turning now to the drawings , there is shown in fig1 the distal end region of an implantable cardiac pacing lead 16 . devices such as lead 16 typically are inserted intravenously , for example into the subclavian vein or the cephalic vein , and progressively moved toward the heart until the distal end reaches a selected cardiac chamber . with the distal tip positioned at a selected location , the lead proximal end , still outside the body , is maneuvered to implant the distal tip into the endocardium . the implanted lead transmits electrical signals between the selected location in the heart and the lead proximal end , for one or both of two purposes : ( a ) to monitor heart electrical activity at the selected location ; and ( b ) to carry stimulating signals to the selected location from a pulse generator ( not shown ) connected to the lead proximal end . to transmit the electrical signals there is provided an electrical conductor , shown in fig1 as a single wound coil 18 formed of a nickel alloy . the coil provides maximum flexibility for conforming to the vein , with minimal stress on the conductor . at the distal end of the lead is an electrode 20 , electrically and mechanically coupled to coil 18 by a platinum alloy crimp tube 22 . a flexible , dielectric sheath 24 surrounds the coil and crimp tube . the sheath is formed of a biocompatible material such as silicone rubber . a plurality of tines 26 are formed as part of sheath 24 near its distal end , and are employed to secure the lead distal end to the selected myocardial location . electrode 20 is constructed of a platinum alloy , stretched to a thin wire 28 , then crumpled and packed against the distal end of crimp tube 22 . a platinum alloy screen 30 , fastened to the periphery of the crimp tube distal end , maintains the crumpled platinum alloy wire in place . so constructed , electrode 20 is highly porous , for example consisting of approximately twenty percent platinum alloy by volume , the remaining eighty percent being open to receive a therapeutic drug in liquid or solid form , or to permit passage of bodily fluids through electrode 20 . crimp tube 22 is elongate and cylindrical , with a radially outward flange 32 at its distal end to serve as an abutment for sheath 24 and an anchor for screen 30 . a medial wall 34 isolates a distal recess 36 of the crimp tube from a proximal recess 38 . the distal end of conductor coil 18 is retained in proximal recess 38 by a core pin 40 and a crimp 42 in the crimp tube wall , just proximally of a radially enlarged head portion of the core pin . a drug loaded matrix 44 , e . g . a biocompatible silicone adhesive compound impregnated with dexamethasone sodium phosphate or other desired therapeutic drug , is loaded into distal cavity 36 and retained within the cavity by electrode 20 . matrix 44 , while pliable and prior to curing , is loaded manually into the assembled lead . loading can occur through a bore 46 formed through electrode 20 between the distal recess and the lead exterior . in the event of such distal loading , a portion of the drug matrix 44 remains in bore 46 as illustrated . alternatively , a load slot 48 is provided through crimp tube 22 along its side and near its distal end . slot 48 provides a larger opening for loading the matrix , and is particularly useful when the matrix has a high viscosity rendering distal loading difficult or impossible . while lead 16 in fig1 is shown having both bore 46 and load slot 48 , it can be appreciated that typically either of these alone would suffice . a drug loaded matrix in uncured or powdered form could be accommodated , as well , by either the bore or slot . loading an uncured , pliable matrix provides substantially perfect conformation of the resulting cured matrix to any recess shape incorporated , for example a recess having an enlarged proximal diameter , holding the final cured matrix in place . additionally , any potential adhesive characteristics of the matrix can provide bonding to the recess wall . it is not always convenient to load a drug impregnated matrix , uncured , into a crimp tube recess . accordingly , fig2 illustrates a second embodiment lead 56 which permits the use of a matrix consisting of a previously formed , solid plug . an electrically conductive coil 58 and porous electrode 60 are mechanically and electrically connected by a crimp tube 62 . a sheath 64 , provided with tines 66 , surrounds the crimp tube and conductive coil . electrode 60 is constructed of crumpled and packed platinum alloy wire 68 , maintained against the distal end surface of crimp tube 62 by a screen 70 fastened to the crimp tube at a radially outwardly extended flange 72 . a cylindrical recess 74 is formed in crimp tube 62 , open to the proximal end of the crimp tube and spanning substantially the length of the tube . a radially inward extension 76 of the crimp tube at its distal end forms a reduced diameter neck in the recess . a core pin 78 and the distal end of coil 58 are fastened in recess 74 by a crimp 80 . prior to the fastening of coil 58 , a preformed , solid drug impregnated matrix 84 is inserted into recess 74 at the proximal end of crimp tube 62 , and positioned near the crimp tube distal end as shown . following matrix insertion , a silicone rubber plug 86 , cylindrical and with a slightly larger diameter than the interior diameter of the crimp tube , is inserted to a position proximally of matrix 84 . plug 86 forms a substantially moisture - tight seal between the distal portion of recess 74 and the proximal portion of the recess . the proximal loading feature of lead 56 permits the use of pre - manufactured , solid drug matrices . this eliminates the need for preparing the drug matrix during lead assembly , reducing manufacturing costs in that the assemblers of such leads need not be skilled in determining the proper combination of drugs and matrix constituents , or in mixing these elements to create the matrix . fig3 illustrates a third embodiment lead 96 similar in construction to lead 56 in fig2 with similar parts indicated by the same numerals . the major difference between leads 56 and 96 is in the provision of three solid , preformed drug matrices 98 , 100 and 102 in lieu of a single matrix . this arrangement results in an essentially consecutive elution process , whereby elution is substantially complete in each particular matrix before it begins in the next proximal matrix . consequently , the matrices can be impregnated with the same drug , but in progressively decreasing concentration sequentially from the most distal matrix 98 to proximal matrix 102 . the size of each matrix relative to the others may as well be selectively varied , to permit a stepped elution involving selected elution rates and for selected durations . alternatively , the elution rate may be varied by constructing the matrices of different constituents . fig4 shows an inside view of a fourth embodiment facing lead 106 similar in construction to lead 16 in fig1 with like parts given like numerals . electrode 110 includes a crumpled platinum alloy wire 28 contained by a screen 30 , but in contrast to lead 20 includes no bore . also , no load slot is formed in crimp tube 112 . rather , a bundle of elongate and cylindrical preformed and solid drug impregnated matrices are loaded into distal recess 114 prior to fastening of electrode 110 to the crimp tube . three of these matrices are shown at 116 , 118 and 120 . the bundle of parallel matrices enables a simultaneous or concurrent elution of drugs from all of the matrices . as an example , matrix 116 can be impregnated with an anti - inflammatory drug , matrix 118 with an anti - arrhythmic drug , and matrix 10 with an antibiotic . a bundle of numerous matrices , of course , affords flexibility in determining the proportion of various drugs selected , permitting the tailoring of treatment to individual patients without the need to create custom matrix constituent and drug mixtures . fig5 is a forward sectional view of lead 106 of fig4 but showing the lead loaded with a series of toroidal , concentric matrices 122 , 124 and 126 . this arrangement results in simultaneous elution and therefore affords the advantages discussed in connection with fig4 . furthermore , the toroidal configuration provides a convenient means for close packing of drug matrices of different sizes . fig6 shows a fifth embodiment pacing lead 136 similar in construction to the leads in fig2 and 3 , with like parts given like numerals . the principal difference is that recess 74 of crimp tube 62 contains three sequentially arranged toroidal drug impregnated matrices 138 , 140 and 142 . this arrangement of matrices combines characteristics of sequential and simultaneous elution configurations , although elution perhaps is more similar to simultaneous elution . furthermore , the arrangement takes advantage of the greater matrix surface exposure , and therefore more rapid elution , of the toroidal matrix . if the matrices are sufficiently small in outside diameter ( relative to recess 74 ) so that their inside and outside surfaces are exposed to bodily fluids , the internal surface area of each matrix is enlarged , while the external surface is reduced , in the course of drug elution . as these changes in surface area substantially offset one another , the drug elution rate is constant . alternatively , if an increasing rate of drug elution is desired , the matrices are provided with an outside diameter substantially equal to the inside diameter of the recess , to expose essentially only their internal surfaces . then , as drug elution proceeds , the exposed internal surface area available for drug elution increases . this feature of the toroidal structure results in a rate of drug delivery which increases with time , assuming a uniform distribution of the drug throughout the matrix . in this manner , drug delivery can be selectively and gradually accelerated . fig7 illustrates a sixth embodiment pacing lead 146 including a conductive coil 148 and porous electrode 150 connected to one another by a crimp tube assembly 152 , consisting of a distal crimp tube section 154 and a proximal section 156 . a dielectric sheath 158 including tines 160 surrounds the crimp tube assembly and conductive coil . distal crimp tube section 154 includes a radially outwardly directed flange 162 for abutment by sheath 158 and for supporting a screen 164 which maintains crumpled platinum alloy wire 166 in its hemispherical configuration against the crimp tube distal end . a recess 168 is formed through crimp tube section 154 , open to electrode 150 and also permitting the loading of a preformed , solid drug impregnated matrix 170 into recess 168 at the proximal end of crimp tube section 154 . recess 168 is widened near the proximal end of section 154 , and internally threaded . a recess 172 is formed in proximal crimp tube section 156 , for containment of conductive coil 148 and core pin 174 , and is crimped near the core pin as shown at 176 . crimp tube section 156 includes a distal extension 178 externally threaded for engagement with distal section 154 . crimp tube sections 154 and 156 , when separated , permit proximal loading of matrix 170 . when threaded together as shown in fig7 these sections form a crimp tube assembly that prevents fluid flow between recesses 168 and 172 . all of the above - described leads are provided with a highly porous electrode as described in connection with lead 16 , and thus are particularly well suited for drug elution , whether from a matrix or directly from the electrode . first , when used in connection with a drug impregnated matrix as described , the electrode permits bodily fluids to flow substantially freely into and out of the matrix containing recess . consequently , while the electrode may maintain the matrix or matrices within the recess , it does not materially hamper elution . secondly , due to its high porosity , the electrode is well suited for loading with a therapeutic drug in liquid or solid form . the result , particularly as compared to utilizing a drug impregnated matrix alone , is a much more rapid and direct delivery of drug to the location of implant . the graph in fig8 illustrates the beneficial result of loading a porous electrode with a steroid . the upper curve , representing a non - loaded control electrode , illustrates an early peak in stimulation threshold of approximately 1 . 8 volts , followed by a chronic threshold of about 1 . 3 volts . by comparison , the steroid loaded electrode does not exhibit an acute peak in stimulation threshold levels , and maintains a relatively stable chronic threshold level of approximately 0 . 7 volts . in addition to the general reduction in stimulation thresholds , the removal of the early stimulation threshold peak is itself a substantial advantage . this eliminates the need to provide a pacing apparatus with a chronic voltage level set high enough to overcome the initial peak , or alternatively to attempt to program an early voltage peak into the pacing regimen , hopefully matching the actual peak in stimulation threshold . thus , in accordance with the present invention , the porous electrode may be loaded with primarily an anti - inflammation drug to reduce or eliminate an acute peak in stimulation threshold level , while an accompanying matrix or matrices can be tailored to chronic treatment . thus , a cardiac pacing lead constructed in accordance with the present invention permits a selection from among various drugs , drug concentrations and drug elution rates , for sequential or simultaneous administration , tailored to the needs of a particular patient . a silicone plug or two - part crimp tube assembly affords the advantage of proximal loading , enabling the use of preformed , solid drug impregnated matrices .
an implantable cardiac pacing lead includes a porous , platinum electrode , a flexible electrically conductive coil , and a crimp tube coupling the electrode to the distal end of the coil . a recess in the crimp tube , open to the electrode at the crimp tube distal end , houses a matrix impregnated with a therapeutic drug . the electrode is highly porous and loaded with a therapeutic drug in liquid or solid form . immediately upon implant of the electrode at a selected myocardial location , the electrode begins dispensing the therapeutic drug . meanwhile , the matrix begins to elute its drug at a rate more suited to chronic treatment . if desired , the recess can extend through the crimp tube to permit proximal end loading of the drug carrying matrix . a silicone plug , or an additional and proximal crimp tube section , is inserted into the recess after loading of the matrix to provide the required fluid seal . finally , a variety of different matrices can be housed in the recess , to provide elution of different drugs and at differing rates .