the sino - atrial ( s - a ) node comprises a small collection of cells disposed within the upper wall of the right atrium . these cells have a property which distinguishes them from other cardiac cell tissue in that they permit a constant , slow leakage of sodium ions through the cell membrane . with other types of cells , however , the cell membrane ordinarily excludes sodium . referring to figure 1 , the waveform of the membrane potential is illustrated for s - a node cells . it will be observed that when the cells have depolarized , the membrane potential falls to approximately - 90 mv and that as positive sodium ions slowly invade the cell , the membrane potential slowly rises until a threshold level is reached ( numeral 10 ) at which time the cell depolarizes and rises rapidly to a positive potential of approximately 20 mv ( numeral 12 ). at this time , the s - a node cells begin to pump out the sodium , thereby repolarizing over the interval spanned by the descending segment 14 to the &# 34 ; resting potential &# 34 ; 16 where the cycle begins anew . since the cells will be clustered , the resulting signal to be sensed by the pulse generator will be a summation of the individual depolarization . the s - a node cells are highly sensitive to changes in oxygen concentration in the blood , to circulating hormones , to certain drugs as well as to impulses coming from the nervous system . the rate at which the node cells fire ( depolarize ) depends upon the above factors , increasing when the body needs more oxygen or is under stress , and decreasing when at rest . in a healthy heart , the depolarization of the s - a node travels as a wave across the muscle tissue comprising the atrium . upon reaching another specialized collection of cells , namely the a - v node , it is made to fire and sends a delayed response through the bundle of his and through the right and left bundle branches and through all purkinge fibers to cause a coordinated contraction of the ventricular myocardium . where because of disease or other reasons , a block exists in the conduction path of the heart or in the case of rhythm disturbances resulting from congenital disorders or otherwise , the conduction paths of the heart are blocked , the patient would be a good candidate for an implantable pacemaker of the type described herein and which is illustrated generally in fig2 of the drawings . the pacing system is indicated generally by numeral 18 and includes an implantable pacemaker 20 contained within a body - compatible , hermetically sealed container 22 . the electrical circuitry housed within the container 22 has input and output connections contained within a header block 24 into which is fitted the terminal connector ( not shown ) of a pacing lead 26 . the pacemaker can 22 may be implanted at any one of a number of locations within the body in accordance with known techniques and the lead 26 is routed through the vascular system and into the heart . referring next to fig3 there is shown an enlarged view of the distal end portion of the lead assembly 26 which , in fig2 is shown as being enclosed within the circle 28 . the lead 26 is seen to comprise an elongated tubular sheath 30 which is preferably fabricated from a suitable , body - compatible , flexible plastic , such as silastic , polyurethane or any of the other plastic materials commonly used in the fabrication of conventional pacing leads . the tubular sheath 30 is seen to surround first and second conductors 32 and 34 which join to the proximal connector of the lead ( not shown ). the conductor 32 also connects internally to a distal tip electrode 36 which functions as the stimulating electrode . conductor 34 connects internally to a surface ring electrode 38 which functions as a sensing electrode . inserted in the lead body between the tip electrode 36 and the sensing electrode 38 is a porous substrate 40 used for in - vitro culture of mammalian anchorage - dependent cells . in one arrangement , the substrate may comprise mitogenic calcium compounds which are non - toxic to cells . the porous calcium substrate is preferably ring - shaped and will have an irregular or textured surface to increase the surface area available for cell growth . a particular solid substrate suitable for use in the present invention comprises porous hydroxyapatite or tricalcium phosphate forms of calcium phosphate made by compacting granules of such compounds , and non - porous granules or solid bodies of calcium carbonate . such substrates have been found to support cell growth in layers many cells thick rather than the monolayer cell growth exhibited by in - vitro cell culture using different substrates . moreover , it has been found that cells grown in the calcium substrate using an appropriate nutrient growth solution maintain their phenotype , meaning that the cultured cells exhibit the same types of characteristics as the natural cells . the substrate 40 , being porous , becomes ingrown with the sinus node cells as indicated by the enlargement of one small area of the substrate 40 and identified by numeral 42 . referring next to fig4 there is shown by means of a block diagram the circuitry comprising the pacing electronics contained within the housing 22 . it is preferably a microprocessor controlled device including a microprocessor controller 44 having associated therewith a memory 46 for storing various programmable parameters , such as stimulating pulse width , escape interval , sensitivity , etc . when operating in a demand mode , the microprocessor controller 44 is configured to receive input signals on line 48 from a r - wave detector circuit 50 . the input to the r - wave detector circuit comes from the sense electrode 38 , via conductor 34 , in the lead 26 . in the event that the escape interval elapses before a natural r - wave signal is detected , the microprocessor controller 44 triggers the stimulation pulse generator 52 to issue a stimulating pulse . this pulse is delivered through the lead 26 on conductor 32 to the tip electrode 36 which , typically , will be positioned at the apex of the right ventricle . the sinus node cell tissue on the substrate 40 is immersed in the bloodstream and , as such , responds to changes in blood oxygen concentration , catecholamines and other hormones to effectively shift the threshold voltage 10 at which cell depolarization takes place . the lead - mounted cell depolarization signal is picked up by the sensing electrode 38 and fed back over lead 34 to the lsn detector circuit 54 which amplifies and shapes the pulse applied to the microprocessor controller 44 . the microprocessor controller is programmed to compute the time interval between successive lsn depolarization signals to , in turn , adjust the escape interval of the demand pacing circuitry . this , in turn , adjusts the rate at which stimulation pulse generator 52 provides ventricular stimulating pulses to the tip electrode 36 when natural r - wave activity is lacking . persons skilled in the field of biochemistry can readily formulate a culture media for maintaining and growing s - a node cells or their equivalent and to devise additives which will reduce adverse immune reactions to the cell structures on the tip of lead 26 . similarly , the cells will reside in a mechanical structure or capsule that will allow the cells to remain viable and protected from mechanical stresses . this invention has been described herein in considerable detail in order to comply with the patent statutes and to provide those skilled in the art with the information needed to apply the novel principles and to construct and use such specialized components as are required . however , it is to be understood that the invention can be carried out by specifically different equipment and devices , and that various modifications , both as to the equipment details and operating procedures , can be accomplished without departing from the scope of the invention itself .
described herein is a cardiac pacer pulse generator and lead combination in which sinus node cell tissue is harvested or cultured and appropriately affixed to the distal end of the pacing lead to generate electrical depolarization signals whose rate changes with blood - oxygen levels and hormonal activity . the lead includes a sensing electrode for picking up these depolarization signals and applying them to the rate adaptive pacemaker . the pacemaker then functions to produce heart tissue stimulating pulses at a rate dictated by the body &# 39 ; s physiologic demand .