referring now to fig1 there is shown a subcutaneous ecg electrode 11 made in accordance with the present invention for chronically deriving electrocardiograms from an active subject . a flexible electrical conductor 12 , some portion of which is covered with insulation 13 to make an electrical lead , is secured to and in electrical contact with pocket - shaped electrically conductive member 14 . the conductor 12 is preferably comprised of multiple strands of stainless steel wire . end 16 of the electrical conductor is adapted to be connected to monitoring equipment such as a high - impedance input amplifier and a recorder ( not illustrated ). at the opposite extremity from end 16 the bare conductor is doubled back on itself to form a loop 17 and the side - by - side segments of the conductor are squeezed together and tightly encircled by a metal crimp band 18 . electrode 14 has an oval cross - section and is made up of substantially parallel and coextensive walls 21 , 22 and curvilinear end sections 23 , 24 ( see fig3 ). pocket - shaped member 14 may be constructed from a flattened tube with a substantially uniform wall thickness . the common practice of using solder and solder flux to make connections to electric leads has been found to cause problems in implants . tissue toxicity responses and deterioration of the connection due to battery potentials that arise from the metal discontinuities and flux chemistry are likely to result when soldered joints are implanted . spot welding and crimping have been found to be better ways of making connections . after member 14 reaches the stage of fabrication where it looks like a squashed metal cylinder , electrical conductor loop 17 is inserted into end 19 of member 14 , end 19 is crimped closed and loop 17 is spot welded to member 14 . for example , the loop may be spot welded to member 14 at points 20 . end 26 of member 14 is left open and forms a mouth or entrance to the pocket so that when the electrode is implanted body fluid may flow into the lumen or chamber 27 ( see fig2 ) and wet the interior surfaces . thus , the body fluid which is an electrolyte , permits electric currents generated by the subject &# 39 ; s heart to reach the inner metal walls of pocket - shaped member 14 . a material which is tolerated reasonably well by tissue and which is suitable for member 14 is stainless steel . silver is less durable and somewhat toxic ; however , it may be used as an alternative . cloth straps 25 are glued to the outer surfaces of electrode walls 21 and 22 near electrode mouth 26 . a second set of cloth straps 28 are glued to the outer surfaces of walls 21 and 22 in the proximity of conductor loop 17 . a polyester cloth such as coarse woven dacron ® has been found to be compatible with tissue . cloth straps 29 are also placed over and normal to straps 28 . these straps are adhered to member 14 , straps 28 and conductor 12 . straps 29 provide stress relief for conductor 12 when forces are exerted on it in the direction away from member 14 . straps 29 also reduce the likelihood of the electrical conductor therebetween kinking and producing a tissue pressure point . once the pocket electrode is in place in a subject , it may be secured to the subject &# 39 ; s tissue by sutures 31 . the exterior surface of member 14 as well as the mouth region , plus the cloth straps and a portion of the electrical conductor adjacent the electrode are covered with a tissue - compatible elastomeric electrical insulation 32 of very high impedance . the preferred insulation is silicone rubber and this material may also be used to glue the cloth straps to electrode 14 . further , silicon rubber may be used as insulation 13 for electrical conductor 12 . when the pocket electrode is implanted in a subject , body fluids touch the exterior surfaces and fill lumen 27 ; however , the body fluids are only permitted electrical contact with the interior walls of member 14 . some of the advantages of the pocket electrode are believed to derive from the creation of an electrode / electrolyte interface that is removed from tissue / electrode disturbances . the portions of the electrode that can be touched by tissue are electrically insulated . lumen 27 can be reached by electrolytes in the form of body fluids , but only by tissue . the mouth of the pocket electrode is relatively large and the exposed electrode surface on the interior of the pocket electrode is smooth and much larger than that of a typical ecg electrode such as a lead loop electrode . accordingly , it is believed that the path of current flux into the lumen is not unduely constricted and thus not easily perturbed , and that any extraneous localized region of cellular or mechanical activity fails to rise to a noticeable level as would be the case with a small conventional ecg electrode . the electrode is easy to clean and to keep clean during surgery . it is not prone to tissue imbedding and thus the electrical performance tends to remain constant with time . the electrode has a thin silhouette with no objectionable protrusions . the electrical lead tends to remain flat where it connects to the electrode . accordingly , the electrode may be subcutaneously accommodated with relative comfort and no fear of unpleasant pressure points or rejection . when implanted , the electrodes should be placed to the side of the skin incision to facilitate revascularization of the skin . the axis of incision when in the anterior - posterior plane minimally disrupts the vascular bed . generally , the larger the size of the electrode , the more the vascularization of the overlying skin is undesirably reduced . in one experiment involving a labrador dog as a test subject , an electrode made in accordance with the subject invention was utilized and it had a wall 21 that was 2 . 5 cm long , 1 cm wide , and 0 . 010 inches thick . the experiment had a duration of three months and when the electrode was examined after the experiment it was noted that no tissue had entered lumen 27 . in vitro tests with two prior art ecg electrodes as well as the present invention were conducted to simulate the in vivo environment . tests were performed on lead loop electrodes , disc electrodes of the type described in u . s . pat . no . 4 , 219 , 027 , and electrodes made in accordance with the subject invention . a pair of each type of ecg electrodes was immersed in an 0 . 9 % saline bath and one electrode was rubbed between two fingers to simulate the electrode / tissue motion encountered in an active subject . the electrodes were coupled to monitoring apparatus with a 10 - megohm input impedance . in one test , the monitoring apparatus included a 10 - 100 hz bandpass filter whereas in another test the filter was removed and the apparatus had d - c response . fig4 a and 4c depict the artifacts generated when the disc electrode was rubbed , and fig4 b and 4d illustrate the artifacts that were produced when the lead loop was rubbed . the bandpass filter was employed when the artifacts of fig4 c and 4d were generated . it is quite evident from the potential plots that the lead loop electrode generates more artifacts , by at least an order of magnitude , than the disc electrode . on the other hand , when a pocket electrode made in accordance with the present invention was rubbed between fingers in the saline solution , neither baseline shifts ( d - c voltages ) nor oscillations ( a - c voltages ) were observed . that is , no artifacts were generated when the electrode perturbed . those skilled in the art will appreciate that the specific structures and methods of operation described herein may be altered without departing from the spirit and scope of the invention .
a low - noise electrode suited for sensing electrocardiograms when chronically and subcutaneously implanted in a free - ranging subject . the electrode comprises a pocket - shaped electrically conductive member with a single entrance adapted to receive body fluids . the exterior of the member and the entrance region is coated with electrical insulation so that the only electrolyte / electrode interface is within the member remote from artifact - generating tissue . cloth straps are bonded to the member to permit the electrode to be sutured to tissue and to provide electrical lead flexure relief .