one of the primary goals of cardiac electrophysiology mapping is to locate with some certainty the position of an electrode within a cardiac cavity . an electric navigation field is created within a patient &# 39 ; s body on each of the three principal axes by driving a constant current . if the impedances measured in the body are constant , then the potentials on each axis measured at a location , with respect to a reference electrode at a static location , will remain at a constant potential over time . thus , if a location site is marked in the heart , one may return to that site with a catheter electrode in the future and be confident that if the measured navigation potentials , or impedance , are the same as before , the anatomic location is the same . fig1 depicts a schematic diagram of an exemplary electrophysiology mapping or ablation system . the patient 11 is depicted as an oval for clarity . three sets of surface electrodes ( e . g ., patch electrodes ) are shown applied to a surface of the patient 11 along an x - axis , a y - axis , and a z - axis . the x - axis surface electrodes 12 , 14 are applied to the patient along a first axis , such as on the lateral sides of the thorax region of the patient ( e . g ., applied to the patient &# 39 ; s skin underneath each arm ) and may be referred to as the left and right electrodes . the y - axis electrodes 18 , 19 are applied to the patient along a second axis generally orthogonal to the x - axis , such as along the sternum and spine of the patient in the thorax region and may be referred to as the chest and back electrodes . the z - axis electrodes 16 , 22 are applied along a third axis generally orthogonal to both the x - axis and the y - axis , such as along the inner thigh and neck regions of the patient , and may be referred to as the left leg and neck electrodes . the heart 10 lies between these pairs of surface electrodes . an additional surface reference electrode ( e . g ., a “ belly patch ”) 21 provides a reference and / or ground electrode for the system 8 . the belly patch electrode 21 is an alternative to a fixed intra - cardiac electrode 31 . it should also be appreciated that in addition , the patient 11 will have most or all of the conventional electrocardiogram ( ecg ) system leads in place . this ecg information is available to the system although not illustrated in the fig1 . a representative catheter 13 with a single , distal measurement electrode 17 is also depicted in fig1 . the catheter 13 may also have additional electrodes in addition to the measurement electrode 17 . a fixed reference electrode 31 may be attached to a heart wall on an independent catheter 29 . in many instances , a coronary sinus electrode or other fixed reference electrode 31 in the heart 10 can be used as a reference for measuring voltages and displacements . for calibration purposes the reference electrode 31 remains stationary on the wall of the heart during the course of the procedure . each surface electrode is independently connected to a multiplex switch 24 . pairs of the surface electrodes are selected by software running on a computer 20 , which couples the surface electrodes 12 , 14 , 16 , 18 , 19 , 21 , 22 to a signal generator 25 . a first pair of surface electrodes , for example , the z - axis electrodes 18 , 19 , is excited by the signal generator 25 . the exited electrodes generate an electric field in the body of the patient 11 and the heart 10 . this electrode excitation process occurs rapidly and sequentially as alternate sets of patch electrodes are selected and one or more of the unexcited surface electrodes are used to measure voltages . during the delivery of a current pulse , the unexcited surface electrodes 12 , 14 , 16 , and 22 are referenced to either the reference electrode 31 or the belly patch 21 and respective voltages are measured across one or more of these unexcited electrodes . in this way , the surface electrodes are divided into driven and non - driven electrode sets . while a pair of electrodes is driven by the current generator 25 , the remaining , non - driven electrodes may be used as references to synthesize the orthogonal drive axes . a low pass filter 27 processes the voltage measurements to remove electronic noise and cardiac motion artifact from the measurement signals . the filtered voltage measurements are transformed to digital data by the analog to digital converter 26 and transmitted to the computer 20 for storage under the direction of software . this collection of voltage measurements is referred to herein as the “ patch data .” the software has access to each individual voltage measurement made at each surface electrode during each excitation of each pair of surface electrodes . the patch data is used to determine a relative location in three dimensions ( x , y , z ) of the measurement electrode 17 . potentials across each of the six orthogonal surface electrodes may be acquired for all samples except when a particular surface electrode pair is driven . sampling while a surface electrode acts as a source or sink in a driven pair is normally avoided as the potential measured at a driven electrode during this time will be skewed by the electrode impedance and the effects of high local current density . generally , three nominally orthogonal electric fields are generated by a series of driven and sensed electric dipoles in order to realize catheter navigation in a biological conductor . alternately , these orthogonal fields can be decomposed and any pairs of surface electrodes can be driven as dipoles to provide effective electrode triangulation . additionally , such nonorthogonal methodologies add to the flexibility of the system and the ability to localize biologic impedance compensation . for any desired axis , the potentials measured across an intra - cardiac electrode 17 resulting from a predetermined set of drive ( source - sink ) configurations are combined algebraically to yield the same effective potential as would be obtained by simply driving a uniform current along the orthogonal axes . thus , any two of the surface electrodes 12 , 14 , 16 , 18 , 19 , 22 may be selected as a dipole source and drain with respect to a ground reference , e . g ., the belly patch 21 , while the unexcited electrodes measure voltage with respect to the ground reference . the measurement electrode 17 placed in the heart 10 is exposed to the field from a current pulse and is measured with respect to ground , e . g ., the belly patch 21 . in practice the catheters within the heart may contain multiple electrodes and each electrode potential may be measured . as previously noted , at least one electrode may be fixed to the interior surface of the heart to form a fixed reference electrode 31 , which is also measured with respect to ground . data sets from each of the surface electrodes and the internal electrodes are all used to determine the location of the measurement electrode 17 within the heart 10 . after the voltage measurements are made , a different pair of surface electrodes is excited by the current source and the voltage measurement process of the remaining patch electrodes and internal electrodes takes place . the sequence occurs rapidly on the order of 100 times per second . to a first approximation the voltage on the electrodes within the heart bears a linear relationship with position between the patch electrodes that establish the field within the heart . correction factors , e . g ., to compensate for respiration , may be applied to the raw location information to improve the accuracy of the location value . when operating with constant currents during an electrophysiology study , the potentials created with respect to any reference will be a function of the intervening impedance . this concept is represented schematically in fig4 . a catheter 13 with a distal measurement electrode 17 is placed within the heart 10 of the patient 11 . a set of orthogonal surface electrodes 12 , 14 , 16 , 18 , 19 , 22 are alternately paired as drive electrodes while the remaining , unexcited electrodes may function as sensor electrodes . the electric potential generated by a dipole pair of electrodes may be measured at a measurement electrode in the body and the path of current through the body between the driven surface electrode and the distal measurement electrode 17 on the catheter may be understood simply as a circuit . for example , as shown in fig4 , when the left leg electrode 18 is driven as a source , a circuit 30 is created between the left leg electrode 18 and the measurement electrode 17 on the catheter 13 within the heart 10 . the body tissue between the skin at the left leg electrode 12 and the heart 10 acts primarily as a resistive impedance and can be viewed functionally as a resistor 32 . however , due to changes in body chemistry during a procedure , the value of this resistor 32 may drift . therefore , the resistor 32 may be more accurately viewed as a variable resistor as depicted . it should be apparent that if the tissue impedance changes , then the voltage data and thus positional data for each electrode , including a reference electrode , e . g ., the belly patch electrode 21 , will similarly drift . to the extent that intervening impedances change after a study begins and sites have been marked , attempts to place the measurement electrode at previously visited locations may not be consistent with earlier marked sites . to the extent that the impedance change for a given pair of drive surface electrode dipoles is tracked by unexcited surface electrodes used as sensors , these data may be used to correct the drift on each driven dipole . this is best done by using the surface electrode patches as sensors in the time slices when they are un - driven . two exemplary methods are discussed herein to account for and correct any potential drift . the first method assumes that the bio - impedance changes are essentially homogeneous . thus all driven dipoles change by the same percentage and averaging of all the patch data returns a single impedance index . the second method does not make this assumption and computes an index for each axis . a first method for biologic impedance scaling assumes that any biologic impedance changes are essentially homogenous within the body , that all dipoles change by the same percentage , and that averaging of all surface electrode data will provide a single , accurate impedance index . the method uses the neck , left , right , chest , and back surface electrode patches as sensors when they are not being driven . the left leg electrode is not generally used for sensing because sensed potentials on it tend to be very small . for pairings of the driven surface electrode excluding the left leg , there are thus three potential sense patches that contribute data . in the case of driven pairings including the left leg , four surface electrode patches are available as sensors . the mean of the absolute values of the measured impedance at the surface electrode sensors is obtained continuously as a function of time . this mean value may be denoted as pm ( t ). immediately after the a study begins , the initial value of pm ( t ) is saved as an initial value pa . on every subsequent sample , all of the measurement electrode data is multiplied by the ratio pa / pm ( t ). the three dimensional impedance measurements are thereby scaled to account for any drift over time . to illustrate , a study begins and , assuming there are no errors due to unconnected surface electrodes , the surface electrodes are each sensed during their un - driven phases and averaged to yield a pa of 10 . 0 ohms , which is saved in the software on the computer 20 . assume an important site is marked with a measurement electrode on a catheter in the heart at a site with the impedance coordinates of ( 1 . 0 , 2 . 0 , 10 . 0 ) ohms with respect to a belly patch reference electrode 21 . these impedance coordinates are translated for the user using a nominal scale factor to yield positional coordinates of ( 25 , 50 , 400 ) millimeters . suppose , due to a saline drip or other factors , an hour later the patient &# 39 ; s biologic impedance has lowered by 2 %. without compensation , a return visit by a catheter electrode to the marked anatomic site would now show up at coordinates of ( 24 . 5 , 49 . 0 , 392 . 0 ) millimeters , a drift of 8 millimeters . however , the same biologic impedance drift will be registered on the data sensed by the surface electrodes . for example , pm ( t ) will now read 9 . 8 ohms . therefore , scaling the catheter electrode coordinate data by pa / pm ( t ), or 1 . 0204 , will ensure the catheter electrode is positioned at its original and correct location . a second method for scaling biologic impedance shifts according to the present invention is generally the same as in the prior embodiment , except that separate scale factors are maintained for each axis or pair of driven electrode dipoles . while it is generally found that biologic impedance drifts are essentially homogeneous , this method may be preferred in the event that biologic impedance drifts are not homogeneous throughout the measurement space . in this instance a value , pm ( i , t ), the mean of the absolute values of the measured impedance of each of the dipole combinations as a function of time , can be calculated . in this method , pm ( i , t ) for each dipole pair are computed , where “ i ” is the dipole number , or “ impedance number ,” by averaging the impedance measurements at the surface electrodes on a dipole basis . notably , not all of the undriven surface electrodes need be averaged to optimally determine the axial impedance for each dipole . thus , the initial value of patch impedance , pa ( i ), and subsequent values of patch impedance , pa ( i , t ), may be measured for each dipole by averaging only the electrode data per the table of fig3 for that dipole . as shown in fig3 , the measured impedance for a first dipole xa - ya ( 0 ) between the left electrode and the back electrode may be obtained by averaging the right electrode ( xb ) and the neck electrode ( za ) data . this dipole is depicted in fig2 a . similarly , the measured impedance for a second dipole xa - yb ( 1 ) between the left electrode and the chest electrode may also be obtained by averaging the right electrode ( xb ) and the neck electrode ( za ) data . this dipole is depicted in fig2 b . the measured impedance for a third dipole xb - ya ( 2 ) driven between the right electrode and the back electrode may be obtained by averaging the left electrode ( xb ) and the chest electrode ( yb ) data . this dipole is depicted in fig2 c . the measured impedance for a fourth dipole xb - yb ( 3 ) driven between the right electrode and the chest electrode may be obtained by averaging the left electrode ( xb ), the back electrode ( ya ) and the neck electrode ( yb ) data together . this dipole is depicted in fig2 d . the measured impedance for a fifth dipole za - ya ( 4 ) driven between the neck electrode and the back electrode may be obtained by averaging the left electrode ( xa ) and the right electrode ( xb ) data together . finally , the measured impedance for a sixth dipole is zb - ya ( 5 ) driven between the left leg electrode and the back electrode may be optimally obtained by recoding data for the chest electrode ( xa ) only . it should be noted that while biologic impedance scaling will have the greatest effect when correcting impedance changes when a body surface electrode is used as a reference electrode , such scaling may also be applied to cases where an intra - cardiac reference electrode is used . there is simply less error to correct for in the latter case . although various embodiments of this invention have been described above with a certain degree of particularity , or with reference to one or more individual embodiments , those skilled in the art could make numerous alterations to the disclosed embodiments without departing from the spirit or scope of this invention . all directional references ( e . g ., proximal , distal , upper , lower , upward , downward , left , right , lateral , front , back , top , bottom , above , below , vertical , horizontal , clockwise , and counterclockwise ) are only used for identification purposes to aid the reader &# 39 ; s understanding of the present invention , and do not create limitations , particularly as to the position , orientation , or use of the invention . connection references ( e . g ., attached , coupled , connected , and joined ) are to be construed broadly and may include intermediate members between a collection of elements and relative movement between elements unless otherwise indicated . as such , connection references do not necessarily infer that two elements are directly connected and in fixed relation to each other . it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative only and not limiting . changes in detail or structure may be made without departing from the basic elements of the invention as defined in the following claims .
a method for scaling the impedance measured during the course of an electrophysiology study accounts for impedance drifts . by scaling the impedance there is greater assurance that previously recorded positional information can be used to accurately relocate an electrode at a prior visited position . the scale factor may be based upon a mean value across several sensing electrodes . alternatively , the scale factor may be calculated specifically with respect to an orientation of a dipole pair of driven electrodes .