there are a great many possible implementations of the invention , too many to describe herein . some possible implementations that are presently preferred are described below . it cannot be emphasized too strongly , however , that these are descriptions of implementations of the invention , and not descriptions of the invention , which is not limited to the detailed implementations described in this section but is described in broader terms in the claims . one implementation is illustrated in fig2 and 15 - 17 . a front electrode 14 includes three monitoring electrodes 20 positioned equidistant from a central stimulation electrode 18 . all three monitoring electrodes 20 and the stimulation electrode 18 are supported on a common assembly . a back electrode 12 includes only a stimulation electrode 16 , but may optionally also include one or more integrated monitoring electrodes as well . the active area of the stimulation electrode is about 10 cm in diameter . the active area of each monitoring electrode is about 2 cm in diameter . the edge of each monitoring electrode active area is spaced about 1 cm from the edge of the adjacent stimulation electrode active area . various constructions are possible for the electrode pad assemblies . fig1 - 17 show one possible construction . materials have been chosen that provide resiliency and compliance to the skin surface . fig1 shows the electrode assembly configured to be applied to the front of the chest . a foam cover ( or backing ) layer 24 ( e . g ., voltek volara ™) extends fully across the back of the electrode ( top surface facing up away from patient ). each of the three monitoring electrodes is formed by securing a nickel plated brass snap 21 to an agcl post 22 through an opening in the cover layer ( alternatively a lead wire may be connected to the monitoring electrodes ). below that is a foam frame layer 28 , which has an opening through which each of the agcl posts ( agcl plated glass filled abs ) contact a porous foam sponge 23 , which is impregnated with ecg gel 35 ( e . g ., pharmaceutical innovations ™). the stimulation electrode 18 is provided by a conductive plate 29 ( e . g ., tin ) in the center of the electrode assembly . the conductive plate is supported beneath the foam frame layer 28 . beneath the conductive plate , and making conductive contact with the patient , is a conductive layer 30 , e . g ., a solid gel such as a hydrogel ( e . g ., ludlow ™ 63t hydrogel ). at one edge , a portion 38 of the conductive plate extends through an opening in the foam frame layer , and is mechanically and electrically connected at 25 to a wire lead 26 . wire lead 26 extends to an electrical connector 40 , to which the wire lead 26 from the back electrode assembly is also connected . various alternatives may be used for the conductive , skin - contacting layers 23 , 30 . thee include , but are not limited to , solid conductive gels ( e . g ., hydrogel ), a porous material filled with a liquid gel , and a porous material soaked in a conductive solution such as saline . fig1 shows the electrode assembly configured to be applied to the back of the chest . it has a similar construction to that of the front electrode , except that it lacks monitoring electrodes , and has a rectangular , rather than circular , stimulation electrode . conductive plate 33 is supported on the underside of a foam backing layer 32 . a conductive layer 34 , e . g ., solid gel such as the same hydrogel as used in the front stimulation electrode , makes contact with the patient . a portion 38 of the conductive plate extends through an opening in backing layer 32 , and is connected to wire lead 26 . an insulator foam backing layer 31 covers the portion 38 of the conductive plate that extends to the top of foam backing layer 32 . in the implementation of fig2 and 15 - 17 , the monitoring electrodes 20 are all positioned the same distance from the stimulation electrode 18 . if the electrical currents flowing between the stimulation electrodes are approximately equal in all directions then the artifact measured by each monitoring electrode will be similar and cancel when a potential difference is formed by subtracting the signals . but placing the monitoring electrodes at equal distances from the stimulation electrode is no guarantee that the measured stimulus artifact will be the same in all three monitoring electrodes . transmission factors , monitoring electrode impedance , the path of current flow , the shape of the electric field , and other variables can influence measured artifact . some of the transmission factors , e . g ., respiration and blood flow , may be time varying . current flow can be influenced by surface properties and anatomical structures in the body . however , positioning the monitoring electrodes equal distances from the stimulation electrode may be sufficient in many cases , as it may result in stimulus artifacts that are sufficiently closely matched as to reduce the level of artifact to an acceptable level in the differential signal ( the difference in potential between two monitoring electrodes ). and the remaining artifact can optionally be reduced further using other methods described below . alternatively , the monitoring electrode spacing can be adjusted based on modeling current flows , experimental results , or a priori knowledge of the transmission factors involved . circuitry in the electrode or in the medical device doing the monitoring may equalize the artifact measured at each electrode by changing a gain or impedance , or by using other known techniques . forming the sum of two monitoring electrodes with artifacts of similar magnitude but opposite polarity will also reduce artifact , e . g ., the sum of monitoring electrodes relative to a common reference where one is positioned near the positive and one near the negative stimulation electrodes . in the implementation of fig2 and 15 - 17 , three monitoring electrodes are provided for the front electrode assembly , which is positioned over the heart . the back electrode assembly does not have any monitoring electrodes . other implementations may use different numbers of monitoring electrodes on the front assembly , and monitoring electrodes could be included on the back assembly . measuring a potential difference requires at least two electrodes . integration of three monitoring electrode with the stimulation electrode over the heart has at least two benefits . first , many ecg monitors use a third electrode to drive common mode signals back to the patient , to improve signal quality in the presence of large common mode signals such as power line interference . if the third electrode is only used for this common mode rejection purpose , its location relative to the stimulation electrode is less important . second , ecg monitors for three - lead monitoring generally display the potential difference between a selected electrode pair . these differences are called lead i , lead ii , and lead iii when the electrodes are positioned in conventional locations on the right and left arms as well as the left leg . the monitoring electrodes in the invention do not represent the standard leads , but still provide three possible potential differences , from which the operator of the ecg monitor may select . the operator may select the view which is most clinically relevant or contains the least artifact during cardiac pacing . in implementations in which three potential differences between pairs of electrodes are sought , the locations of all three monitoring electrodes can be selected to improve artifact cancellation ( e . g ., each may be equidistant from the central stimulation electrode ), so that a choice can be made as to the best two electrodes to use for canceling the stimulus artifact . more than three monitoring electrodes may also be provided . the monitoring hardware may be configured to detect the artifact - reducing electrode assembly . if it recognizes such an electrode assembly , the hardware may process signals differently and / or change labeling on displays , strip chart recorders , storage devices and external interfaces . the change in labeling will prevent those reviewing the signals from trying to interpret them as a standard electrode configuration ( e . g ., standard 3 - lead ). the electrode assembly identification would typically be made through the monitoring portion of the assembly rather than through the stimulation portion , because in some implementations electrical stimulation is allowed to continue even if a switch is made to standard monitoring electrodes when time permits . various methods may be used to identify the electrode assembly , including , for example , the following : ( 1 ) specific resistances between connector pins are detected by the monitor ; ( 2 ) voltages , currents , or specific waveforms input to the monitor from the electrode assembly ; ( 3 ) interfaces to nonvolatile memory or a microprocessor contained within the electrode connector or assembly ; ( 4 ) pulling unused monitoring channels to specific voltages ( currents , or known waveforms ) that can be used to identify the cable . an example of the fourth option is connecting a three lead ecg cable to a 10 wire monitor with a special electrode connector so that certain unused inputs are shorted to ground while others are shorted to a specified voltage . any condition that is unlikely to occur without the connector in place can be used for identification . fig1 illustrates the physics underlying the ability of some implementations to cancel much of the stimulus artifact from the ecg signal . a stimulus is applied to the patient ( represented by the dashed line rectangle ) using a pair of stimulus electrodes ( therapy pads ). current flows from one therapy pad to another along varying current paths . the figure shows the overly simplified case of there being just three current paths , with one monitoring electrode positioned along each of two of the paths . resistors are shown along the current paths to represent the resistance experienced by current flowing along particular paths . the values of the resistors are dependent on the placement of the electrodes , and the physical properties of the patient , and the values of the resistors may be time varying ( e . g ., as the result of respiration ). each monitoring electrode records some potential owing to the current flowing during a therapy pulse . after the pulse , the therapy pads may remain polarized . the polarization equalizes over time , and the monitoring electrodes record the potential difference due to the polarization . if the polarization has an equal effect on both monitoring electrodes , then the effect of the polarization ( what we have called the stimulus artifact ) will cancel , and the differential signal will be due to the electrical activity within the body such as the ecg . another possible implementation is shown in fig3 . the monitoring electrodes 20 are supported on a common assembly separate from ( but , in this example , surrounding ) the stimulation electrode 18 . in fig3 the common assembly is an annular in shape to surround a circular stimulation electrode , but other shapes may be used . an advantage of the shapes used in fig3 is that they help guarantee that the monitoring electrodes are equally spaced from the stimulation electrode . two of the monitoring electrodes are positioned to be equidistant from the stimulation electrode . the third monitoring electrode ( at the top of the figure ) is shown in a position slightly away from an equidistant location . if the third electrode is not used to form an ecg signal , but is used only for common mode rejection purposes , then it is not necessary that it be equidistant . in other implementations , in which it was sometimes desirable to use the third electrode for forming an ecg signal , it may be decided to place it in an equidistant location just as the other two monitoring electrodes . other methods may also be suitable for positioning the electrodes . they may be positioned at set distances from the therapy pad using constant length cables or other physical connection to the therapy pad that allows easy placement at a pre - determined distance . fig4 illustrates one implementation in which placement of the monitoring electrodes around the stimulation electrode is regulated by the length of electrode lead wires extending from the central stimulation electrode . when the wires are fully extended in a radial direction , the electrode positions will be equidistant from the stimulation electrode . similarly , the wires may be slightly different lengths in order to equalize artifact based on a priori knowledge of the current flow . alternatively , the separation between monitoring and stimulation electrodes could be prescribed by a mechanical element ( e . g ., a mechanical cable ), rather than by the electrical leads . the electrodes might initially be affixed ( prior to their extension ) to the therapy pad by an adhesive or a mechanical device ( i . e . clip , velcro , etc ). another implementation is shown in fig6 . three groups of multiple monitoring electrodes — with the electrodes in each group being at different locations relative to the stimulation electrode — are shown . in fig6 , there are two monitoring electrodes 20 in each of group a , b and c . analog or digital signal processing may be used to produce a combination of the electrodes in a group , so that when the difference between the processed ( or weighted ) combinations from two groups is taken the artifact is better cancelled in the differential signal . this method provides compensation for irregularities in the current flow or electric field originating from the stimulation electrode , and may be time - varying to compensate for time - varying parameters such as respiration . more than two electrodes may be provided in each group . not all locations will require forming a weighted combination of a plurality of electrodes . good electrode placements for artifact rejection may not be ideal for analysis of monitored signals . e . g ., they may not provide a standard clinical ecg signal . signal processing may be used to derive or synthesize improved or more clinically standardized looking waveforms from the actual monitoring electrodes . this may be accomplished by , in effect , creating a derived ( or synthesized ) monitoring electrode from combinations of actual monitoring electrodes . a block diagram of one cardiac pacing implementation of this procedure is shown in fig7 . signals 90 from monitoring electrodes ( e . g ., ones of the type shown in fig2 ) may optionally be combined with signals from one ( or both ) stimulation electrode ( which during intervals between stimulation pulses can also serve as a monitoring electrode ). a signal processing block 92 produces estimates 94 of standard ecg vectors that are more familiar to the user than potential differences formed directly from the nonstandard electrode locations of fig2 . a preferred implementation is to transform the signals from the monitoring electrodes ( and optionally the stimulation electrode ) into ecg signals comparable to what would have been detected using the standard 3 - lead placement of ecg electrodes ( two near the arms , and one near a leg ). to perform the transformation , the coefficients of a linear transformation matrix are derived from a statistically meaningful population of patients , from whom ecg measurements have been taken at both the new monitoring electrode ( and stimulation electrode ) locations and the conventional 3 - lead locations . a least squares fit is done to derive coefficients of the linear transformation matrix . the prior art taught several methods of synthesizing leads from a reduced or alternate set of electrodes . for instance , dower &# 39 ; s easi system ( u . s . pat . no . 5 , 711 , 304 ) used five electrodes in non - standard locations on the body to synthesize an estimate of the 12 - lead ecg . dower placed the electrodes far apart on the body in locations selected for ease of placement and 12 - lead synthesis . the prior art also taught transformations from implanted leads to standardized leads . implanted electrodes are fixed in position . it is possible to attach standard surface electrodes to the patient and derive the optimal transform . fig1 and the following discussion provide one mathematical basis for the lead synthesis . the figure shows a round stimulation ( pacing / defibrillation ) electrode surrounded by three equidistant monitoring electrodes , each spaced from the others at 120 degree angles . the objective of lead synthesis is to convert the monitoring signals from these nonstandard locations to estimates of the standardized lead i , lead ii , and lead iii difference signals that would be derived from electrodes positioned at the standard right arm ( ra ), left arm ( la ), and left leg ( ll ) locations and also shown in the figure . the signals from the nonstandard monitoring locations can be represented by a matrix x containing samples from the monitored electrodes as column vectors . these signals are high pass filtered or processed so that their mean value is zero . in the example set out below , x is an n × 2 matrix where n represents the number of samples and two columns are formed from three ecg electrodes . since the electrodes need a reference voltage , two independent ecg vectors ( v n ) may be produced from these three monitoring electrodes ( e n ). the third vector may be derived from the other two as follows and is omitted from matrix x to avoid a singular or ill - conditioned system of equations below v 3 = v 1 − v 2 = e 1 − e 2 −( e 1 − e 3 )= e 1 − e 3 the desired standardized signals can be represented by a matrix y containing each of the output signals as column vectors with the mean removed , for instance an n × 3 matrix where columns 1 , 2 , and 3 represent leads i , ii , iii respective . the goal of lead synthesis is to find a transformation matrix c such that [ x 11 x 12 x 21 x 22 x 31 x 32 ⋮ ⋮ x n ⁢ ⁢ 1 x n ⁢ ⁢ 2 ] ⁡ [ c 11 c 12 c 13 c 21 c 22 c 23 ] = [ y 11 y 12 y 13 y 21 y 22 y 23 y 31 y 32 y 33 ⋮ ⋮ ⋮ y n ⁢ ⁢ 1 y n ⁢ ⁢ 2 y n ⁢ ⁢ 3 ] the squared error between measured ecg vectors y and estimated ecg vectors ŷ can be calculated as ŷ is the estimate of y and can be calculated using the following equation . the optimal transformation matrix c will generally vary from patient to patient and is based on the relative placement of the electrode assembly and the standard three lead electrodes . however , c can be estimated from a database of known ecg signals and used generically . the operator may have the ability to switch between the sampled ecg vectors and the synthesized leads so the most usefull view may be selected . fig1 illustrates an example of monitoring signals . the signals in the column labeled “ custom leads ” are potential differences measured from a nonstandard electrode configuration such as fig2 . the column labeled “ standard leads i , ii , iii ” are measured by monitoring electrodes such as the ones in fig1 . the far right column includes estimates of the standard leads synthesized from the nonstandard signals by a matrix transformation . although not exactly the same as the signals from standardized locations , they are close enough in appearance for many clinical purposes such as calculation of heart rate . the monitoring and stimulation electrodes may vary in composition . rather than use a conductive polymer material ( sometimes called , “ solid gel ” or “ hydrogel ”) for both types of electrodes , liquid gel could be used for the monitoring electrodes , and conductive polymer material only for the stimulation electrodes . this has the advantage of better impedance and signal quality shortly after applying the monitoring electrodes to the skin . conductive polymer pads typically require time for the skin to warm the gel and reduce impedance , whereas liquid gel does not suffer from such delays . many situations including emergency cardiac pacing or defibrillation are time critical and there may be an advantage to using liquid gels . it may also be advantageous in some circumstances to use liquid gels ( or other conductive agents ) for the stimulation electrodes . for instance , a liquid gel may be beneficial if the stimulation electrode is intended for cardiac pacing only . other applications may require electrodes ( stimulation or monitoring ) with a conductive surface ( s ) but no gel . conductive gel or electrode paste may be applied to the conductive surface or to the skin as needed . this is generally the preferred method for re - usable ecg or eeg electrodes . electrode assemblies containing more than one electrode ( e . g ., a stimulation 18 and one or more monitoring electrodes 20 ) may include multiple conductors ( e . g ., tin layers ) that contact the skin through a common gel ( polymer pad or other ) layer . fig1 shows such an implementation . two therapy pad assemblies are used for stimulation . each pad assembly includes a stimulation electrode , and at least one of the pad assemblies includes one or more monitoring electrodes . electrical current is driven between the conductive plates of the stimulation electrodes ; the plates are in electrical contact with the skin through a conductive gel . during the electrical stimulus a potential difference exists between the plates . the polarization of the plates may persist for some time after the stimulus . measuring small changes in voltage between the plates of the stimulation electrodes may be difficult because of the relatively large potential differences . but the potential difference between the plate of a stimulation electrode and the plate of an adjacent monitoring electrode is less than the difference between the plates of two stimulation electrodes , and thus monitoring small voltage changes is more feasible . the relative positions of the stimulation and monitoring electrodes may be determined by the measurements of interest , for instance across the heart . if the same conductive gel sheet covers the plates of the stimulation electrode and the plate of the monitoring electrode then the polarization effect will be similar and the differential signal will be less contaminated with artifact . in some implementations in which the same electrode assembly has more than one type of gel , a vapor barrier may be provided to retard moisture transfer from one gel to another . the electrode assemblies are typically sealed within a package until use , but while this retards moisture from leaving the interior of the sealed package , it does not prevent moisture transfers within the assembly . a lower moisture gel such as a hydrogel may absorb water from a second ( e . g . liquid gel or different hydrogel ) electrode over the life of the packaged electrode . a vapor barrier inside the package may be used to seal one gel type from the other to increase shelf life . the vapor barrier may be implemented in a variety of ways including the method described by dupelle and white in u . s . pat . nos . 6 , 453 , 205 and 6 , 280 , 463 , in which a sealed cup is used to contain a liquid gel . the vapor barrier may be made from commonly used materials such as mylar and aluminum . an aluminum thin film layer may be deposited by the so - called thermal evaporation method whereby aluminum wire is evaporated onto a heated crucible in a vacuum chamber . some implementations may use an inert material that is non conductive , such as a thin film deposition of siox ( typically via sputter deposition ). other vapor barriers may also be used . an exploded cross - sectional view of an electrode with a vapor barrier 50 is shown in fig2 ( patient contact surface at top ). the outer surface of the electrode is made from a layer of adhesive backed closed cell foam 52 such as voltek volara . a second layer of insulating foam 54 creates recessed wells for the monitoring ( ecg ) electrodes . a thin vapor barrier 50 surrounds the ecg well and adheres to the surrounding foam . the ecg electrode wire may be riveted ( 56 ) through an insulated vapor barrier to maintain the seal . alternately , the vapor barrier may be a conductive metal such as tin and the wire may be soldered or otherwise connected directly to the vapor barrier . the entire electrode assembly is placed on a release liner 58 ( e . g ., silicon impregnated polyethylene ). peeling off the release liner also uncovers the ecg electrodes since the top of the vapor barrier has a stronger bond to the liner than it does to the bottom part of the vapor barrier . the vapor barrier may be constructed in various ways , but one possibility is shown in fig2 , wherein two layers of polyester ( e . g ., mylar ) and aluminum are bonded face to face . fig9 shows one possible analog electrical circuit that can be used to implement artifact rejection . the signal from each of two monitoring electrodes ( electrode 1 and electrode 2 ) is buffered through inverting amplifiers a 1 and a 3 , respectively . the buffered signal is fed through a low pass filter , which may be tuned using a variable capacitor to change the time constant . although a variable capacitor 60 is shown in the circuit , a variable resistor or some other combination of tunable circuit elements may be used . the filters may be set to adjust for different delays in the two input signals so that most of the energy from the artifact will cancel when the signals are subtracted . fig9 also shows variable gain amplifiers a 2 and a 4 , which allow the signal from each electrode to be scaled so that the magnitude of the artifact is similar in both signals and will cancel when subtracted in the output amplifier a 5 . with sufficient signal to noise ratio ( snr ), only one programmable gain amplifier is needed , provided it can attenuate the signal as well as amplify it . one example of scaling is an implementation in which two monitoring vectors p 1 and p 2 are calculated from the signals detected at three electrodes , as follows . the constants c 1 and c 2 may be selected in some implementations so that the magnitude of the artifact in the two monitoring vectors p 1 , p 2 are approximately equal so that the artifact cancels in the scaled output y . in other implementations , one of the vectors is used without being scaled ( so that no constant is necessary ). fig2 shows an example of how the artifact may be reduced in some implementations with a 100 ma stimulus . the unscaled difference signals ( vectors ) p 1 and p 2 ( the top two signals ) both contain appreciable artifact . but the scaled difference signal ( bottom signal ) has a substantially reduced artifact , less than in either of the original difference signals . the constants c 1 and / or c 2 may be derived mathematically by comparing the artifact or may be tuned by the operator ( e . g . by twisting a dial ) to minimize artifact . the scaling may affect the shape of the resultant monitoring signal ( e . g ., it may not have magnitudes relevant for diagnostic purposes ), but it may be useful for determining heart rate or the general shape of the ecg . the scaling may be implemented in hardware or software , and the constants may be positive or negative depending on the direction of the artifact in each monitored difference signal ( vector ). to make it possible for the circuitry to adapt quickly to new patients and new electrodes , some implementations would use digitally controlled components such as ( but not limited to ) programmable gain amplifiers , digitally controlled variable resistors , and capacitors or inductors that can be switched in or out of the circuit ( e . g ., with analog switches ). manually adjustable components may also be used , and set by the operator . the signal processing shown in fig9 may alternatively be implemented using digital or software processing of the sampled signal , or with a combination of analog and digital signal processing . the circuit is preferably designed so that the artifact will not saturate the input amplifiers or converters during periods of interest for monitoring . digital signal processing may allow for more flexibility in delaying or processing the signals . digital processing requires sampling the raw signals from each electrode relative to a common reference . complex filters and / or adaptive gain estimates may also be used in either a digital or analog implementation . another technique for reducing the artifact in the differential signal acquired from two monitoring electrodes is adjusting the input impedance in the electrical circuit that detects the potential at the monitoring electrodes . the artifact will be minimized if the impedance of each monitoring electrode is equal . electrode impedance can be directly measured or estimated from the artifact . the impedance can be measured by applying a therapy pulse at low or full power , or by using sine waves , chirps , or other arbitrary waveforms suitable for this purpose . the resulting voltage or current waveform measured at each monitoring electrode ( or between the two electrodes ) can be used to estimate the impedance ( or impedance mismatch ). one implementation of an impedance balancing circuit is shown in fig5 , in which the level of the stimulus artifact at the two electrodes is better equalized , thereby reducing the level of artifact in the differential signal . by balancing the impedance of the monitoring electrodes , it may not be necessary to connect a patient drive electrode to assist in common mode rejection . the circuit in fig5 shows two monitoring electrodes 70 , 72 ( upper left ) and a patient drive electrode 74 ( lower left ). each electrode is shown as an rc element , and it should be noted that these values may vary over time ( e . g ., from respiration ). the conductive gel &# 39 ; s contact with the skin may be different at each electrode , resulting in an impedance imbalance between electrodes . the monitored signal is typically of small magnitude compared to the stimulus artifact and other common mode signals such as power line interference . during or shortly after a stimulation pulse , two monitoring electrodes positioned at the same potential in the electric field between the two stimulation electrodes will measure a very large common mode signal as the result of the polarization on the two stimulation electrodes , i . e ., the stimulus artifact . but the circuit measuring the difference between the two monitoring electrodes may not reject the large stimulus artifact if the impedances are not properly balanced . several methods are known in the art for canceling common mode signals , and these may be applied to improve cancellation of the stimulus artifact . each of the leads running to the stimulation and monitoring electrodes has a cable shield 76 surrounding it . the circuit drives the cable shield with the common mode signal through amplifier a 1 . this reduces the effect of cable capacitance by maintaining signal and shield at similar potentials . the shield drive is also integrated and inverted by a 2 and driven back to the patient . this has the effect of reducing common mode signals by moving the reference level of the circuit close to the common mode of the patient . currents due to common mode signals may flow from the patient through various return paths including the patient or shield drive . circuit elements may be adjusted to correct for imbalances in electrode impedance to reduce common mode signals . the figure shows variable resistors and capacitors controlled by an impedance compensation circuit 78 . the impedance matching circuit may be simpler to implement if placed directly between the two monitoring electrodes , but this may require complex cabling and not be as practical . impedance matching elements may include components commonly used in the art , including ( but is not limited to ) trim pots , manually - adjustable capacitors , digitally - controlled variable resistors or capacitors , or one or more rc elements with analog switches . inductors or other passive components may also be used . the impedance compensation controller may include a mix of analog and / or digital processing . the impedance may be measured directly by applying a current to the patient in the form of sine waves , chirps , or therapy pulses at full or reduced intensity . there are other methods well known in the state of the art . it may also be measured indirectly by estimating the imbalance from power line interference or artifact from pulses delivered during therapy . the controller adjusts the digitally controller circuit elements and may monitor changes in common mode artifact . alternately , the impedance may be adjusted manually by the operator of the device , but this may be time consuming and require some expertise not shared by all device operators . various arrangements of cables and connectors can be used for connecting the stimulation ( therapy pads ) and monitoring electrodes to their associated medical device ( e . g ., a combined cpr prompting , defibrillation , and pacing device ). for example , the therapy electrodes may be wired to a therapy connector . the wires may be made from any electrically conductive material and may be permanently attached to the electrodes or may attach to some or all of the electrodes using a connector . a connector allows the wires to be reused but may be less reliable and takes time for connection . it has the disadvantage of allowing the operator to make a mistake by forgetting to connect a wire or by connecting a wire to the wrong electrode . the monitoring electrodes may be wired to a monitoring connector . the therapy and monitoring connectors may be physically separate or combined into a unified connector . the unified connector may be one piece or made up of a monitoring connector and therapy connector that can come apart or move in such a way that one or both of the connectors will be attached to the medical device . this may be accomplished with wires permanently attached to the electrodes and running individually , or attached together in a single cable , to a connector . attaching the single connector to the medical devices ensures that all connections are properly made . however , this requires a separate input on the medical device for standard leads to be connected , and an internal switching mechanism capable of selecting between electrodes or displaying both sets of leads . alternatively , separate connectors may be used for the therapy pad and monitoring electrodes . this has the advantage of allowing the operator to replace one or both of these cables with other monitoring electrodes or pads , and eliminates the need for a switching mechanism . this type of cabling may allow the electrodes to be used on devices not originally designed for this purpose . some of the multi - lead cable constructions shown in the application of peter a . lund et al ., entitled , “ medical cable ”, filed on even date herewith ( and herein incorporated by reference ), may be used in some implementations . to simplify cabling and reduce cost , therapy pad wires may be shared with the monitoring electrodes in certain applications . this is especially relevant where pulses are applied to the therapy pads for short durations , and monitoring is not required during this time . switching circuitry or non - linear circuit elements including but not limited to , diodes or gas discharge tubes may be used for this purpose . a possible implementation of a shared wire electrode assembly is shown in fig8 . diodes 81 , 82 allow the flow of current to the stimulation electrode 85 ( which conducts to the patient through gel layer 86 ) but block a reverse flow of current back to the monitoring channel during the monitoring phase following stimulation . using diodes oriented in both directions allows the delivery of biphasic stimulation waveforms while preventing polarizations ( e . g . of less than a diode drop ) from being measured by the monitoring circuit . more than one diode may be used in either direction to split high currents or for fault tolerance . optionally , a resistor 83 or high impedance monitoring electrode 84 may reduce current flow through the monitoring electrode 84 during therapy . implementations such as that of fig8 , as well as other shared wire implementations , have the advantages of reduced clutter and reduced chance of wrong connections . in some implementations , the elimination of additional cables may also reduce overall manufacturing cost . the configuration shown in fig1 uses a plurality ( two shown ) of stimulation electrodes 18 of one polarity . current flows from one positive stimulation electrode 16 to two negative stimulation electrodes 18 . in other implementations both the positive and negative stimulation electrodes could be divided into two or more electrodes . a monitoring electrode 20 is positioned in the center ( but not in contact with ) each negative stimulation electrode . if both of the negative stimulation electrodes are positioned so that each receives ( sinks ) approximately the same current , the artifact measured by the two monitoring electrodes will be approximately equal and will thus cancel when the difference is taken between the two electrodes . another configuration using a plurality of stimulation electrodes 18 of the same polarity is shown in fig1 . in this implementation , an electrode of one polarity is divided into three stimulation electrodes 18 ( even more separate electrodes could be used ), and the other polarity is handled by just a single electrode 16 ( but alternatively this polarity could , also , be handled by a plurality of electrodes ). the multiple stimulation electrodes of the same polarity have separate conductive plates ( e . g ., tin ), and may have a common conductive gel underlying them , or separate gel areas . all three of the stimulation electrodes are used together during stimulation , and two of the stimulation electrodes are also used for monitoring ( as shown by the left and right electrodes leading to the differential amplifier ). the two electrodes used for monitoring are positioned across the heart and aligned to produce an ecg vector of interest . these two electrodes may be smaller than the central electrode . using the two stimulation electrodes for monitoring is possible because the polarization on the two electrodes is approximately equal and of the same polarity . the three electrodes may be part of one assembly , or be split into two or three assemblies for flexibility in placement . the combined area of the three electrodes is made sufficient for the therapy being delivered ( e . g ., defibrillation or pacing ). still another implementation is shown in fig1 . here the central electrode has been eliminated , and one stimulation polarity is divided into two electrodes 18 positioned at two sides of the heart , and aligned to produce an ecg vector of interest . both electrodes are using during stimulation , and monitoring is done by forming the difference between the two electrodes . the stimulus artifact may be mitigated further using analog or digital signal processing . such processing may include adaptive blanking of the artifact where filter inputs , displays , or strip chart recorders are blanked , zeroed , or otherwise modified during the artifact . an algorithm or adaptive method may be used to adjust the blanking time based on the measured signals . this may allow the operator to view more of the monitored signal if the artifact is cancelled quickly and to prevent confusing artifacts from being displayed if the artifact takes longer to dissipate . many other implementations other than those described above are within the invention , which is defined by the following claims .
electrodes and circuitry for monitoring and stimulating the exterior of the human body , comprising delivering stimulation pulses to stimulation electrodes applied to the exterior of the body , detecting an electrical potential at monitoring electrodes applied to the exterior of the body , positioning at least a first and second monitoring electrode at locations at which an electrical artifact caused by the electrical stimulation pulses is substantially cancelled in a signal formed from the electrical potentials detected at the first and second monitoring electrodes .