Abstract:
Implantable medical systems include implantable medical leads that have magnetic orientation-independent magnetically actuated switches that are placed in the conduction path to the electrode of the lead. Thus, regardless of the orientation of a substantial magnetic field like that from an MRI machine to the lead and switch within the lead, the switch opens when in the presence of that substantial magnetic field. The switch may be placed in close proximity to the electrode such that the opening of the switch disconnects the electrode from the majority of the conduction path which thereby produces a high impedance for RF current and reduces the amount of heating that may occur at the electrode when in the presence of substantial levels of RF electromagnetic energy as may occur within an MRI machine.

Description:
TECHNICAL FIELD 
     Embodiments relate to implantable medical leads and systems. More particularly, embodiments relate to the creation of a high impedance within a conduction path of an implantable medical lead when in the presence of a magnetic field of a given strength that is significantly greater than is present in typical ambient conditions, such as magnetic fields that may be encountered within a bore of a magnetic resonance imaging (MRI) machine. 
     BACKGROUND 
     An implantable medical lead of an implantable medical system carries electrical stimulation signals from a pulse generator located at an implantation site of a patient to one or more electrodes at a distal end of the implantable medical lead that are located at a stimulation site of the patient. Electrical conductors within an insulative body of the implantable medical lead provide a conduction path for the electrical stimulation signals to traverse. 
     Patients having implantable medical systems that include implantable medical leads may have the need to undergo MRI scans. During the MRI scan, the patient is exposed to a relatively large static magnetic field of 1.5 Tesla or more as well as a smaller gradient magnetic field. While the magnetic fields may have a specific and known orientation relative to the scanning bore of the MRI machine and therefore to the body of the patient, it is noteworthy that the implantable medical lead and particularly the distal end within the patient may have any number of orientations relative to the magnetic fields. 
     Within the MRI, the patient is also exposed to a relatively high power radio frequency (RF) electromagnetic energy in the megahertz frequency range. This RF electromagnetic energy presents a potentially dangerous situation for the patient during the MRI scan. The RF electromagnetic energy may couple to the conductors within the implantable medical lead that provide the conduction path to the electrodes and thereby create relatively high levels of RF electrical current that produces heating of the tissue surrounding the electrodes. This is especially problematic for implantable neurostimulation systems where the electrodes are positioned in highly vulnerable stimulation sites such as within the brain or adjacent to the spinal cord. 
     SUMMARY 
     Embodiments address issues such as these and others by providing multiple actuators within the lead that are responsive to a magnetic field where each actuator is capable of opening a switch within the conduction path over a particular range of magnetic field orientation. Thus regardless of the orientation of the magnetic field to the lead and the actuators within the lead, the magnetic field causes at least one actuator to open the switch and thereby disconnect the electrode from the remainder of the conduction path on the opposite side of the switch. The open switch creates a low capacitance, high impedance path for any RF energy coupled to the lead conduction path. Including the switch in close proximity to the electrode leaves only an insignificant short conductive path length that remains connected to the electrode. The amount of heating that may be generated at the electrode is thereby reduced to a safer level. 
     Embodiments provide a method of creating a high impedance within a conduction path of an implantable medical lead. The method involves providing a first actuator that when in the presence of a magnetic field attempts to move from a first start position to a first stop position and reaches the first stop position when a force acting on the first actuator due to the presence of the magnetic field is adequate to produce such movement, the first actuator being responsive to magnetic fields that are not oriented normal to a direction of movement of the first actuator. The method further involves providing a second actuator that when in the presence of a magnetic field attempts to move from a second start position to a second stop position and reaches the second stop position when a force acting on the second actuator due to the presence of the magnetic field is adequate to produce such movement, the second actuator being responsive to magnetic fields that are not oriented parallel to a direction of movement of the second actuator. The method also involves providing at least one switch in series with the conduction path that resides in a closed state and achieves an open state to create the high impedance when the first actuator reaches the first stop position and/or when the second actuator reaches the second stop position. 
     Embodiments provide an implantable medical lead that includes a lead body, a conductor surrounded by the lead body, and an electrode coupled to the distal end of the lead body. The lead further includes a first actuator within the lead body that when in the presence of a magnetic field attempts to move from a first start position to a first stop position and reaches the first stop position when a force acting on the first actuator due to the presence of the magnetic field is adequate to produce such movement, the first actuator being responsive to magnetic fields that are not oriented normal to a direction of movement of the first actuator. The lead also includes a second actuator within the lead body that when in the presence of a magnetic field attempts to move from a second start position to a second stop position and reaches the second stop position when a force acting on the second actuator due to the presence of the magnetic field is adequate to produce such movement, the second actuator being responsive to magnetic fields that are not oriented parallel to a direction of movement of the second actuator. Additionally, the lead includes at least one switch within the lead body and in series between the conductor and the electrode on the distal end of the lead body, the at least one switch residing in a closed state and achieving an open state to create a high impedance between the conductor and the electrode on the distal end of the lead body when the first actuator reaches the first stop position and/or when the second actuator reaches the second stop position. 
     Embodiments provide a medical system that includes a pulse generator and an implantable medical lead. The lead includes a lead body, a conductor surrounded by the lead body, the conductor being electrically coupled to the pulse generator, and an electrode coupled to the distal end of the lead body. The lead further includes a first actuator within the lead body that when in the presence of a magnetic field attempts to move from a first start position to a first stop position and reaches the first stop position when a force acting on the first actuator due to the presence of the magnetic field is adequate to produce such movement, the first actuator being responsive to magnetic fields that are not oriented normal to a direction of movement of the first actuator. The lead also includes a second actuator within the lead body that when in the presence of a magnetic field attempts to move from a second start position to a second stop position and reaches the second stop position when a force acting on the second actuator due to the presence of the magnetic field is adequate to produce such movement, the second actuator being responsive to magnetic fields that are not oriented parallel to a direction of movement of the second actuator. Additionally, the lead includes at least one switch within the lead body and in series between the conductor and the electrode on the distal end of the lead body, the at least one switch residing in a closed state and achieving an open state to create a high impedance between the conductor and the electrode on the distal end of the lead body when the first actuator reaches the first stop position and/or when the second actuator reaches the second stop position. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an operating environment for various embodiments of implantable medical systems including implantable leads having magnetic orientation-independent magnetically actuated switches. 
         FIG. 2  shows an example of the placement of magnetic orientation-independent magnetically actuated switches within an implantable medical lead. 
         FIG. 3  shows an example of a magnetic orientation-independent magnetically actuated switch within an implantable medical lead while in a closed state. 
         FIG. 4  shows an example of a magnetic orientation-independent magnetically actuated switch within an implantable medical lead with the switch in an open state for purposes of illustrating details of an actuator. 
         FIG. 5  shows a cross-sectional view of an example of the actuator of the magnetic orientation-independent magnetically actuated switch when a significant magnetic field is not present. 
         FIG. 6  shows a cross-sectional view of an example of the actuator of the magnetic orientation-independent magnetically actuated switch when a significant magnetic field is oriented in a direction normal to a direction of movement of an actuator of the switch. 
         FIG. 7  shows a cross-sectional view of an example of the actuator of the magnetic orientation-independent magnetically actuated switch when a significant magnetic field is oriented in a direction diagonal to a direction of movement of an actuator of the switch. 
         FIG. 8  shows a cross-sectional view of an example of the actuator of the magnetic orientation-independent magnetically actuated switch when a significant magnetic field is oriented parallel to a direction of movement of the switch. 
         FIG. 9  shows a first example of a manufacturing process for the actuator of the magnetic orientation-independent magnetically actuated switch. 
         FIG. 10  shows a second example of a manufacturing process for the actuator of the magnetic orientation-independent magnetically actuated switch. 
         FIG. 11  shows a third example of a manufacturing process for the actuator of the magnetic orientation-independent magnetically actuated switch. 
         FIG. 12  shows a fourth example of a manufacturing process for the actuator of the magnetic orientation-independent magnetically actuated switch. 
         FIG. 13  shows another example of a magnetic orientation-independent magnetically actuated switch configuration within an implantable medical lead. 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments provide implantable medical systems that have implantable leads containing a magnetic orientation-independent magnetically actuated switch within the conduction path to an electrode of the lead. In these various embodiments, the switch is magnetic orientation-independent by operating independently of orientation of the magnetic field to a direction of movement of one or more actuators of the switch, although the switch may be more sensitive to certain orientations of the magnetic field than others. According to these embodiments, when the implantable lead is brought into the presence of a significant magnetic field, such as within an MRI machine, the magnetic orientation-independent magnetically actuated switch is forced into an open state to thereby electrically disconnect the electrode from the remainder of the conduction path and create a high impedance for RF currents. Heating of tissue at the electrode is reduced to a safe level as a result of the conduction path being disconnected from the electrode by the switch. 
       FIG. 1  shows a typical operating environment for embodiments of the medical lead having the magnetic orientation-independent magnetically actuated switch. An implantable medical system  100  is implanted within a patient  112  and includes an implantable pulse generator  102  and an implantable medical lead  104  that is coupled to the implantable pulse generator  102 . The lead  104  includes a distal end  106  where one or more electrodes  108 ,  110  are present. Electrical stimulation signals from the pulse generator  102  are carried by the lead  104  to the electrodes  108 ,  110  where the stimulation is delivered to the tissue at the stimulation site within the patient  112 . 
     The distal end  106  of this example is shown in more detail in  FIG. 2 . Here, it can be seen that the lead  104  has an insulative lead body  202  that contains electrical conductors  204  and  206 . The electrical conductors  204 ,  206  may be cables or coils. The electrical conductor  204  is electrically connected on a first end to a proximal contact or other electrical connection established with the pulse generator  102 . The conductor  204  is electrically connected on the end shown in  FIG. 2  to a magnetic orientation-independent magnetically actuated switch  208  which is in turn electrically connected to the electrode  110 . The electrical conductor  206  is connected to a magnetic orientation-independent magnetically actuated switch  210  which is in turn connected to the electrode  108 . These switches  208 ,  210  are also contained within the lead body  202 . It will be appreciated that either of the electrodes  108 ,  110  may be a most distal electrode or may be located proximal of other distal electrodes that may or may not have switches in the corresponding conduction path. 
     The positioning of the distal end  106  of the lead  104  may vary from one patient to another. While one patient may have the distal end  106  positioned as shown in  FIG. 1 , another patient may have the distal end positioned at a different angle relative to a reference axis of the patient  112 . So even though the direction of the magnetic field within an MRI machine may be a known constant, the orientation of the magnetic field of the MRI machine to the distal end  106  of the lead varying from one patient to the next results in the orientation of the switches  208 ,  210  relative to the magnetic field of the MRI machine also varying from one patient to the next. Therefore, the switches  208 ,  210  are constructed so as to be magnetic orientation-independent. Examples of such a magnetic orientation-independent construction are discussed below. 
       FIGS. 3 and 4  show one example of the magnetic orientation-independent magnetically actuated switch  208  in more detail. In this example, the switch  208  is shaped as a cylinder to facilitate inclusion within the lead body  104 . On one end, a ferromagnetic body  302  is positioned beside another ferromagnetic body  304  with a gap  306  present between to allow the ferromagnetic body  302  to move toward the ferromagnetic body  304 . The switch  208  further includes series of non-ferromagnetic bodies  308  separated by ferromagnetic bodies  310 . On the end opposite the body  302  is an actuator end  318 . The internal details of the switch  208  and the operation of the actuator end  318  are discussed in more detail below with reference to  FIGS. 5-8 . 
     This example of the switch  208  also includes a conductor  312  that includes an orthogonal spring loaded portion  314  that spans the actuator end  318 . As discussed below, one or more actuators extend from the actuator end  318  during operation of the switch  208  in the presence of a significant magnetic field. However, these one or more actuators should not extend from the actuator end  318  when not in the presence of the magnetic field. The spring loaded portion  314  applies a bias to the actuator end  318  to return the actuators to a start position once removed from the magnetic field and to maintain those actuators in the start position to allow the connectivity to the electrode  110  to be maintained. The bias of the spring loaded portion  314  is chosen to be great enough to overcome any frictional resistance plus resistance from any residual magnetic forces to returning the actuators to the start position while being low enough to be overcome by the force the expected magnetic field of the MRI machine or other concern produces in the actuators. 
     The conductor  312  and spring loaded portion  314  may also serve as a conductive portion of the switch  208 . The conductor  204  of the lead  104  may be electrically connected to the conductor  312 . A conductor  205  that extends to the electrode  110  is also connected to an electrical contact  316  mounted on the actuator end  318  of the switch  208 . The electrical contact  316  may be electrically isolated from any conductive surfaces of the switch other than the spring loaded portion  314  for instance by being mounted on a non-conductive surface. When the spring loaded portion  314  is holding the actuators in the start position as in  FIG. 3 , the spring loaded portion  314  contacts the electrical contact  316  so that a conductive path is completed from the conductor  204  to the conductor  205  so that stimulation signals may proceed to the electrode  110 . However, in the presence of a significant magnetic field that produced forces in the actuators of the switch  208 ′ to overcome the bias of the spring loaded portion  314 ′ as shown in  FIG. 4 , the spring loaded portion  314  is disconnected from the electrical contact  316  at the actuator end  318 ′ to thereby disconnect the conductor  205  and electrode  110  from the remainder of the conductor  204 . This disconnection substantially reduces the heating that occurs at the electrode  110  due to ambient RF electromagnetic energy. 
       FIG. 5  shows a longitudinal cross-sectional view of the switch  208  which shows actuators  317 ,  319  in the starting position where no significant magnetic field is present and where the ends of the actuators  317 ,  319  do not extend beyond the actuator end  318  as shown in  FIG. 3 . The actuator  317  is a pin that is mechanically coupled to the ferromagnetic body  302 . The actuator  319  is a cylinder constructed of a series of ferromagnetic bodies separated by non-ferromagnetic bodies, where the ferromagnetic bodies of the actuator  319  are offset from the outer ferromagnetic bodies included in the outer cylinder surrounding the actuator  319 . For instance, ferromagnetic body  313  is offset from ferromagnetic body  310  while ferromagnetic body  315  is offset from ferromagnetic body  310  and ferromagnetic body  311 , albeit with ferromagnetic body  315  being closer to ferromagnetic body  311  than to ferromagnetic body  310 . Ferromagnetic bodies  310  and  311  are separated by the non-ferromagnetic body  309 . In this example, the actuator  317  is positioned coaxially with the actuator  319  by being located within a bore through the actuator  319 . 
       FIG. 6  shows a longitudinal cross-sectional view of the switch  208 ′ where a significant magnetic field  602  is present in a lateral orientation  604  to the switch  208 ′, the lateral orientation  604  being normal to a direction of movement of the actuator  319 ′ and actuator  317  in this example, where the direction of movement of the actuators  319 ′ and  317  are parallel to one another. It will be appreciated that other embodiments of the switch  208  may be designed where the direction of movement of the actuators  317  and  319  are not parallel to one another. The actuator  317  is in the starting position and does not extend beyond the actuator end  318  as the lateral orientation  604  creates lateral poles in the ferromagnetic bodies  302  and  304  such that they do not attract to one and another. The actuator  319 ′ is in a stop position where the actuator  319 ′ extends beyond the actuator end  318  to thereby open the switch  208 ′ to disconnect the electrode  110  from the conductor  204  and create a high impedance for RF currents. The actuator  319 ′ is extended because the lateral orientation  604  of the magnetic field  602  produces magnetic poles in the ferromagnetic bodies  310 ,  313  and  311 ,  315  which pulls the bodies  310  and  313  into alignment of their longitudinal positions and also pulls the bodies  311  and  315  into alignment of their longitudinal positions. This alignment occurs as a result of the body  315  being closer to body  311  than to the body  310  such that the body  315  is pulled into alignment of longitudinal position with the body  311 . This alignment creates longitudinal motion of the actuator  319 ′. In this example, the force acting on the actuator  319 ′ is at a maximum due to the lateral orientation  604  while the force acting on the other actuator  317  is at a minimum. In other embodiments of the switch, when the force on the actuator  319 ′ is at a maximum, the force on the other actuator  317  may be greater than a minimum but less than a maximum of force that the actuator  317  ever receives. 
       FIG. 7  shows a longitudinal cross-sectional view of the switch  208 ″ where a significant magnetic field  602  is present in a diagonal orientation  606  to the switch  208 ″, the diagonal orientation  606  being diagonal to a direction of movement to the actuator  317 ′ as well as being diagonal to a direction of movement to the actuator  319 ′ in this example. The actuator  317 ′ and the actuator  319 ′ are moved to the stop position where both are extended to open the switch  208 ″ and to thereby disconnect the electrode  110  from the conductor  204  and create a high impedance for RF currents. The actuator  319 ′ is extended because the diagonal orientation  606  of the magnetic field  602  still produces magnetic poles in the ferromagnetic bodies  310 ,  313  and  311 ,  315  which are adequate to pull the bodies  310  and  313  into alignment of their longitudinal positions and also pulls the bodies  311  and  315  into alignment of their longitudinal positions. This alignment creates longitudinal motion of the actuator  319 ′. Furthermore, the actuator  317 ′ is extended because the diagonal orientation  606  of the magnetic field  602  to the longitudinal direction of movement of the actuator  317 ′ still produces magnetic poles in the ferromagnetic bodies  302 ,  304  which is adequate to move the body  302  longitudinally to bring the bodies  302 ,  304  closer together by closing the gap  306 ′. This longitudinal movement creates longitudinal motion of the actuator  317 ′. 
       FIG. 8  shows a longitudinal cross-sectional view of the switch  208  where a significant magnetic field  602  is present in a longitudinal orientation  608  to the switch  208 ″′, the longitudinal orientation  608  being parallel to the direction of movement of the actuator  319  and of the actuator  317 ′ in this particular example. As discussed above, in other embodiments of the switch  208  the direction of movement of the actuators may not be parallel to one another. The actuator  319  is in the start position and does not extend beyond the actuator end  318  as the longitudinal orientation  608  creates longitudinal poles in the ferromagnetic bodies  310 ,  313  and  311 ,  315  such that they do not attract to one and another. The actuator  317 ′ has moved to the stop position where the actuator  317 ′ is extended to open the switch  208 ′″ to thereby disconnect the electrode  110  from the conductor  204  and create a high impedance for RF currents. The actuator  317 ′ is extended because the longitudinal orientation  606  of the magnetic field  602  produces longitudinal magnetic poles in the ferromagnetic bodies  302  and  304  which is adequate to move the body  302  longitudinally to bring the bodies  302 ,  304  closer together by closing the gap  306 ′. This longitudinal movement creates longitudinal motion of the actuator  317 ′. In this particular example, the force acting on the actuator  317 ′ is at a maximum due to the longitudinal orientation  608  while the force acting on the other actuator  319  is at a minimum. In other embodiments of the switch, when the force on the actuator  317 ′ is at a maximum, the force on the other actuator  319  may be greater than a minimum but less than a maximum of force that the actuator  319  ever receives. 
     Thus, as can be seen in  FIGS. 6-8 , regardless of the orientation of the magnetic field  602  to the switch  208 , the magnetic field  602  causes one or both actuators  317 ,  319  to extend. Therefore, the electrode  110  is disconnected from the conduction path  204  regardless of the orientation of the magnetic field  602  to the switch  208 . 
       FIG. 9  shows a first example of a manufacturing process  900  for the switch  208 . Initially, alternating sheets  902  of ferromagnetic and non-ferromagnetic layers are stacked. Examples of the ferromagnetic material include the Permenorm® alloys by the Vacuumschmelze GMBH Corporation and the like. Examples of the non-ferromagnetic material include brass, annealed 300 series stainless steel, titanium, polyurethane, PEEK, polysulfone, and the like. These sheets are then affixed in a bonding operation  903  to produce a bonded stack  904 . Examples of the bonding agent include epoxies, cyanoacrylates, and the like. The individual outer cylinder  906  and actuator cylinder  908  are created through a cutting operation  905 , such as by watercutting or wire erosion of the bonded stack  904 . The outer cylinder  906  and actuator cylinder  908  are then turned down to more precise outer diameters and to produce longitudinal bores where the longitudinal bore of the outer cylinder  910  is sized to receive the actuator cylinder  912  at a turning operation  907 . 
     At a final assembly operation  909  the ferromagnetic body  304  is attached to the outer cylinder  910  and the actuator cylinder  912  is positioned within the bore of the outer cylinder  910 . Also at the final assembly operation  909 , the actuator pin  317  that is coupled to the ferromagnetic body  302  is inserted into the bore through the ferromagnetic body  304  and the bore of the actuator cylinder  912  to complete the magnetic orientation-independent magnetically operated actuators  317 ,  319  of the switch  208 . The conductor  312  and spring loaded portion  314  may then be attached to complete the switch  208 . 
       FIG. 10  shows a second example of a manufacturing process  1000  for the switch  208 . Initially, alternating sheets  1002  of ferromagnetic and non-ferromagnetic layers are watercut or wire eroded into individual components  1004  having appropriate diameters at a cutting operation  1003 . These individual components  1004  are then affixed in a bonding operation  1005  to produce the individual outer cylinder  1006  and actuator cylinder  1008 . The outer cylinder  1006  and actuator cylinder  1008  are then turned down to more precise outer diameters and to produce longitudinal bores where the longitudinal bore of the outer cylinder  1010  is sized to receive the actuator cylinder  1012  at a turning operation  1007 . 
     At a final assembly operation  1009  the ferromagnetic body  304  is attached to the outer cylinder  1010  and the actuator cylinder  1012  is positioned within the bore of the outer cylinder  1010 . Also at the final assembly operation  1009 , the actuator pin  317  that is coupled to the ferromagnetic body  302  is inserted into the bore through the ferromagnetic body  304  and the bore of the actuator cylinder  1012  to complete the magnetic orientation-independent magnetically operated actuators  317 ,  319  of the switch  208 . The conductor  312  and spring loaded portion  314  may then be attached to complete the switch  208 . 
       FIG. 11  shows a third example of a manufacturing process  1100  for the switch  208 . Initially, bars  1102  of ferromagnetic and non-ferromagnetic layers are precision turned into individual components  1104  having appropriate diameters at a turning operation  1103 . These individual components  1104  are then affixed in a bonding operation  1105  to produce the individual outer cylinder  1106  and actuator cylinder  1108 . The outer cylinder  1106  and actuator cylinder  1108  are then drilled and reamed to produce longitudinal bores where the longitudinal bore of the outer cylinder  1010  is sized to receive the actuator cylinder  1012  at a drilling and reaming operation  1107 . 
     At a final assembly operation  1109  the ferromagnetic body  304  is attached to the outer cylinder  1110  and the actuator cylinder  1112  is positioned within the bore of the outer cylinder  1110 . Also at the final assembly operation  1109 , the actuator pin  317  that is coupled to the ferromagnetic body  302  is inserted into the bore through the ferromagnetic body  304  and the bore of the actuator cylinder  1112  to complete the magnetic orientation-independent magnetically operated actuators  317 ,  319  of the switch  208 . The conductor  312  and spring loaded portion  314  may then be attached to complete the switch  208 . 
       FIG. 12  shows a fourth example of a manufacturing process  1200  for the switch  208 . Initially, alternating sheets  1202  of ferromagnetic and non-ferromagnetic layers are watercut or wire eroded into individual components  1204  having appropriate outer diameters and bores with appropriate inside diameters at a cutting operation  1203 . These individual components  1204  are then affixed in a bonding operation  1205  to produce the individual outer cylinder  1210  and actuator cylinder  1212 . This is done by placing the individual components on stacking pins  1206 ,  1208 , which are constructed of a material such as polyoxymethylene or polytetrafluoroethylene, to ensure the resulting bores of the outer cylinder  1210  and actuator cylinder  1212  are consistent where the longitudinal bore of the outer cylinder  1210  is sized to receive the actuator cylinder  1212 . The material of the pins  1206 ,  1208  may be chosen to prevent bonding of the small parts to the pins. The outer cylinder  1210  and actuator cylinder  1212  are then ground down to provide outer cylinder  1214  and actuator cylinder  1216  with more precise outside diameters at a grinding operation  1207 . 
     At a final assembly operation  1209  the ferromagnetic body  304  is attached to the outer cylinder  1214  and the actuator cylinder  1216  is positioned within the bore of the outer cylinder  1214 . Also at the final assembly operation  1209 , the actuator pin  317  that is coupled to the ferromagnetic body  302  is inserted into the bore through the ferromagnetic body  304  and the bore of the actuator cylinder  1216  to complete the magnetic orientation-independent magnetically operated actuators  317 ,  319  of the switch  208 . The conductor  312  and spring loaded portion  314  may then be attached to complete the switch  208 . 
     The prior embodiments of the switch  208  have illustrated the actuators  317 ,  319  as being packaged together where both actuators  317 ,  319  operate upon the same switch established by the spring loaded portion  314  and the electrical contact  316  of  FIG. 3 . However, other configurations are also feasible, such as a multi-switch configuration shown in  FIG. 13 . Here, a first switch  1302  is a cylinder of alternating ferromagnetic layers  1304  and non-ferromagnetic layers  1306  that define a bore. Within the bore is positioned an actuator  1312  that is also a cylinder of alternating ferromagnetic layers and non-ferromagnetic layers. The large cylinders of the prior embodiment that are responsive to longitudinally oriented magnetic field are not present for the first switch  1302 . 
     In the presence of a magnetic field that is not longitudinal to the switch  1302 , a force causes longitudinal movement of the actuator  1312  to cause the actuator  1312  to extend. A conductor  1308  that has a spring loaded portion  1310  is affixed to the outer cylinder where the spring loaded portion  1310  contacts an electrical contact  1314  when in the start position. A conductor  207  is electrically coupled to the electrical contact  1314  and the conductor  207  extends further distally. The spring loaded portion  1310  biases the actuator  1312  into the non-extended start position but the force from the non-longitudinal magnetic field moves the actuator  1312  to a stop position which causes the spring loaded portion  1310  to separate from the electrical contact  1314  to disconnect the electrode  110 . 
     A second switch  1303  is a cylinder with a ferromagnetic body  1320  with a ferromagnetic body  1316  separated from the ferromagnetic body  1320  by a gap  1318 . The second switch  1303  lacks all of the individual cylinders that are responsive to the laterally oriented magnetic field such that the overall length of the second switch  1303  may be reduced relative to the prior embodiments. An actuator  1326  is connected to the ferromagnetic body  1316 . In the presence of a magnetic field that is not lateral to the switch  1303 , a force causes longitudinal movement of the actuator  1326  to cause the actuator  1326  to extend. A conductor  1322  that has a spring loaded portion  1324  is affixed to the outer cylinder where the spring loaded portion  1324  contacts an electrical contact  1328  when in the start position. The conductor  205  is electrically coupled to the electrical contact  1314  and the conductor  205  extends further distally to the electrode  110 . The spring loaded portion  1324  biases the actuator  1326  into the non-extended start position but the force from the non-lateral magnetic field moves the actuator  1326  to a stop position which causes the spring loaded portion  1324  to separate from the electrical contact  1314  to disconnect the electrode  110 . 
     Thus, the switch  1302  and/or the switch  1303  may serve to disconnect the electrode  110  from the conduction path  204 . This configuration may be appropriate where a smaller diameter lead body is desired, where the series combination of the switch  1302  and switch  1303  may utilize a smaller diameter than a combined switch  208 . However, the combined switch  208  may benefit from a shorter length and may be suitable for situations where a larger diameter lead body may be acceptable, such as for peripheral nerve stimulation applications. 
     While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.