with regard to its software aspects , the present invention can be implemented by suitable programming of the control software of known pacemakers . the invention can in particular be applied to the implantable devices marketed by ela médical , montrouge , france , such as the symphony and rhapsody branded devices . these are programmable devices with memory and microprocessors comprising circuits for receiving , formatting , and processing electric signals collected by implanted probes bearing cardiac electrodes , and delivering stimulation impulses to these electrodes . it is possible to transmit to the device by telemetry the software , which will be stored and carried out to implement the functions of the invention described below . adaptation of these apparatuses and creation of suitable software programming to implement the functions of the inventions described herein are deemed to be within the abilities of a person of ordinary skill in the art and a matter of design choice and will not be described in detail herein . in fig1 , reference number 10 indicates , generally , an intracardiac bio - impedance measuring circuit , incorporated in the circuit of the implanted generator . reference number 12 indicates an endocavitary probe implanted in the myocardium 14 and comprising , in the illustrated example , an atrial electrode 16 , a proximal ventricular electrode 18 , and a distal ventricular electrode 20 . for measurement of the intracardiac impedance ( in this example a trans - valvular impedance ), the device injects by means of generator 22 a current i inj between the atrial electrode 16 and the distal ventricular electrode 20 . a corresponding voltage v in is then collected between the atrial electrode 16 ( which is thus an electrode common to both the injection and the collection ) and the proximal ventricular electrode 18 . this voltage is amplified by an amplifier 24 and digitized by an analog / digital converter 26 , to give a coded value representative of the voltage v in . this measurement technique in itself is known and therefore will not be described more in anymore detail . one will note that the described technique is only one example of an intracardiac configuration of bio - impedance measurement . the first stage ( in itself known ) of treatment of the signals consists of evaluating the impedance starting from the values of the injected current i inj and of the collected differential voltage v in . this impedance is given by the relation : z = k 1 * ( v in - k 3 ) i inj - k 2 ( k 1 being the factor of proportionality between the input voltage and a step of converter 26 ( expressed in μv ), k 2 being the internal impedance of the implant , symbolically illustrated as 28 by resistance r ( expressed in ohms ), and k 3 being the shift value , i . e ., the value delivered by converter 26 for a null voltage v in in input ( expressed in step of code ). to determine the three constants k 1 , k 2 , and k 3 , it is necessary to operate beforehand three measurements with different values i inj1 and i inj2 from the injection current and values different from the impedance between the terminals . this calibration can be carried out , for example , by substituting at probe 12 a resistance 30 of a fixed known value r 0 or r 1 , one of the two values being able to be for example r 0 = 0 ( short - circuiting the input ). calibration can be carried out for each implant , during a test automated on a production line , the specific values of the constants k 1 , k 2 , and k 3 being memorized in the implant after being determined : k 1 is the drift of the input voltage for step i of the converter , expressed in μv by step of code : k 2 is the internal impedance of the implant , expressed in ohms : k 3 is the value of exit of the converter for a voltage of null input ( expressed in step of code ): in the alternative , it is possible to carry out a calibration only for some representative samples of implants , to determine an average value for the constants k 1 , k 2 , and k 3 , and to then apply these constants to all implants . this last manner of proceeding is less precise , but appears in practice sufficient for most current needs , except for some research protocols requiring a higher precision . the following stage , characteristic of the present invention , concerns determining the absolute dynamic value of the endocavitary volume starting from the impedance signal thus obtained . in the first embodiment , volume ( vol ) is given starting from a linear volume / impedance characteristic , i . e ., a relation of the general form vol =( a * z )+ b . to determine the two constants a and b , a calibration is carried out starting from two initial known points , determined for each patient , for example , the systolic volume vol 1 and diastolic volume vol 2 , determined by echography . these two volume values vol 1 and vol 2 , correspond to impedance values z 1 and z 2 , as follows : in the second , preferred , embodiment , the volume is given starting from a hyperbolic volume / impedance characteristic , i . e . a relation of the general form vol = a /( z + b ) or , in other words : the two constants k 4 and k 5 are obtained starting from initial measurements obtained for each patient : k 4 is the constant of proportionality of volume , expressed in milliliters per ohm : k 5 represents the level of the asymptote for a volume tending towards infinite , expressed in ohms : fig2 shows the results of an estimate of the volume carried out in the way indicated above , either by a linear conversion ( shown by dashes ), or by a hyperbolic conversion ( shown by full lines ). the squares represent the points of measurement of a test carried out in vitro on a dog heart , documented in the article by raul chirife titled “ intracardiac impedance for hemodynamic assessment ,” cardiac pacing , rome ( 2004 ). as one can see , the value of the volume estimated by using a hyperbolic conversion is extremely close to physical reality , though a linear conversion , which is simpler to implement , can also give satisfactory results when less precision is required , or when the range of variation of volume is relatively small that the linear characteristic will suffice . fig3 illustrates , as for fig2 , the results of an estimate of volume carried out in the way indicated above , by a hyperbolic conversion , for two values d = 22 mm and d = 44 mm of the diameter of a test chamber made of silicone and filled with a saline solution , compared to : ( i ) on the one hand the in vitro values actually measured ( squares and stars ) and , ( ii ) on the other hand the values resulting from a digital simulation by a finite elements analysis ( diamonds and triangles ). the measured and estimated comparative values are documented in the article of k . hoekstein and g . f . inbar , titled “ cardiac systolic volume estimate ,” ee pub . no 974 , february 1994 , technion , israel . analysis of this figure shows the relevance of a volume / impedance conversion realized according to the teachings of the invention , particularly of a conversion of a hyperbolic type . one skilled in the art will appreciate that the present invention can be practiced by other than the described embodiments , which are presented for purposes of illustration and not of limitation .
an active implantable medical device having a plurality of connection terminals able to be connected to electrodes placed in at least three distinct respective sites in a myocardium ; circuits for measuring an intracardiac bio - impedance , comprising circuits for injecting a current and circuits for collecting a voltage at respective poles of a configuration of said connection terminals , and circuits able to deliver at an output a dynamic impedance signal that is a function of the injected current and the collected voltage ; and circuits for evaluating an intracardiac volume , receiving at in input the impedance signal and delivering at an output a dynamic value of volume representing an instantaneous absolute value of intracardiac volume .