the implantable cardiac stimulator 10 may be a cardiac pacemaker or a cardioverter / defibrillator ( icd ). in the illustrated exemplary embodiment , the cardiac stimulator 10 is a ventricular cardiac pacemaker and defibrillator . other known cardiac stimulators are dual - chamber cardiac pacemakers for stimulating the right atrium and the right ventricle , or biventricular cardiac pacemakers , which in addition to the right ventricle are also able to stimulate the left ventricle . such stimulators typically have a housing 12 , which is generally made of metal and is therefore electrically conductive , and which may be used as a large - surface electrode pole . a connector housing 14 , also referred to as a header , is typically affixed to the exterior of the housing 12 . such a header typically has contact sockets for accommodating plug contacts . the contact sockets have electrical contacts 16 which are connected via appropriate conductors to an electronics system situated in the housing 12 of the cardiac stimulator 10 . the electrode line 20 likewise represents an implantable medical device within the meaning of at least one embodiment of the invention . electrode poles in the form of a point electrode or tip electrode 22 and an annular electrode 24 present in the vicinity thereof are situated in a manner known per se at the distal end of the electrode line 20 . the electrode poles 22 and 24 are designed in such a way that , depending on the function of a cardiac stimulator to which the electrode line 20 is connected , they are provided to sense electrical potentials of the cardiac tissue ( myocardium ) or to supply electrical signals , for example for delivering stimulation pulses to the cardiac tissue which surrounds them . fig1 shows the manner in which the electrode poles , i . e ., the tip electrode 22 and the annular electrode 24 , and for the present application , the electrode line 20 , are located in the apex of a right ventricle of a heart . the tip electrode 22 and the annular electrode 24 are in each case electrically connected via at least one electrical conductor 26 to a plug contact 28 at the proximal end of the electrode line 20 . the plug contact 28 has electrical contacts which correspond to the electrical contacts 16 of the contact socket in the connector housing 14 of the implantable cardiac stimulator . the electrical conductors 26 in the electrode line 20 may be designed as somewhat elongated cable conductors or as helically coiled conductors . such conductors , which connect functional electrode poles to electrical contacts of the plug contact at the proximal end of the electrode line 20 in an electrically conductive manner , are referred to as function conductors within the scope of this description , since , for example , they transmit electrical signals used for the treatment from the plug contact to the particular electrode pole , or conduct sensed signals which represent electrical potentials from the particular electrode pole to the plug contact , and are thus used for the fundamental function of the medical device . the electrical conductors 26 which connect the electrode poles 22 and 24 to the electrical contacts of the plug 28 of the electrode line 20 are enclosed over most of their length by an insulating sheath , resulting in targeted electrical contact with the tissue of the heart via the electrode poles . in addition to the electrode poles 22 and 24 , which are typically used for stimulation ( in this case , ventricular ) of the cardiac tissue , the electrode line 20 has two large - surface electrode poles 30 and 32 , which are used as defibrillation electrodes and are formed by at least one bare helically wound wire . it is pointed out that within the scope of this exemplary embodiment the invention is explained with reference to a right ventricular cardiac pacemaker and defibrillator . as a medical device within the meaning of at least one embodiment of the invention , however , in principle an ablation electrode line may also be used , which in the application likewise extends into the heart of a patient and is controlled by a device located outside the patient , and for this purpose is connected to the device . fig2 illustrates a typical temperature curve 100 of a conventional pacemaker / icd electrode in magnetic resonance imaging ( mri ). the temperature increases rapidly when the high - frequency alternating field in the mri unit is switched on at time 110 , the steepness of the increase and the maximum achievable temperature being greatly dependent on the electrode position relative to the high - frequency alternating fields of the mri . when the high - frequency alternating field is switched off at time 120 , the electrode tip quickly cools due to its relatively low heat capacity . fig3 schematically shows an electrode pole in the form of a tip electrode 210 , and two electrode poles in the form of annular electrodes 220 and 230 , each of which is connected to a function conductor ( supply line ) zl 1 , zl 2 , and zl 3 , respectively . the schematically illustrated electrode poles are typically located at the distal end of an electrode line , which also has further components , for example an insulating sheath , which are not shown in the schematic illustration . it is known that the function conductors zl 1 , zl 2 or zl 3 may assume various shapes , and may be designed as cable feeds or also as helically shaped feeds . fig3 shows the manner in which a field - generating electrode pole , in this case the tip electrode 210 , cooperates with sensing electrode poles in the form of annular electrodes 220 and 230 when the function conductors zl 1 , zl 2 , zl 3 are exposed to intense interference injection , for example as the result of externally acting high - frequency alternating fields in a magnetic resonance imaging ( mri ) unit . the electrical field around the tip electrode 210 is indicated by arrows and the letter e . the product of e and the distance dl ( e * dl ) results in the voltage which is present between the annular electrodes 220 and 230 due to the field , having field strength e , emanating from the tip electrode 210 . thus , a measure of the field strength of the electrical field e around the tip electrode 210 may be derived from the difference in potentials between the annular electrodes 220 and 230 . the tip electrodes and annular electrodes 310 and 320 , respectively , and their respective supply lines ( function conductors ) zl 1 and zl 2 are schematically illustrated in fig4 a and 4b , similarly as in fig3 . fig4 a and 4b show that the current intensity emanating at the electrode pole is of different magnitudes , depending on the strength of the acting electromagnetic fields . this is correlated with a change in the strength of the electrical field e as well as the heating which is produced . for coaxial electrode lines ( the predominant design in current clinical practice ), for example the degree of heating ( and therefore the electrical field ) of the annular electrode ( s ) is usually much lower than that of the tip electrode . therefore , it may be ignored as an approximation . however , the annular electrode ( s ) 320 is / are located in the field of the tip electrode 310 , and are therefore at a different potential as a function of the field strength . the corresponding voltage is calculated from the integral ∫ e ( l ) dl . this signal reaches the ipg via function line zl 2 ( the supply line for the annular electrode 320 ), where it may be evaluated . the fact that the annular electrode ( s ) is / are at a given potential in the field of the tip electrode may be determined in relation to other reference potentials ( thus , for example , the tip itself , another ring , or the ipg housing ). fig5 a and 5b each show a block diagram of the cardiac stimulator 10 , which in the present case is referred to as an implantable pulse generator ( ipg ). the ipg has a sensing device for evaluating an electrical signal of the electrode which is correlated with the heating of an electrode pole . the ipg housing 400 of the cardiac stimulator 10 is schematically indicated in fig5 a and 5b . the components which are relevant to the present exemplary embodiment are illustrated as blocks in the ipg housing 400 . therefore , the possibility of further components of a typical cardiac stimulator being present in addition to the illustrated components is not excluded . an electrode line 405 is connected to the ipg housing 400 . the electrode line 405 is likewise schematically illustrated in the sense of the diagram in fig3 , and in the illustrated exemplary embodiment has an electrode pole in the form of a tip electrode , and two electrode poles in the form of annular electrodes , each of which is connected via its own supply line ( function conductor ) 421 , 422 , 423 , respectively , to an optional filter 420 . a mixer 430 is connected downstream from the optional filter 420 , and downstream from the mixer 430 is a threshold value detector 440 . the output signal of the threshold value detector 440 is delivered to an evaluation unit 450 , which in turn acts on the remaining electronics system of the cardiac pacemaker , referred to in general as the ipg electronics system 410 . the arrow 411 indicates that the ipg electronics system 410 is externally programmable . the function conductor ( supply lines ) 421 - 423 of the electrode line 405 extend to the interior of the ipg , and arrive at that location . the optional filter 420 results in only signals in specific frequency ranges , for example greater than 1 mhz , or bands having bandwidths of less than 10 mhz around typical magnet resonance hf frequencies such as 42 , 64 , 128 mhz , for example , being further processed . the ipg housing 400 may likewise be used as an electrode pole , for example as a reference electrode , and for this purpose is connected via a line 424 to the filter 420 . these signals are added on a weighted basis in the mixer 430 , and this result is added to the threshold value detector 440 . when the threshold conditions are met , the threshold value detector 440 generates a corresponding detector output signal . this detector output signal is further processed in the evaluation unit 450 , and a control signal is sent to the ipg electronics system 410 . the ipg electronics system 410 determines the particular function conductors which are to be connected to the filter as input lines . this may once again be externally programmed ( see arrow 411 ). in the exemplary embodiments illustrated in fig5 a and 5b , the threshold value detector 440 and the evaluation unit 450 together form an evaluation device 460 which responds to an exceedance of the threshold value . the evaluation device 460 may also be designed in such a way that it determines and evaluates the cumulative effect of the signal by integration , averaging , or determining the effective value of the signal , and outputs an output signal as a function of the magnitude of the cumulative effect . the threshold value detector 440 may be designed in such a way that it responds to the maximum value of its input signal ( the output signal of the mixer 430 ), and in this sense functions as a maximum value detector . the ipg electronics system 410 , as part of the control device of the cardiac stimulator 10 , may be designed in such a way that it always switches the cardiac stimulator 10 to a safe operating mode in the event of an intense electromagnetic field effect , as a function of the output signal of the evaluation unit 450 or of the evaluation device 460 , and does so , for example , for a specified , preprogrammed period of time . the evaluation device 460 may also be connected to a telemetry unit 470 to allow the output signal of the evaluation device to be transmitted to an external device , and thus to provide information for a medical practitioner via a corresponding central service center . a converter 480 may be mounted at the distal end of the electrode line 405 in the immediate proximity of the particular sensing electrode pole , and the converter converts the signals ( differences in potential , for example ) detected by the sensing electrode poles in such a way that the signals may be transmitted to the evaluation unit without interference . such a converter 480 is shown as an optional component in fig5 a and 5b , and is connected to the evaluation unit 450 via a signal line sl . the converter 480 may be a transformer , for example , which transforms the signal detected by the sensing electrode poles to a higher value in order to conduct the signal to the evaluation unit 450 via high - impedance lines ( signal line sl ) with immunity to interference from high - frequency alternating fields . alternatively , the converter 480 may be a piezoelectric converter which converts the signals detected by the sensing electrode poles into mechanical , specifically acoustic , output signals , for example , which may be conducted to the evaluation unit 450 via a suitable signal line . the evaluation unit 450 may also have an acoustoelectric converter which converts acoustic signals back to electrical signals . according to another alternative , the converter 480 may be a rectifier or demodulator whose output signal is a direct current signal , so that any high - frequency interferences of this direct current signal may be easily filtered out by the evaluation unit 450 . according to another alternative , the converter 480 may be an electrooptical converter , for example a light - emitting diode ( led ), which converts the signal detected by the sensing electrode poles to a light signal , which may be conducted without interference as a signal line sl to the evaluation unit 450 via an optical fiber . in this case the evaluation unit 450 has a complementary converter , for example a photodiode , which converts the light signals back to electrical signals . the converter for converting the acoustic signal or the light signal to an electrical signal may also be provided at the proximal end of the electrode line 405 . this is indicated in fig5 b by the converter 490 , which is connected to the evaluation unit 450 via an electrical signal line sl ′. it will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teaching . the disclosed examples and embodiments are presented for purposes of illustration only . therefore , it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention .
an implantable medical device having at least one elongated electrical function conductor and an electrode pole connected to the at least one elongated function conductor . the at least one elongated function conductor transmits therapeutic signals or diagnostic signals or both . the electrode pole delivers electrical current or field or sense electrical potentials , or both , in surrounding tissue during use . the implantable medical device includes a sensing device connected to a field - generating electrode pole , a potential - sensing electrode pole , and a reference pole . the sensing device detects generated electrical potentials via the potential - sensing electrode pole in relation to the reference pole , and generates an output signal that represents a detected electrical potential . the implantable medical device includes a control device connected to the sensing device to evaluate an output signal generated by the sensing device , and to control the medical device as a function of the potential detected by the sensing device .