as are known in the art. Upon activation, the energizing circuit energizes electrodes  14  with radio frequency (RF) energy, as is known in the art. Using separate ablation controls, the electrophysiologist may activate electrodes  14  together or separately (if selective ablation is desired) to ablate a target tissue, as described in detail below.  
         [0047]    As known in the art, electrodes  14  may be associated with temperature sensors (not shown in the drawings) which may be connected to temperature monitoring circuitry for monitoring the temperature of the tissue in contact with electrodes  14 . An output of the temperature monitoring circuitry may be visually displayed to the electrophysiologist, as is known in the art, to provide the electrophysiologist with on-line indication of the electrode temperatures, which are indicative of adjacent tissue temperatures. If temperature sensors are used, they may be connected to the monitoring circuitry via connectors  56  and additional conductors (not shown) in catheter shaft  11 .  
         [0048]    According to the present invention, catheter  10  is used for ablating tissue on the endocardium isthmus of tissue between the inferior vena cava and the tricuspid annulus of a patient suffering from aberrant heart activity, such as atrial flutter or fibrillation, as described below.  
         [0049]    FIGS.  5 - 8  schematically illustrate a procedure for introducing catheter  10  into the right atrium and subsequently guiding distal end portion  12  to securely engage a portion of the endocardium tissue  108  between the inferior vena cava and the tricuspid annulus.  
         [0050]    As shown in FIG. 5, distal end portion  12  is first guided into the right atrium of the patient&#39;s heart  100  from the inferior vena cava. Once catheter  10  is introduced into the right atrium, the electrophysiologist proceeds to deflect distal end portion  12  towards the right ventricle  104 , using the proximal steering mechanism of catheter  10 . Distal end portion  12  enters the right ventricle via the tricuspid valve  110 , as shown in FIG. 6. If necessary, end shaft  13  may be rotated to assist in the manipulation of distal end portion  12 .  
         [0051]    After distal end portion  12  is inserted into the right ventricle, the electrophysiologist uses the distal steering mechanism to deflect tip  16  into the hook-shaped configuration described above, as shown in FIG. 7. Then, the catheter is pulled back, i.e., in the direction of inferior vena cava  112 , until a portion of the tricuspid annulus  16  is grasped by the hook-shaped tip  16 , as shown in FIG. 8.  
         [0052]    Once tip  16  is anchored at the tricuspid annulus, the catheter may be pulled further back and the curvature of distal end portion  12  may be adjusted, using the proximal steering mechanism, until electrodes  14  of elongated configuration  40  securely engage a portion of the isthmus of tissue  108  between tricuspid annulus  106  and inferior vena cava  112 . At this point, the electrophysiologist activates some or all of electrodes  14  to ablate a substantially continuous, substantially linear, lesion on the endocardial wall of the isthmus of tissue  108 .  
         [0053]    As described above, electrodes  14  may be associated with temperature sensors. These sensors may include thermocouples or any other temperature sensing means known in the art. Based on the temperatures measured by these optional temperature sensors, the electrophysiologist may deactivate some or all of electrodes  14  when the temperature of the ablated tissue site exceeds a predetermined threshold. Then, when the temperature of the ablated sites drops below the threshold, the electrophysiologist may reactivate electrodes  14  if further ablation is required.  
         [0054]    As mentioned above, tip  16  may optionally include a sensor electrode for monitoring/mapping the electrical potential of tissue adjacent tip  16 , e.g., to enable more accurate and/or more efficient positioning of end portion  12  against isthmus of tissue  108 . Sensor electrodes may also be included in electrode configuration  40 , e.g., for mapping the electrical potential along isthmus of tissue  108 , during or between ablation sessions, to determine whether further ablation may be necessary.  
         [0055]    Reference is now made to FIG. 9 which schematically illustrates a distal end portion  212  of an ablation catheter in accordance with another embodiment of the present invention, having an elongated electrode configuration  240  including a plurality of electrodes  214  and a tip  216 . In the embodiment of FIG. 9, distal end potion  212  is adapted to be steerable or deflectable at three regions, namely, a distal tip deflection region  260 , an intermediate deflection region  250  and a proximal deflection region  262 . Regions  260  and  262  are generally analogous to the distal and proximal deflection regions  60  and  62 , respectively, of distal end portion  12 , as described above with reference to FIGS.  2 - 8 . Intermediate deflection region  250  may be located at a predetermined position along electrode configuration  240 . The mechanisms for deflecting end portion  212  at regions  260  and  262  may be similar to those used for deflecting end portion  12  at regions  60  and  62 , respectively, as described in detail above with reference to FIGS.  2 - 8 . The mechanism for deflecting distal end portion  212  at intermediate region  250  may include any suitable deflection mechanism, for example, a control wire (not shown) extending through the hollow interior of end portion  212 , analogous to control wires  55  and  64  in the embodiment of FIGS.  2 - 8 .  
         [0056]    The curvature of end portion  212  at any or all of regions  260 ,  250  and  262  may be controlled by the electrophysiologist using any suitable controls (not shown), for example, handle controls similar to controls  26  and  27  in the embodiment of FIGS.  2 - 8 . Thus, in this embodiment, the electrophysiologist may control the curvature of distal end portion  212  at region  250 , in addition to controlling the curvature of distal and proximal regions  260  and  262 . The addition of intermediate deflection region  250  enables more flexibility in conforming the shape of distal end portion  212  to the shape of isthmus of tissue  108  during ablation treatment. In an embodiment of the present invention, intermediate deflection region  250  is adapted to be deflected in the direction indicated by arrow  270 , so as to provide improved contact with isthmus of tissue  108  when end portion  212  is urged against the tissue.  
         [0057]    In yet another embodiment of the present invention, end portion  212  is not deflectable at region  250  but, rather, end portion  212  is formed of a resilient material and is pre-shaped to have a predetermined curvature at region  250 , as shown generally in FIG. 9. In this embodiment of the invention, when end portion  212  is urged against a target tissue, such as isthmus of tissue  108 , the curvature of region  250  changes until electrode configuration  240  conforms to the shape of the target tissue. This ensures urged contact between electrodes  214  and the target tissue without an additional steering mechanism.  
         [0058]    In still another embodiment of the present invention, end portion  212  is deflectable only at distal region  260 , to assume a hook-shaped configuration as described above, but is not deflectable at proximal region  262 . End portion  212  may also be pre-shaped or deflectable at intermediate region  250 , as described above. In this embodiment, once the tricuspid annulus is grasped by the hook-shaped tip of the catheter, it is primarily the backward pulling force applied by the electrophysiologist that brings electrodes  214  into urged contact with the target endocardial tissue.  
         [0059]    It will be appreciated by persons skilled in the art that the present invention may be carried out using any of the above described configurations of electrodes and/or deflection regions and/or pre-shaped regions, as well as any other suitable configuration of electrodes and/or deflectable/pre-shaped regions.  
         [0060]    It should be appreciated that the present invention is not limited to the specific embodiments described herein with reference to the accompanying drawing. Rather, the scope of the present invention is limited only by the following claims: