the following description is of the best mode presently contemplated for carrying out 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 determined with reference to the claims . from fig1 it is apparent that stimulator 10 comprises a case 12 and header 14 . the heart stimulator 10 is connected to three electrode leads , namely a right ventricular electrode lead 16 , a right atrial electrode lead 18 and a left ventricular electrode lead 20 . the left ventricular electrode lead 20 is designed to pass through the coronary sinus of heart 22 . a typical electrode suitable for use with heart stimulator 10 is the electrode lead corox + up / bb ( biotronik ). left ventricular electrode lead 20 comprises a left ventricular tip electrode 24 at the distal end of the left ventricular electrode lead 20 and a left ventricular ring electrode 26 . atrial electrode lead 18 comprises a right atrial tip electrode 28 at the distal end of the right atrial electrode lead 18 and a right atrial ring electrode 30 . the right ventricular electrode lead 16 comprises right ventricular tip electrode 32 at the distal end of the right ventricular electrode lead 16 and a right ventricular ring electrode 34 . in order to illustrate that heart stimulator 10 may be adapted to act as an implantable cardioverter / defibrillator ( icd ) ventricular electrode lead 16 also includes a ventricular shock coil 36 for the delivery of defibrillation shocks to the right ventricle 38 of the heart 22 and an atrial shock coil 40 for the delivery of atrial defibrillation shocks to a right atrium 42 of the heart 22 . each electrode and shock coil of electrode leads 16 to 20 is separately connected to an electric circuit enclosed by case 12 of heart stimulator 10 by way of electrical contacts of a plaque ( not shown ) at the proximal end of each electrode lead 16 to 20 and corresponding contacts ( not shown ) in header 14 of heart stimulator 10 . right atrial shock coil 40 is connected to right atrial shock generator 50 ( see fig2 ) that is controlled by a control unit 52 of heart stimulator 10 . similarly right ventricular shock coil 36 is connected to a right ventricular shock generator 54 that is also connected to control unit 52 . right atrial tip electrode 28 and right atrial ring electrode 30 are both connected to right atrial stimulation pulse generator 56 and to a right atrial sensing stage 58 that are in turn connected to control unit 52 . right atrial stimulation pulse generator 56 is adapted to generate atrial stimulation pulses of sufficient strength to cause an excitation of atrial myocardium by an electrical stimulation pulse delivered via right atrial tip electrode 28 and right atrial ring electrode 30 . preferably , means are provided to adapt the right atrial stimulation pulse strength to the stimulation threshold . right atrial sensing stage 58 is adapted to pick up myocard cardial potentials indicating an intrinsic atrial excitation that corresponds to a natural atrial contraction . by way of right atrial sensing stage 58 , it is possible to stimulate the right atrium 42 of heart 22 in a demand mode wherein a right atrial stimulation pulse is inhibited if an intrinsic atrial event ( intrinsic atrial excitation ) is sensed by right atrial sensing stage 58 prior to expiration of an atrial escape interval . in a similar manner , right ventricular ring electrode 34 and right ventricular tip electrode 32 are connected to a right ventricular stimulation pulse generator 60 and to a right ventricular sensing stage 62 that in turn are connected to control unit 52 . by way of right ventricular tip electrode 32 , right ventricular ring electrode 34 , right ventricular stimulation generator 60 and right ventricular sensing stage 62 , right ventricular stimulation pulses can be delivered in a demand mode to the right ventricle 38 of heart 22 . in the same way left ventricular tip electrode 32 and left ventricular ring electrode 26 are connected to the left ventricular stimulation pulse generator 64 and the left ventricular sensing stage 66 that are in turn connected to control unit 52 to allow for stimulation of a left ventricle 70 of heart 22 . triggering and inhibition of delivery of stimulation pulses to the right atrium , the right ventricle or the left ventricle is controlled by control unit 52 , in a manner known to the skilled artisan . the timing that schedules delivery of stimulation pulses if needed is controlled by a number of intervals which may at least partly depend on a hemodynamic demand of a patient that is sensed by means of an activity sensor 72 connected to control unit 52 . activity sensor 72 allows for rate adaptive pacing wherein a pacing rate ( the rate of consecutive ventricular stimulation pulses for a duration of consecutive atrial stimulation pulses ) depends on a physiological demand of a patient that is sensed by the activity sensor 72 . details of rate adaptation are known to the skilled artisan and need not to be explained in detail in this description . whereas an actual stimulation rate determines the timing from one ( paced ) heart cycle to another , intervals like an atrioventricular delay interval and an interventricular delay interval determine the timing within one heart cycle . starting with an atrial event , the right ventricle would be excited ( either intrinsically or paced ) at the end of an atrioventricular delay interval . a left ventricular contraction should follow the right ventricular contraction at the end of an interventricular delay interval . this includes the case wherein the right ventricle and the left ventricle are excited at the same time , resulting in an interventricular delay interval duration of zero . also , it is possible that the left ventricle may be excited prior to the right ventricle resulting in an negative interventricular delay interval duration . in any case , the atrioventricular delay interval duration and the interventricular delay interval duration need to be adapted to an individual heart in order to achieve an optimized cardiac output . heart stimulator 10 is adapted to determine an optimal atrioventricular delay interval duration and an optimal interventricular delay interval duration automatically . for this purpose , heart stimulator 10 includes an impedance determination unit 74 that includes a constant current source 76 and a voltage measuring unit 78 . the constant current source 74 generates a pulsed biphasic current that comprises pairs of current pulses that have an alternating polarity and the same absolute current strength . an impedance determination stage 80 calculates an impedance value based on the current strength and the voltage measured . as will be appreciated by the skilled artisan , instead of measuring the impedance , the inverse value of the impedance , equivalent values such as male conductivity or conductance , respectively , can be used throughout the invention in an analogous manner taking into account the inverse relationship . control unit 40 is adapted to determine a quality factor that represents the efficiency of biventricular pacing . in order to determine the quality factor , impedance measurements are performed while applying a transient pacing protocol controlled by the control unit 40 . the transient pacing protocol causes the control unit to switch several times between intrinsic ventricular rhythm ( no triggering of stimulation pulses for a number of n intrinsic heart cycles ) and biventricular paced rhythm . the number of consecutive paced cycles , n paced , is preferably smaller than 10 cycles ( see below ). the number of consecutive cycles in intrinsic rhythm , n intrinsic , is larger than the number of consecutive cycles in biv paced rhythm , n paced , preferably by a factor of 2 to 5 . ( for example , where n paced = 5 , n intrinsic = 15 , a transient protocol may take the form : 5 cycles paced — 15 cycles intrinsic — 5 cycles paced — 15 cycles intrinsic — . . . ). the number of transient repetitions , n transition , ( i . e . how often the rhythm changes from intrinsic to biv stimulation ) needs to be large enough that the total number of biv paced cycles is sufficient to cover several respiration cycles ( in order to be able to remove the effect of respiration on the impedance curve by averaging ), preferably 4 to 16 repetitions . to achieve an implicit avd and / or vvd optimization , the control unit is adapted to perform the described transient pacing protocol at each lv lead site several times for a set of atrioventricular delays ( all shorter than intrinsic conduction ) and take the best quality factor found as quality factor for that position . since this represents an implicit avd - optimization for each test - site , the avd values corresponding to the best quality factor can be memorized for each lv lead position so that they may be used for programming the avd after final lead positioning . similarly , the control unit is adapted to perform the described transient pacing protocol at each lv lead site several times for a set of interventricular delays ( vvd &# 39 ; s ) and take the best quality factor found as quality factor for that position . since this represents an implicit vvd optimization for each test - site , the vvd values corresponding to the best quality factor can be memorized for each lv lead position so that they may be used for programming the vvd after final lead positioning . both avd and vvd variation are combined by first varying the avd , then using the avd with the best quality factor and performing the pacing protocol again with variation of the vvd . alternatively , in a more simple embodiment , both the atrioventricular delay ( avd ) and the interventricular delay ( vvd ) can be held constant . then , the atrioventricular delay ( avd ) is sufficiently shorter than the intrinsic atrioventricular ( av ) conduction time , preferably about 60 % of intrinsic av conduction time . the interventricular delay ( vvd ) preferably is about 0 ms . during performing the the transient pacing protocol either sinus rhythm is used throughout ( the right atrium is not paced by setting the basic rate of the pacemaker below the sinus rate ), or , preferably , the atrium is stimulated throughout the transient protocol at a constant rate ( 5 - 30 ppm ) sufficiently above the sinus rate in order to prevent a take - over by the intrinsic rhythm . for each intrinsic to biv paced transition , two averaged impedance curves are calculated : one from the n paced cyles of the biv paced rhythm after the transition , and one from the n reference intrinsic cycles immediately before the transition thus , a number of ( n intrinsic − n reference ) intrinsic cycles following the biv paced cycles are discarded . as a preferred example , n paced = 5 , n intrinsic = 15 , n reference = 5 ( i . e ., 10 intrinsic cycles are not included in the analysis ). the control is further adapted to calculate averaged impedance curves ( z - curves ) while performing the transient pacing protocol as follows : for calculating averaged impedance curves , the ventricular event from the iegm or pacemaker timing is used for synchronization . this results in n averages = n transitions pairs of averaged impedance curves . alternatively also n group = 2 or 4 subsequent transitions may be grouped for the calculation of averaged impedance curves ( resulting in n averages = n transitions / 2 or n transitions / 4 pairs of averaged impedance curves ). additionally the resulting averaged curves may be processed using the following methods : smoothing removal of known technically caused artifacts removal of artifacts caused by the ventricular iegm curve fitting the preprocessed z - curves are analyzed and one or several parameters are extracted , that characterize certain attributes of the impedance curve ( separately for the naverages bivpaced curves and intrinsic curves ). preferably a subset ( that is needed for calculation of the quality factor ) from the following list of parameters is calculated for both the intrinsic and the biv impedance curves of each intrinsic - biv pacing transition : edz : end diastolic impedance ( minimum impedance in diastolic time window , may also be approximated by the minimum impedance or even by z mean ) esz : end systolic impedance ( maximum impedance in systolic time window , may also be approximated by the maximum impedance ) sz : esz - edz ( may also be approximated by the impedance amplitude or standard deviation , the maximum or minimum or maximum absolute impedance slope , or another quantity characterizing the impedance variation width ) these parameter values are used by the control unit to determine the quality factors by calculating the ratio ( quotient ) of these parameter values during both rhythms . preferably the quality factor is calculated by one of the following methods : alternatively the corresponding quantities may be calculated using the conductivity y = 1 / z ( taking care that in the quality factors described in the first two lines of the table numerator and denominator change places , and ef = sy / edy or sy / y mean ). the quality factor is now calculated separately for each intrinsic - bivpacing transition ( or each group of intrinsic - bivpacing transitions ), resulting in n averages values for the quality factor . the quality factors from all transition groups are then averaged to form the one resulting quality factor for the current lv lead position ( and the current avd and vvd values ). a schematic overview of the impedance analysis ( for n intrinsic = 15 , n reference = 5 , n paced = 5 , n transitions = 8 , n group = 2 , n averages = 4 , extracted parameter = sz ) is shown in fig3 . alternatively , the parameter values may be first averaged over all intrinsic - bivpacing transitions , and then the quality factors may be calculated from the averaged parameters for intrinsic and for bivpaced rhythm . this approach needs less computing power ; since only one division is required for the quality factor calculation . although an exemplary embodiment of the present invention has been shown and described , it should be apparent to those of ordinary skill that a number of changes and modifications to the invention may be made without departing from the spirit and scope of the invention . in particular , it is possible to implement the features of the claimed transceiver unit into state of the art implantable medical devices such as implantable pacemakers or implantable cardioverter / defibrillator . this invention can readily be adapted to such devices by following the present teachings . all such changes , modifications and alterations should therefore be recognized as falling within the scope of the present invention .
a device is connected to electrode leads which performs intracardiac impedance measurements , conducts a transient pacing protocol , analyses the impedance measurements , and generates an lv lead position quality factor . the transient pacing protocol includes a repeated change between ventricular intrinsic rhythm and biventricular paced rhythm and may also include a variation of the atrioventricular delay and / or the interventricular delay . the quality factor expresses the degree to which hemodynamic properties have improved due to biv stimulation for the current lv lead position compared to intrinsic ventricular rhythm .