Abstract:
A method and assembly for selectively actuating features of implanted medical devices with a magneto-static field. The method includes selectively exposing the implanted device to a static magnetic field source, selectively shielding the magnetic field source, and distancing the shielded magnetic field source from the medical device. One version of the assembly includes a permanent magnet and a displaceable shield assembly that shields the magnetic field generated by the magnet in one configuration and is displaceable to a second configuration wherein the magnetic field is at least partially exposed. In another version, the assembly is an electromagnet that can be selectively activated and deactivated. The electromagnet generates minimal magnet field when it is off.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of implantable cardiac devices and, in particular, to a small permanent magnet provided with a displaceable, conformal magneto-static shield to inhibit unintentional exposure of the magnetic field that is suitable for carrying on the person.  
         BACKGROUND OF THE INVENTION  
         [0002]    Cardiac devices are known assemblies implanted in patients to monitor the heart and provide therapeutic stimuli to treat a variety of arrhythmias. Many of these devices also have features that may be selectively activated by exposure to a magneto-static field. Typically, these devices include well-known reed switches that can be closed by exposure to a magnetic field of a given threshold value. A typical use of a magnetically activated reed switch in an implanted cardiac device is to enable a telemetry circuit within the device so that data indicative of the function of the patient&#39;s heart, as it is sensed by the implanted cardiac stimulation device, as well as data indicative of the function of the implanted device can be telemetered to an external programmer. This data can be reviewed by a treating medical professional. The advantage of using a magnetically activated switch in this circumstance is that it permits the selective activation of a particular function of the device that is implanted within the patient in a simple, non-invasive manner.  
           [0003]    Magneto-static fields are chosen to activate these selectable features for several reasons. A patient is not likely to encounter strong magneto-static fields (&gt;0.5 Gauss) inadvertently. Magneto-static fields pass relatively readily through the body and thus to the implanted device. Magneto-static fields of reasonable strength have no known injurious effects on the human body. A small, high strength permanent magnet can be readily carried on the person and used by the patient to activate the selectable features of the cardiac device when desired.  
           [0004]    However, several problems occur with carrying a permanent magnet on the person. If the magnet is inadvertently brought too close to the device, the selectable features of the device can be unintentionally activated. Also, strong magneto-static fields can irreparably scramble data stored on magnetic recording media. In fact, exposure to high gauss fields is a known manner of wiping magnetic recording media, such as computer diskettes, audio tapes and the like. Credit cards are also typically provided with magnetic strips with account holder information encoded therein and exposure to a permanent magnet can erase this information from the card.  
           [0005]    An additional liability to permanent magnets carried on the person is that they are attracted to and can adhere to ferrous material. For example, a magnet carried in the person&#39;s pocket can be attracted and stick to a steel structure. It will be appreciated that a magnet, unexpectedly adhering to a steel railing on a stairway, for example, could induce a person to stumble and fall, possibly leading to injury. A permanent magnet would also be attracted to ferrous items such as keys, pocketknives, pens, and fingernail files that are often carried in a purse or pocket. A magnet could further attract and knock over steel objects such as cans, medical instruments, etc. as a person carrying a magnet walks by.  
           [0006]    In addition, exposing certain materials, the most common of which are ferrous materials, to a magnetic field causes the materials so exposed to become magnetized themselves. Thus a steel key and key ring, for example, placed in proximity to a permanent magnet, would become partially magnetized themselves and would have similar characteristics to those of the original magnet.  
           [0007]    A further difficulty that occurs with these magnets in connection with implantable cardiac devices is that the unshielded magnets are strong enough to result in inadvertent activation of the reed switches in an implanted device while the medical professional carrying the magnet is in the presence of the patient. This can result in undesired operation of the device resulting in undesired drain of limited battery resources. Moreover, the magnets are also strong enough that the magnets can affect the operation of external programmers that are used to evaluate the operation of the cardiac stimulation device implanted within the patient.  
           [0008]    Unfortunately, while these magnets are necessary to permit remote activation of functions within the implanted cardiac stimulation device, there is no way to deactivate the magnets. Hence, the problems associated with carrying around magnets of sufficient strength to activate functions within an implanted cardiac stimulation device have not been readily addressed in the prior art.  
           [0009]    From the foregoing it will be appreciated that there is an ongoing need for a small, permanent magnet that can be readily carried on a person to enable a person implanted with a cardiac stimulation device to employ the magnet to selectively activate certain features and functions of the implanted device. Moreover, there is still an ongoing need to develop a magnet device suitable for activation of magnetic switches in implanted cardiac devices that can also be shielded when the magnet device is not being used to avoid the difficulties associated with medical professionals carrying around powerful magnets.  
         SUMMARY OF THE INVENTION  
         [0010]    The aforementioned needs are satisfied by the magnet device of the present invention which in one aspect is comprised of a magnet and a configurable container. The magnet can be exposed wherein it produces a magnetic field of a first strength sufficient to activate a magnetic switch within an implanted cardiac stimulation device to thereby induce the implanted cardiac stimulation device to perform a selected function. The magnet can also be shielded within the container such that the magnet produces a field of second strength that is sufficiently less than the first strength such that the magnet does not activate the magnetic switch within the implanted cardiac stimulation device.  
           [0011]    Preferably, the container defines a high magnetic permeability path through which a substantial portion of the flux flows to thereby reduce the strength of the magnetic field outside the container. Preferably, the container is made of a material that has a high level of magnetic permeability. Magnetic permeability in the context of magnetic fields is analogous to electrical conductivity in the context of electrical current. Given alternative paths with high and low conductivity, electrical current will predominantly flow through the path with high conductivity (low resistance). In a similar manner, magnetic fields will predominantly pass through regions of high permeability in preference to regions of low permeability. Air and most common materials have relatively low permeabilities on the order of 1. However, materials such as iron and MuMetal® have permeabilities on the order of tens of thousands. Thus, in one embodiment, if the container has sufficient quantities of high permeability material that is placed about the permanent magnet, the magnetic field will predominately pass within the highly permeable path and thus reduce the magnetic field strength induced by the permanent magnet outside of the container. Advantageously, the high permeability material does not damage magnetic field strength, it is simply providing a more permeable path for the magnetic flux in the container material.  
           [0012]    In one embodiment, the magnet device produces a magnetic field of at least 10 Gauss measured 7.6 cm from the magnet. When the magnet is shielded within the container, in this embodiment, the magnet produces a magnetic field of less than 2 Gauss measured 7.6 cm from the magnet.  
           [0013]    The container can have a variety of different configurations. The magnet can be positioned within a container such that it can be removed from the container. The magnet can also be fixedly mounted within the container and a lid of the container can be removed or the magnet can be otherwise exposed to produce the larger magnetic field.  
           [0014]    In another aspect, the magnetic device can include an electromagnet assembly for selectively activating features of an implanted cardiac stimulation device. In this aspect, the magnet device is electrically actuated to produce a stronger magnetic field having a magnetic field strength sufficient to activate a magnetic switch in an implanted device. When the device is not actuated, the magnetic field strength is low enough not to result in activation of the magnetic switches and also reduces the inconvenience of having a strong magnet in the presence of other metal objects.  
           [0015]    The present invention therefore provides a mechanism that reduces the negative effects of magnetic fields emanating from magnets that are used to activate selected functions of implanted medical devices, such as implanted cardiac stimulation devices. These and other objects and advantages will be more apparent from the following discussion taken in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    Further features and advantages of the present invention may be more readily understood by reference to the following description taken in conjunction with the accompanying drawings, in which:  
         [0017]    FIGS.  1 A- 1 D illustrate in four views one embodiment of a shielded magnet assembly in a shielded configuration;  
         [0018]    [0018]FIG. 2 illustrates the shielded magnet assembly of FIG. 1 in an exposed configuration;  
         [0019]    [0019]FIG. 3 illustrates an alternative embodiment of a shielded magnet assembly in an exposed configuration;  
         [0020]    [0020]FIG. 4 illustrates another alternative embodiment of a shielded magnet assembly in an exposed configuration;  
         [0021]    [0021]FIG. 5 illustrates yet another alternative embodiment of a shielded magnet assembly in an exposed configuration;  
         [0022]    [0022]FIG. 6A illustrates an embodiment of a shielded magnet assembly in a shielded configuration;  
         [0023]    [0023]FIG. 6B illustrates the shielded magnet assembly of FIG. 6A in an exposed configuration;  
         [0024]    [0024]FIG. 7 illustrates a further embodiment of a shielded magnet assembly;  
         [0025]    [0025]FIG. 8A illustrates one more embodiment of a shielded magnet assembly in a shielded configuration;  
         [0026]    [0026]FIG. 8B illustrates the shielded magnet assembly of FIG. 8A in an exposed configuration; and  
         [0027]    [0027]FIG. 9 illustrates a method of employing a shielded magnet assembly to selectively activate features of an implanted cardiac device. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0028]    The following description is of the best mode presently contemplated for practicing the invention. This description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the issued claims. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout.  
         [0029]    FIGS.  1 A- 1 D and  2  illustrate a shielded magnet assembly  100  in a shielded configuration  112  (FIGS.  1 A- 1 D) and an exposed configuration  114  (FIG. 2). The shielded magnet assembly  100  comprises a permanent magnet  102  that is shielded in the shielded configuration  112  such that the magneto-static field developed by the magnet  102  is substantially directed within the shielded magnet assembly  100  such that a reduced magnetic field appears beyond the envelope of the assembly  100 . A reduced magnetic field should be construed throughout this description that follows as meaning less than approximately 2 Gauss measured 7.6 cm in any direction from any exposed surface of the magnet  102 .  
         [0030]    More specifically, the permanent magnet  102  in the exposed configuration produces a magnetic field having a strength selected to activate a magnetic switch  117  (FIG. 10), such as a reed switch in an implanted cardiac stimulation device to thereby induce the implanted cardiac stimulation device to perform a pre-selected function. The magnetic field strength needed to activate the magnetic switch in an implanted device must be relatively strong as the magnetic switches are typically configured to only be activated by strong magnetic fields to reduce the risk of inadvertent triggering of the switches resulting from the patient being exposed to stray magnetic fields.  
         [0031]    The magnet assembly  100  in the shielded configuration results in the magnet  102  being shielded sufficiently so that the magnetic field produced by the permanent magnet  102  outside of the envelope of the assembly  100  is low enough to reduce the inconvenience of carrying a strong magnet on the person of the treating medical professional. As will be described in greater detail below, the assembly  100  is preferably configured such that the permanent magnet does not produce a field outside of the envelope of the assembly  100  when the assembly is in the shielded configuration  112  that would be strong enough to activate the magnetic switches in the implanted device when the assembly  100  is positioned adjacent the skin of the patient proximate the implanted device.  
         [0032]    As is illustrated in FIG. 2, the magnet  102  may also be exposed in the exposed configuration  114  such that the magnetic field developed by the magnet  102  is substantially unshielded on at least one pole to facilitate activating features and functions of an implanted cardiac device  116 . The construction, use, and selectable features of the cardiac device  116  are well known to those of ordinary skill in the art.  
         [0033]    The magnet  102 , of this and all following embodiments, is made of a permanently magnetic material, such as Samarium-Cobalt, Neodymium-Iron-Boron, or other elements or alloys that are well known in the art. The size of the magnet  102  is chosen with respect to the intrinsic properties of the particular material chosen to develop a magnetic field of at least 10 Gauss as measured 7.6 cm from the surface of the magnet  102  of this and all following embodiments in the exposed configuration  114 . The edges and corners of the magnet  102  are rounded or beveled in a known manner to avoid sharp and pointed edges which might otherwise cause injury to a user of the assembly  100 .  
         [0034]    The shielded magnet assembly  100  of this embodiment also comprises a magnet holder  104 . The magnet holder  104  is a made of a material with a relatively high magnetic permeability, such as iron or an alloy of 77% Nickel, 14% Iron, 5% Copper, and 4% Molybdenum sold under the trademark MuMetal®. The magnet holder  104  defines a cavity  108 . The cavity  108  in this embodiment is a generally rectangular opening extending into one end of the magnet holder  104 . The size of the cavity  108  is chosen to closely conform to the size and shape of one end of the magnet  102 . The magnet  102  is inserted into the cavity  108  so as to achieve a friction fit in a known manner. In an alternative embodiment, the magnet  102  is inserted into the cavity  108  and secured with an adhesive. The magnet holder  104 , when attached to the magnet  102 , provides a gripping surface for the user to manipulate the assembly  100 .  
         [0035]    The shielded magnet assembly also comprises a shield liner  106  and cover  110 . The shield liner  106  is made of a material with a relatively high magnetic permeability, such as iron or an alloy of 77% Nickel, 14% Iron, 5% Copper, and 4% Molybdenum sold under the trademark MuMetal®. The cover  110  is made of a durable, smooth material such as plastic. The shield liner  106  and cover  110  are made such that the shield liner  106  fits tightly inside the cover  110  in a friction fit so as to fixedly attach the cover  110  to the shield liner  106 .  
         [0036]    The shield liner  106  defines a cavity  109 . The cavity  109  is sized to closely conform to the contour of the magnet  102  so as to form a removable friction fit between the cavity  109  of the shield liner  106  and the magnet  102 . The friction fit between the shield liner  106  and the magnet  102  retains the shield liner  106  and cover  1   10  in contact with the magnet  102  in the shielded configuration  112 . However, the friction fit is such that the shield liner  106  and cover  110  can be readily removed from the magnet  102  to achieve the exposed configuration  114 .  
         [0037]    The shield liner  106  and magnet holder  104  are adapted such that, in the shielded configuration  112 , the shield liner  106  and the magnet holder  104  are in continuous, adjacent contact. Since the magnet  102  is in physical contact with both the shield liner  106  and the magnet holder  104 , the magnetic field developed by the magnet  102  will predominantly pass within the relatively high permeability material of the shield liner  106  and the magnet holder  104 . As previously mentioned, the size, shape, and material of the shield liner  106  and the magnet holder  104  are chosen to limit the magnetic field beyond the envelope of the shield liner  106  and the magnet holder  104  to no more than 2 Gauss as measured 7.6 cm away.  
         [0038]    Hence, the shield liner  106  and magnet holder  104  provide a high magnetic permeability path for the magnetic flux that is produced by the permanent magnet when it is in the shielded configuration  112 . This path results in much of the magnetic flux generated by the magnet  102  being confined within the shield liner  106  thereby decreasing the strength of the magnetic field beyond the assembly  100 .  
         [0039]    The size and exact materials used to construct the magnet  102 , magnet holder  104 , and shield liner  106  are chosen to meet the shielded and exposed magnetic field requirements noted previously. It should be appreciated that the greater the intrinsic magnetic strength of the material used to construct the magnet  102  and the higher the magnetic permeability of the material used to construct the magnet holder  104  and the shield liner  106 , the smaller the shielded magnet assembly  100  can be made. Smaller sizes of the assembly  100  improve convenience for a patient/user. Material choice and shape are chosen with other design constraints including material cost, availability, and ease of construction by one of skill in the art.  
         [0040]    [0040]FIG. 3 illustrates an alternative embodiment of a shielded magnet assembly  200  in an exposed configuration  114 . The shielded magnet assembly  200  of this embodiment comprises the magnet  102  substantially identical to the magnet  102  of the shielded magnet assembly  100  previously described. The shielded magnet assembly  200  also comprises a first  204  and a second  206  enclosure half. The first  204  and second  206  enclosure halves are made of a high permeability material, such as those previously described with respect to the magnet holder  104 . The first and second enclosure halves  204 ,  206  thereby provide the high magnetic permeability path through which the magnetic flux flows when the assembly  200  is in the closed configuration to thereby reduce the strength of the magnetic field outside of the assembly  200 . The first  204  and second  206  enclosure halves of this embodiment are substantially identical oblate members and are adapted to closely mate together.  
         [0041]    The first  204  and second  206  enclosure halves of this embodiment are not attached, although, in alternative embodiments, the first  204  and second  206  enclosure halves are hingedly connected. The first  204  and second  206  enclosure halves each define a cavity  210 . The cavity  210  in each of the first  204  and second  206  enclosure halves is configured to closely conform to the contour of the magnet  102 . The first  204  and second  206  enclosure shells are each attached to the magnet  102 , and thus held in adjacent contact with each other, by a friction fit with the magnet  102 .  
         [0042]    In an alternative embodiment, the first  204  and second  206  enclosure halves are held together in adjacent contact by hook and loop fastener secured and employed in a well known manner to adjacent faces of the first  204  and second  206  enclosure halves wherein the cavity  210  is sized with respect to the magnet  102  such that the magnet  102  is readily removable from both the first  204  and second  206  enclosure halves. In yet another alternative embodiment, the cavities  210  in the first  204  and second  206  enclosure halves are sized such that the magnet  102  fits tightly in a friction fit with one of the first  204  and second  206  enclosure halves and is thus fixedly attached to the one of the first  204  and second  206  enclosure halves. The cavity  210  in the other one of the first  204  and second  206  enclosure halves is sized such the magnet  102  and attached first  204  or second  206  enclosure half is readily removable from the other first  204  or second  206  enclosure half. The first  204  and second  206  enclosure halves, when positioned adjacent each other in the shielded configuration  112 , shield the magnetic field developed by the magnet  102  in a similar manner to that previously described with respect to the shield liner  106  and magnet holder  104 .  
         [0043]    [0043]FIG. 4 illustrates another alternative embodiment of a shielded magnet assembly  300  in the exposed configuration  114 . The shielded magnet assembly  300  of this embodiment comprises the magnet  102 , a base member  304 , and a cover  306 . The base member  304  and cover  306  are made of a high permeability material, such as those previously described with respect to the magnet holder  104 , and are hingedly attached in a well known manner. The hinged connection of the base member  304  and the cover  306  preferably includes a spring pre-load assembly  310  of a type well known in the art to bias the shielded magnet assembly  300  into either the shielded configuration  112  or the exposed configuration  114 .  
         [0044]    The exposed configuration  114  comprises distancing the cover  306  from the base member  304  as illustrated in FIG. 4. The shielded configuration  112  comprises rotating the cover  306  about the hinged connection to the base member  304  such that the cover  306  is adjacent and in continuous contact with the base member  304 . The adjacent positioning of the cover  306  and the base member  304  in the shielded configuration  112  shields the magnetic field developed by the magnet  102  in a similar manner to that previously described with respect to the shield liner  106  and magnet holder  104 .  
         [0045]    [0045]FIG. 5 illustrates yet another alternative embodiment of a shielded magnet assembly  400  in the exposed configuration  114 . The shielded magnet assembly  400  of this embodiment comprises the magnet  102 , a base member  404 , and a lid  406 . The base member  404  and lid  406  are made of a high permeability material, such as those previously described with respect to the magnet holder  104 . The base member  404  is a generally cylindrical member and defines a cavity  410  adjacent a first end  412  of the base member  404 . The cavity  410  is sized and configured to hold the magnet  102  in a friction fit such that the magnet  102  is at least partially exposed above the first end  412  of the base member  404 . The base member  404  is provided with external threads of a known configuration about the circumference of the base member  404  adjacent the first end  412 .  
         [0046]    The lid  406  is a generally cylindrical, hollow member open on one end and closed on the opposite end. The open end of the lid  406  is provided with internal threads configured to mate with the threads of the base member  404 .  
         [0047]    The exposed configuration  114  of the shielded magnet assembly  400  comprises distancing the lid  406  from the base member  404  as illustrated in FIG. 5. In the exposed configuration  114 , the base member  404  serves as a gripping surface for a user of the shielded magnet assembly  400 . The shielded configuration  112  is achieved by threading the lid  406  onto the base member  404  in a known manner so as to bring the lid  406  and the base member  404  into adjacent, continuous contact along the respective threads provided on each. Thus, in similar manner to that previously described with the alternative embodiments of the shielded magnet assembly  100 ,  200 , and  300 , the magnetic field developed by the magnet  102  is substantially directed through the lid  406  and the base member  404  such that minimal magnetic field extends beyond the envelope of the shielded magnet assembly  400 .  
         [0048]    [0048]FIGS. 6A and 6B illustrate one more embodiment of a shielded magnet assembly  500  in the shielded configuration  112  (FIG. 6A) and the exposed configuration  114  (FIG. 6B). The shielded magnet assembly  500  comprises the magnet  102  and an enclosure  504 . The enclosure  504  is a hollow, cylindrical elongate member approximately 1.25-1.5 cm in outer diameter and approximately 13-15 cm long. The enclosure  504  is open on a first end  506  and closed on a second end  510  opposite the first end  506 . The enclosure  504  also defines a slot  520  extending along the major axis of the enclosure  504  from a point approximately midway between the first  506  and second  510  ends to the first end  506 . The enclosure  504  is made of a high permeability material, such as those previously described with respect to the magnet holder  104 . The magnet  102  of this embodiment is generally cylindrical and sized to conform closely to the interior of the enclosure  504  and to be approximately one-half the length of the enclosure  504 , which, in this embodiment, corresponds to a magnet  102  of approximately 6-7 cm long.  
         [0049]    The shielded magnet assembly  500  also comprises a spring  512 . The spring  512  of this embodiment is a coil spring of a type well known in the art. The spring  512  is sized to closely fit within the interior of the enclosure  504 . The spring  512  is positioned inside the enclosure  504  between the magnet  102  and the interior of the second end  510  of the enclosure  504 . The spring  512  is further sized so as to have a free length of approximately 14 cm so as to apply a pre-load force on the magnet  102  when the magnet  102  is positioned so as to not protrude beyond the first end  506  of the enclosure  504  (i.e. in the shielded configuration  112 ) without coil-binding the spring  512 .  
         [0050]    In one embodiment, the magnet  102  is substantially of uniform diameter along its length and of such a diameter as to snuggly fit within the enclosure  504  so as to inhibit the magnet  102  inadvertently exiting the enclosure  504 . In an alternative embodiment, the magnet  102  defines an annular region of greater diameter than the remainder of the magnet  102  thereby defining a flange adjacent a first end  503  of the magnet  102 . The first end  506  of the enclosure  504  is slightly crimped after insertion of the spring  512  and magnet  102  into the interior of the enclosure  504  to thereby inhibit exiting of the magnet  102  and spring  512  from the enclosure  504 .  
         [0051]    The shielded magnet assembly  500  also comprises a clip  514 . The clip  514  is an elongate member approximately 5 cm long and is made of an elastic, rigid material such as plastic or steel. The clip  514  is fixedly attached at a first end adjacent the first end  506  of the enclosure  504  so as to extend along the major axis of the enclosure  504  towards the second end  510  of the enclosure  504  and is positioned opposite the slot  520 . The clip  514  is adapted such that a second end of the clip  514 , opposite the first end of the clip  514 , bears against the outside of the enclosure  504  in a spring-loaded fashion. The clip  514  facilitates securing the assembly  500  to a shirt pocket in a well understood manner.  
         [0052]    The shielded magnet assembly  500  also comprises a thumbslide  516 . The thumbslide  516  is a generally rectangular member and is made of a rigid, durable material such as plastic or steel. The thumbslide  516  is fixedly attached to the magnet  102  adjacent the first end  503  with a high strength adhesive so as to extend radially outward from the enclosure  504  through the slot  520 . It should be noted that certain known methods of attaching a steel piece, in particular high temperature processes such as welding and brazing, are not appropriate methods for securing the thumbpiece  516  to the magnet  102  due to the possibility of exposing the magnet  102  to temperatures in excess of its Curie temperature and thereby reducing the magnetic field developed by the magnet  102 . The thumbslide  516  facilitates extending and retracting the magnet  102  within the enclosure  504  in a well understood manner.  
         [0053]    The shielded configuration  112  is achieved by manipulating the thumbslide  516  so as to draw the attached magnet  102  within the interior of the enclosure  504 . When the magnet  102  is positioned within the enclosure  504 , the magnetic field developed by the magnet  102  will substantially pass within the material of the enclosure  504  such that a reduced magnetic field appears beyond the envelope of the enclosure  504 . The exposed configuration  114  is achieved by manipulating the thumbslide  516  to extend the magnet  102  beyond the first end  506  of the enclosure  504 . In the exposed configuration  114 , the magnetic field developed by the magnet  102  is exposed on a second end opposite the first end  503 .  
         [0054]    [0054]FIG. 7 illustrates yet even one more embodiment of a shielded magnet assembly  600 . The shielded magnet assembly  600  of this embodiment comprises the magnet  102 , an enclosure body  604 , a knob  606 , and a cap  610 . The magnet  102  of this embodiment is a generally rectangular elongate member and is provided with internal, female threads (obscured from view) extending along the major central axis of the magnet  102 . The enclosure body  604  is a elongate member of cylindrical outer contour and with a rectangular cavity  612  configured so as to allow the magnet  102  to freely move back and forth axially within the cavity  612  and further configured to inhibit rotation of the magnet  102  within the cavity  612 . The enclosure body  604  is made of a high permeability material, such as those previously described with respect to the magnet holder  104 .  
         [0055]    The knob  606  comprises a cylindrical portion (visible in FIG. 7) and an elongate portion (obscured from view in FIG. 7) extending outward from the cylindrical portion wherein the elongate portion of the knob  606  is externally threaded to mate with the internal threading of the magnet  102 . The knob  606  is threaded into the magnet  102  and secured to a first end  614  of the enclosure body  604  in a known manner such that the knob  606  is free to rotate and is inhibited from axial translation with respect to the enclosure body  604 . Thus, rotation of the knob  606  will induce the magnet  102  to extend and retract axially from the cavity  612  in response to actuation of the knob  606 . The threading of the magnet  102  and the knob  606  is preferably of a rapid twist such that movement of the magnet  102  between the shielded  112  and exposed  114  configurations can be achieved by rotating the knob  606  no more than a full turn. FIG. 7 illustrates the magnet  102  in an intermediate position between the shielded  112  and the exposed  114  configurations.  
         [0056]    The cap  610  is a hollow, cylindrical elongate member and is configured to friction fit with the exterior of the enclosure body  604  in a known manner. In one embodiment, the enclosure body  604  is of adequate size to effectively shield the magnet  102  in the shielded configuration  112  by itself. In this embodiment, the cap  610  is made of a less expensive material such as plastic. In an alternative embodiment, the cap  610  is also made of a high permeability material, such as those previously described with respect to the magnet holder  104 . When positioned in friction fit with the enclosure body  604 , the cap  610  acts in concert with the enclosure body  604  to shield the magnetic field developed by the magnet  102  in a similar manner to that previously described with respect to other embodiments of the shielded magnet assembly  100 ,  200 ,  300 ,  400 , and  500 . The cap  610  of both embodiments also obscures the magnet  102  from view and inhibits entrance of debris into the cavity  612 .  
         [0057]    [0057]FIG. 8 illustrates a further embodiment of a shielded magnet assembly  700  in the shielded  112  (FIG. 8A) and exposed  114  (FIG. 8B) configurations. The shielded magnet assembly  700 , of this embodiment, comprises the magnet  102 , an enclosure body  704 , a shield cover  706 , and a shield actuator  710 . The enclosure body  704  and the shield cover  706  are made of a high permeability material, such as those previously described with respect to the magnet holder  104 . The shield actuator  710  may be made of a high permeability material, such as those previously described with respect to the magnet holder  104 , or other rigid material such as steel or plastic.  
         [0058]    The enclosure body  704  is generally rectangular and defines a rectangular cavity  712  extending into one face of the enclosure body  704 . The cavity  712  is sized and configured to securely retain the magnet  102  in a friction fit such that the magnet  102  is positioned at least 1 cm below the face of the enclosure body  704 . The enclosure body  704  of this embodiment further defines a clearance groove  730  extending across the enclosure body  704 , adjacent the cavity  712 , approximately midway between opposite ends of the enclosure body  704 . The clearance groove  730  provides clearance for a user to grasp the shield cover  706  and shield actuator  710 .  
         [0059]    The shield cover  706  and the shield actuator  710  are elongate, rigid members of approximately the same length. The shield cover  706  is hingedly attached at a first end  714  to a first end  720  of the shield actuator  710 . A second end  722  of the shield actuator  710 , opposite the first end  720 , is hingedly attached to the enclosure body  704 , thereby defining a toggle joint  724  structure of a type known in the art. A second end  716  of the shield cover  706  opposite the first end  714  is free to move. The hinged connection of the shield cover  706  to the shield actuator  710  defines a knurled gripping surface  726 .  
         [0060]    In the shielded configuration  112  as illustrated in FIG. 8A, the shield cover  706  and the shield actuator  710  are collinear and extend along the face of the enclosure body  704  with the cavity  712 . The shield cover  706  is positioned and is of such a configuration as to substantially cover the magnet  102  positioned within the cavity  712 . Thus, in the shielded configuration  112 , the magnetic field developed by the magnet  102  is substantially directed within the shield cover  706  and the enclosure body  704 . In one embodiment, the hinged connection of the shield actuator  710  to the enclosure body  704  includes a spring connected between the shield actuator  710  and the enclosure body  704  in a known manner so as to bias the shielded magnet assembly  700  in the shielded configuration  112 .  
         [0061]    Drawing the gripping surface  726  away from the enclosure body  704  will thus induce the shield actuator  710  to pivot about the second end  722  which is hingedly attached to the enclosure body  706 . Drawing the gripping surface  726  away from the enclosure body  704  will further induce the shield cover  706  to pivot with respect to the shield actuator  710  and thus draw the shield cover  706  away from the cavity  712 . Drawing the shield cover  706  away from the cavity  712  will thus expose the magnet  102  so as to achieve the exposed configuration  114 .  
         [0062]    [0062]FIG. 9 illustrates a method of selectively activating features of the implanted cardiac device  116 . A user positions the assembly  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 , or  800  adjacent the chest of the patient provided with the cardiac device  116 . The user then manipulates the assembly  100 ,  200 ,  300 ,  400 ,  500 ,  600 , or  700  to the exposed configuration  114  or activates the assembly  800  via the switch  802 . The user then manipulates the assembly  100 ,  200 ,  300 ,  400 ,  500 ,  600 , or  700  to the shielded configuration  112  or deactivates the assembly  800  via the switch  802  and distances the assembly  100 ,  200 ,  300 ,  400 ,  500 ,  600 ,  700 , or  800  from the patient&#39;s chest.  
         [0063]    As is schematically illustrated in FIG. 9, the implanted cardiac, stimulation device  116  includes at least one magnetic switch  117 . The at least one magnetic switch  117  can be a well known Reed switch that is activated when exposed to a magnetic field having a threshold value. Activation of the Reed switch results in a microprocessor of the implanted cardiac stimulation device  116  initiating a function. One common function is the enabling of a telemetry circuit to permit RF transmission of data from the implanted device to an external programmer. The use of such Reed switches is preferred as it permits selective activation of a device function without requiring continuous consumption of power by the implanted device or an invasive procedure. The magnetic device of the illustrated embodiments is configured to be used with any magnetic switch, including Reed switches, known in the art without departing from the spirit of the present invention.  
         [0064]    It should be appreciated that in the embodiments of the shielded magnet assemblies  100 ,  200 ,  300 ,  400 ,  500 ,  600 , and  700  previously described, the orientation of the magnet  102  with respect to polarity in the exposed configuration  114  is not important to the use of the shielded magnet assemblies  100 ,  200 ,  300 ,  400 ,  500 ,  600 , and  700 . However, it should also be appreciated that in alternative embodiments wherein the polarity of the magnet  102  is important, it is well within the skill of a person of ordinary skill in the art to orient the magnet  102  in a particular fashion without detracting from the scope of the invention. Although the preferred embodiments of the present invention have shown, described and pointed out the fundamental novel features of the invention as applied to those embodiments, it will be understood that various omissions, substitutions and changes in the form of the detail of the device illustrated may be made by those skilled in the art without departing from the spirit of the present invention. Consequently, the scope of the invention should not be limited to the foregoing description but is to be defined by the appended claims.