turning to the figures of the drawings , there is seen in fig1 an electrosurgical system , generally designated 10 , embodying the present invention . system 10 comprises a pair of substantially identical electrodes , or electrode leads generally at 12 , 14 , each of which is adapted to be connected to one of the positive terminal 16 or negative terminal 18 of an electrosurgical bipolar rf generator 20 . the connections between the electrodes and the generator , as well as the structure of the generator itself , are well known and are not described in detail herein . although each electrode in fig1 is preferably identical , such may not be required in accordance with broader aspects of the invention , and only one electrode embodying the present invention may be employed . there may be circumstances where electrode of different shape may be employed . similarly , there may be circumstances where more than two electrodes are used simultaneously — for example one ( or more ) electrode may be connected to one terminal of the generator and two ( or more ) electrodes connected to the other terminal to form two or more ablation line segments at the same time . each illustrated electrode 12 , 14 has a proximal end 22 , which is adapted to be connected to one of the positive or negative terminals of the rf generator , and a distal end 24 that terminates in an electrode 26 that generates low current density and that is adapted to be inserted into tissue , such as that of the heart . with reference to fig2 , each electrode preferably comprises an outer tubular insulating member 28 that extends from the proximal end to the electrode 26 at the distal end . the insulating tube 28 is preferably made of a polymer such as ptfe , nylon or silicon and has an outside diameter of typically 1 mm to 4 mm , although the material and size may vary . a conductive member or wire 30 extends through the insulating tube 28 to conduct rf energy from the generator to the conductive terminal electrode 26 . alternatively , the elongated conductor may be referred to as a lead or wire and the terminal member 26 referred to as the electrode . the terminal end electrode 26 may be separable from the elongated lead or conductor for replacement as needed . the conductive member 30 is preferably in the form of a separate copper wire , but may alternatively comprise a thin conductive film , or other conductive element or means that , for example , may extend along or within the wall of tube 28 . the low current density electrode 26 may comprise a low pitch , small diameter coil or spiral wire that is integral with or otherwise in electrical contact with the distal end of the conductive wire 30 . the coil is also preferably made of solid copper , but may also be a plated copper , such as gold plated , stainless steel , or other conductive materials such as aluminum or beryllium - copper of sufficient strength to allow it to be threaded or screwed into the cardiac tissue . to this end , the tip of the electrode may be tapered to a point for piercing the myocardium . the diameter of the wire comprising the coil or spiral is preferably about 0 . 01 inches ( 0 . 254 mm ), but may range between about 0 . 005 inches ( 0 . 127 mm ) and 0 . 025 inches ( 0 . 635 mm ), while the diameter of the coil itself may be , for example , between about 0 . 080 inches ( 2 mm ) and 0 . 20 inches ( 5 mm ). the pitch of the coil electrode is defined as the axial distance between adjacent coils of the wire . for purposes of clarity , it should be noted that , as the number of coils increases per unit of axial length , the pitch ( or distance between adjacent coils ) decreases . conversely , as the number of coils decreases per unit of axial length , the pitch ( or distance between adjacent coils ) increases . low pitch for purposes of this description is preferably lower than about 3 coils per 5 mm , with the lower pitch spiral being preferred . the pitch of the thread of the conductive coil or screw is preferably about 1 coil ( 360 ° extent ) per 1 mm of the axial extent of the coil or more , but may be about 2 coils per 1 mm . the small diameter of the wire and the low pitch tend to increase and maximize the contact area between the electrode with the tissue to which it is attached . low current density for purposes of this description is about 20 milliamperes per square millimeter ( 20 ma / mm 2 ) or less . by way of example , and not limitation , a current density of approximately 16 . 2 ma / mm 2 is provided by an electrode that has a wire diameter of 0 . 01 inches ( 0 . 254 mm ), a pitch of about 1 coil / mm , a coil diameter of about 0 . 20 inches ( 5 mm ), which electrode extends through approximately 0 . 20 inches ( 5 mm ) of tissue ( with about 5 coils located in the tissue ) and with a current flow of about 1 ampere ( 1000 ma ). such an electrode has about 62 mm 2 of surface area in contact with the cardiac tissue . the current density is calculated by dividing the current flow by the electrode surface area ( or current density = current flow / electrode surface area ). for a current flow of 1 ampere ( 1000 ma ) and an electrode surface area of 62 mm 2 , the current density is 1 ampere ( 1000 ma )/ 62 mm 2 or about 16 . 2 ma / mm 2 . other current densities are possible for other configurations of the electrode and also will depend on the number of electrodes employed . typically , for electrodes located about 2 cm apart in cardiac tissue , a current flow of about 1 ampere is needed to create an ablation line therebetween . in addition , the distal end of each electrode may include a slug 34 or a mass of material ( or “ sink ”) such as copper or a copper / aluminum alloy that surrounds the conductive wire 30 . the distal end 36 of the slug 34 may be coterminus with , or extend slightly beyond , the distal end of the outer insulating member 28 so that when the electrode is screwed into tissue , the surface 36 of the slug 34 contacts the surface of the tissue . thus , when the electrode is attached to the tissue , the slug or mass 34 optionally serves to even further increase the contact area of the electrode , and to provide a mass of material that acts as a heat sink to limit tissue temperature increases . as seen in fig3 , the electrodes 12 and 14 may include a series of slugs or masses 34 a , 34 b , 34 c at the distal end 24 connected in series to the conductive wire 30 to provide for a higher rate of thermal dissipation . as illustrated , the masses or slugs 34 a , 34 b , 34 c are spaced along the wire 30 to provide for flexibility of the electrode . thus , in conjunction with the low pitch , low diameter wire coil 26 , the slug serves as a “ heat sink ”, and also reducing the degree of tissue coagulation at the electrode site due to the minimization of current density at the electrode / tissue interface . more particularly , the distal end 36 of the slug 34 may present a surface area of between about 0 . 01 in 2 and 0 . 1 in 2 , while the slug 34 may have a mass of about 1 gram to 10 grams . with reference to fig4 , an electrode 46 according to the present invention is shown which includes a hand - piece 48 with an elongated shaft or catheter tubing 50 extending distally therefrom and terminating in a screw - type electrode 52 that extends beyond the distal end of the catheter tubing 50 . the electrode 52 is adapted to be conductively connected to one of the outputs of an rf generator ( such as the generator 20 shown in fig1 ) by means of an insulated conductive cable 54 extending from the proximal end of the hand - piece 48 . the hand - piece 48 includes a thumb wheel 56 rotatably secured to the distal end thereof and fixedly connected to the screw electrode 52 ( by means including the conductive wire or other drive member not shown ) for rotating the electrode 52 with respect to the catheter tubing , thus facilitating screw attachment of the electrode 52 to the heart wall . to facilitate the steering or guiding of the catheter 50 / electrode 52 during a minimally - invasive procedure , any well - known structure may be employed . as illustrated , a wire 58 may be secured to the catheter tubing 50 so as to extend between the distal end of the tubing 50 and a thumb lever 60 rotatably mounted to the hand - piece 48 . actuation of the thumb lever 60 selectively extends or retracts the wire 58 to steer the distal end of the catheter tubing 50 , as indicated by the double - headed arrow 62 . thus , a pair of devices such as that shown in fig4 could be introduced through small incisions and positioned on the heart wall using the thumb lever 60 to steer the electrode 52 to the desired position and the thumb wheel 56 to screw the electrode 52 to the heart wall . when used for ablating cardiac tissue , it is desirable that the conductive screw 26 of the electrode penetrate as much of the thickness of the cardiac tissue as possible , without completely puncturing the heart wall and protruding into the interior or exterior of the heart . with reference to fig5 , to this end , each electrode may also include an ekg wire 38 extending through the outer insulating member 28 and terminating in a tip 40 that extends distally beyond the distal - most portion of the conductive screw 26 . ( for purposes of clarity , the slug 34 is not shown in the embodiment of fig5 . however , the spiral of the conductive coil encircles the slug at the distal end of the insulating member 28 while , in the embodiment employing a slug , the ekg wire 36 may extend through a central hole in the conductive slug 34 .) the structure shown in fig3 could also be used to provide for a bipolar ekg , in which case wire 38 with its exposed tip 40 would be connected to a first ekg and the coiled wire 30 connected to a second ekg . the ekg wire 38 is preferably covered by an insulating material , except for the distal tip 40 , which is free of insulation . this permits the ekg reading to be taken only at the very tip of the ekg wire . accordingly , as the electrode is screwed into the heart tissue , the electrode and ekg wires advance simultaneously . as the electrode wire passes out of the myocardium and penetrates the epicardium , the ekg reading abruptly decreases or disappears , thus signaling to the cardiologist or electrophysiologist that the conductive screw has penetrated nearly the entire tissue thickness . as noted above , an alternative arrangement provides an ekg sensor on the distal end of the electrode 26 so that the physician can immediately detect piercing of the heart wall by the electrode . alternatively , more than two electrodes may be used to form two or more ablation segments simultaneously . the low - pitch electrodes of the present invention can be used to make the series of lesions commonly associated with the maze procedure . with reference to fig6 , there is seen a posterior view of a heart showing a series of lesions 1 - 11 that may be made when performing the maze procuedure . the electrodes of the present invention may be used to make one or more of these lesions from the interior of the heart , which may be relatively easier to access than the exterior surface . however , the procedure can be performed on the exterior surface as well . the procedure will be described with respect only to the creation of lesion 1 , which encircles the right pair of pulmonary veins , lesion 2 , which encircles the left pulmonary veins , and lesion 3 , which connects lesions 1 and 2 . each of these lesions may be made in a series or sequence of steps in which the electrodes are attached to the heart and activated to form a first lesion segment . then one of the electrodes is “ unscrewed ” and moved step - wise , or “ leap - frogged ”, with respect to the other electrode . the electrodes are then activated again to form a second lesion segment connected to the first , and the steps repeated again a sufficient number of times to complete the desired lesion . alternatively more than two electrodes may be used to form two or more ablation segments simultaneously . with reference to fig7 , a schematic representation of the right pulmonary veins 42 is shown . the lesion 1 is created by attaching one of the electrodes 12 , 14 to location a 1 and the other to location b 1 , and activating the electrode to form a lesion between locations a 1 and b 1 . typically , the electrodes can be placed from 1 to 4 cm apart to provide a satisfactory transmural lesion between the electrodes . for example , the electrodes may be positioned about 2 cm ( 20 mm ) apart , and a current flow therebetween of about 1 ampere ( 1000 milliamperes ) could be employed to create a line of ablation therebetween . then the electrode from location a 1 is moved to location c 1 , the electrodes activated , thus forming the lesion between points b 1 and c 1 . then the electrode from point b 1 is moved to location d 1 and the electrodes activated to form the lesion between points c 1 and d 1 . then the electrode from location c 1 is moved to location a 1 and the electrodes activated to make the lesion from location d 1 to location a 1 , thus completely encircling the right pulmonary veins with lesion number 1 . then , with reference to fig8 , lesion 2 may be made in a similar fashion about the other pair of pulmonary veins 44 by moving the electrodes serially from locations a 2 - d 2 , and activating the electrodes in the same manner as described above with respect to the formation of lesion 1 . once the pulmonary veins 44 are encircled by lesion 2 , the electrode from location d 2 can be moved to location f and then activated to make the lesion a 2 - f . then the electrode from location a 2 can be moved to location a 1 and then activated to make the lesion connecting f with a 1 , thus completing lesion 3 , and connecting lesions 1 and 2 . as noted above , the other lesions required for the maze or other cardiac procedures can be made in a similar step - wise fashion . with the present invention , it is contemplated that a lesion line of about 100 mm could be made with about seven lead placements . while this may seem laborious , it is a relatively easy procedure to employ , which forms a reliable lesion line even with the movement of a beating heart , and without the trauma , cost and recovery of an open heart maze procedure .
a method and apparatus for creating transmural ablations in heart tissue , for example , may include two or more electrodes adapted to be connected to opposite poles of a bipolar rf generator so as to energize the electrodes to ablate cardiac tissue between the electrodes . a first electrode may be inserted into cardiac tissue at a first location and a second electrode may be inserted into cardiac tissue at a second location which is spaced from the first location . at least one of the electrodes may be adapted to provide a sufficiently low current density in the vicinity of the electrode to avoid substantial tissue desiccation when energized . alternatively , the apparatus may include at least one bipolar ablation electrode which comprises an elongated conductive member of spiral shape having a pitch sufficiently small to provide sufficient surface area to avoid substantial desiccation of tissue when energized by a bipolar rf generator .