the following is a description of preferred embodiments in accordance with the present invention . this description is intended for describing the general principles of the invention and is not to be taken in a limiting sense . thus , even though a biventricular heart stimulator for sensing and stimulating in both ventricles is being illustrated , the invention is also applicable to heart stimulators arranged for sensing and stimulating in one ventricle only , and / or for stimulating in the atria of a human heart . furthermore , the invention is also applicable to implantable cardioverter - defibrillators ( icd ). referring first to fig1 , there is shown an implantable heart stimulator 10 in electrical communication with a human heart 1 via two cardiac leads 20 , 30 arranged for stimulation and sensing . moreover , the heart stimulator 10 comprises electronic circuitry and a battery contained within a hermetically sealed pacemaker housing 12 . the housing 12 has a metallic casing of titanium , enclosing the electronic circuitry and battery , and a molded plastic header , comprising connector blocks and apertures for receiving the connectors at the proximal ends of the cardiac leads . the header is attached to the metallic casing in a fixed and sealed arrangement . the electronic circuitry comprises at least one pulse generator for generating stimulation pulses , sensing circuitry for receiving cardiac signals sensed by the cardiac leads 20 , 30 , and a controller . the controller controls both the sensing of cardiac signals and the delivery of stimulation pulses , for instance as to the duration , energy content and timing of the stimulation pulses . the stimulation pulses generated by the pulse generator are transmitted via the cardiac leads 20 , 30 and delivered to the cardiac tissue of the left and right ventricles of the heart , respectively , by the use of tip electrodes 26 , 36 . generally , the tip electrode 26 , 36 acts as the cathode when the cardiac pulse is delivered . furthermore , in unipolar cardiac systems , the casing 12 acts as the anode , while in bipolar cardiac systems , the anode is provided by an annular electrode ( not shown ) arranged on the cardiac lead at a small distance from the tip electrode . it should be noted that even though no annular or ring electrode is illustrated in the greatly simplified drawing of fig1 , the present invention is equally applicable to unipolar and bipolar systems . turning now to fig2 a and 2 b , there is shown an implantable lead 20 in accordance with a preferred embodiment of the present invention . the lead 20 has an inner conductor 21 and an outer conductor 22 , both in the form of helical coils . the inner conductor 21 is connected to the tip electrode 26 and the outer conductor 22 is connected to a piezoelectric pressure sensor 27 , which will be described further below . the two conductors 21 , 22 are coaxially arranged and separated by a flexible inner tubing 23 made of an electrically insulating polymeric material , which in the preferred embodiments is silicone rubber or polyurethane rubber ( pur ). thus , the conductors 21 , 22 are completely separated and electrically insulated from each other . furthermore , an outer tubing 24 is co - axially arranged about the outer conductor 22 for separating and insulating the outer conductor 22 from surrounding tissue 2 , generally blood . furthermore , an electrically insulating sleeve 28 is arranged between the sensor 27 and the tip electrode in order to provide the distal portion of the lead 20 with a uniform diameter . in this embodiment , the outer tubing 24 and the distal sleeve 28 is also made of silicone rubber or pur . moreover , the surfaces of the inner tubing 23 and the outer tubing 24 facing the inner and outer conductors 21 , 22 , respectively , are provided with electrically conductive surface layers 25 . thereby , no portions of the conductors 21 , 22 are in direct contact with the electrically insulating portions of the tubing 23 , 24 , and there will be no accumulation of electrical charges due to motion artifacts between the conductors 21 , 22 and the tubes 23 , 24 . in one preferred embodiment , the electrically conducting layers 25 are provided by coating the corresponding surfaces of the polymeric tubing 23 , 24 with graphite . the graphite layer is provided as a graphite powder . the graphite layers 25 are provided prior to assembling the cardiac lead . in addition to the graphite providing the tubing 23 , 24 with electrically conductive surfaces , the graphite also has lubricant properties . thereby , the friction between the tubing 23 , 24 and the conductors 21 , 22 is reduced , which inter alia facilitates the assembly of the cardiac lead 20 , in particular when the conductors are fed into and through the lumen of the cardiac lead during assembly thereof . as noted above , in the embodiment shown in fig2 a and 2 b , the cardiac lead 20 has two helically wound conductors 21 , 22 . however , even though the lead comprises dual conductors , the lead 20 is a unipolar lead . this is due to the fact that the term “ unipolar ” only refers to the electrodes or “ poles ” that are being used for the delivery of cardiac pulses . in the present embodiment , the outer conductor 22 is arranged for transmitting electrical signals between the pressure sensor 27 and the heart stimulator 10 , and not for delivering stimulation pulses to an electrode positioned in the human heart . accordingly , in the presently described embodiment , there is only one stimulating electrode 26 located in the human heart and the disclosed lead is a unipolar lead . however , the invention is of course not limited to unipolar leads . on the contrary , the cardiac lead could be provided with further conductors connected to further sensors and / or stimulating electrodes . in particular , a further conductor connected to a ring electrode provided for delivering stimulation pulses is contemplated . the pressure sensor 27 is coaxially arranged about the inner tubing 23 distally of the outer conductor 22 , which is connected to the sensor 27 via a sensor electrode ( not shown ) provided on the sensor . in this embodiment , the sensor 27 is a piezoelectric pressure sensor of the type shown in the international application published under no . wo 02 / 34130 , which hereby is incorporated herein by reference . the sensor 27 comprises a piezoelectric element disposed coaxially around a rigid supporting structure . changes in pressure acting on the piezoelectric sensor 27 results in transport of electrical charges across the sensor , which would cause small changes in the voltage across the sensor . however , since the sensor 27 is connected to a charge amplifier 14 , contained within the stimulator housing 12 , these charges are conducted to the charge amplifier 14 . the charge amplifier 14 absorbs the charges and keeps the voltage across the sensor 27 constant . furthermore , the charge amplifier 14 outputs a voltage signal corresponding to the transported charges . thus , in order for the transport of electrical charges across the sensor 27 to be transmitted to the charge amplifier , two conductors , electrically insulated from each other , are connected to electrodes provided on the sensor and to the inputs of the voltage amplifier . a first electrode ( not shown ) of the sensor is connected to the outer conductor 22 of the cardiac lead 20 . furthermore , a second electrode ( not shown ) of the sensor 27 is electrically connected via the sleeve 28 to the tip electrode 26 , and thereby to the inner conductor 21 . thus , the inner conductor 21 is arranged for transmitting stimulation pulses from the stimulator 10 to the tip electrode 26 , as well as for transmitting electrical signals from the sensor 27 to the stimulator 10 . in this embodiment , the sleeve 28 is provided with an electrical conductor , connecting the tip electrode with the sensor electrode , and insulated from the surrounding tissue . also , the sensor 27 is provided with an outer silicone rubber layer in order to insulate the sensor 27 from the surrounding tissue 2 , in particular the electrodes of the sensor 27 . this embodiment will be further described below in relation to fig5 . however , in an alternative embodiment , the sensor 27 is in direct contact with the surrounding blood or tissue 2 . thus , the second electrode ( not shown ) is provided on the circumference of the sensor 27 such that an electrical contact with the metal casing of the stimulator housing 12 is established via the surrounding tissue . then , there is no need for providing the sleeve 28 with an electrical conductor . accordingly , the sleeve 28 is merely made in an insulating material , ensuring that there is no electrical connection between the sensor 27 and the tip electrode 26 . as readily understood by the person skilled in the art , such a circumferential electrode facing the surrounding tissue must be provided in a material which is biocompatible . this embodiment will be further described below in relation to fig4 . turning now to fig3 , there is illustrated a cardiac lead 40 according to an alternative embodiment of the present invention . in this embodiment , the cardiac lead 40 comprises a silicone rubber tubing 44 provided with a plurality of circumferentially spaced lumina 41 , as well as a central lumen 43 arranged at the axial centre of the lead 40 . the central lumen 43 is arranged for receiving a guide wire or stylet , which is used for guiding and controlling the cardiac lead during implantation thereof . following implantation , the guide wire is removed from the cardiac lead . alternatively , one of the circumferentially arranged lumina 41 could be used for receiving a guide wire . in the shown embodiment , conductors 42 are provided within the circumferentially arranged lumina 41 , wherein two of the conductors 42 are connected to a pressure sensor 27 . in the same manner as is described above in relation to fig2 a and 2 b , the surfaces of the lumina 41 are provided with electrically conducting surface layers 45 . in the preferred embodiments , the surface layers 45 are provided by coating the tubing surfaces with graphite . thereby , the friction between the conductors 42 and the tubing 44 is reduced and assembly of the lead is facilitated in the manner as described above . turning now to fig4 and 5 , there is illustrated in simplified form a circuit diagram of the charge amplifier and the sensor . first , a single conductor system is shown in fig4 . the system comprises a piezoelectric pressure sensor 27 , a conductor 22 located within a cardiac lead , and a charge amplifier 14 . one input of the charge amplifier 14 is connected via the conductor 22 to a first electrode of the sensor 27 . furthermore , the other input of the charge amplifier 14 is connected to a second electrode of the sensor 27 , via a metallic , electrically conducting portion of the stimulator housing 12 and intervening blood and tissue . thus , the respective inputs to the charge amplifier 14 are connected across the sensor 27 . thereby , changes in pressure acting upon the sensor results in transport of electrical charges across the sensor 27 . the charge amplifier 14 receives the transported charges , maintains the voltage across the sensor at a constant level , and outputs an amplified voltage signal corresponding to the transport of charges and variations in pressure to control circuitry . with reference to fig5 , a two conductor system is illustrated . this system corresponds to the system illustrated in fig4 , except for the fact that the inputs of the charge amplifier 14 are both connected to the sensor 27 via conductors 21 , 22 , respectively , located within the cardiac lead . thus , the charge amplifier 14 operates in similar manner to output a voltage signal corresponding to pressure changes acting on the piezoelectric pressure sensor 27 . it should be noted that even though the present invention has been described above in relation to exemplifying embodiments thereof , alterations and modifications may be made within the scope of the invention , as defined in the accompanying claims . for instance , the invention is not restricted to a specific method of providing the polymeric tubes or tubing with an electrically conductive surface layer . a variety of different suitable methods could be used , depending on the chosen conducting material , as understood by the person skilled in the art . furthermore , different methods could be used for different surfaces . for instance , one method could be most appropriate for the inner surface layers , while another method is more appropriate for the outer surface layers . likewise , the material for providing the electrical conductivity of the surface layers is by no means limited to graphite . a variety of suitable materials are contemplated within the scope of the present invention . moreover , different materials could be used for the different surfaces within the same cardiac lead . although modifications and changes may be suggested by those skilled in the art , it is the intention of the inventors to embody within the patent warranted heron all changes and modifications as reasonably and properly come within the scope of their contribution to the art .
an implantable lead for sensing mechanical activity of a human heart has an insulating polymeric tube extending from a proximal end to a distal end of the lead , an electrical conductor provided in the lumen of the polymeric tube , and a sensor connected to the conductor at the distal end thereof . the polymeric tube is provided with a conductive surface layer along the inner face between the polymeric tube and the electrical conductor , the conductive surface layer being in electrical contact with this conductor . accumulation of electrical charges between the electric conductor and the polymeric tube is thereby prevented .