Temporary Medical Electrical Lead

The present invention relates to the field of cardiac stimulation and specifically to the field of temporary stimulation of cardiac tissue through a medical electrical lead.

Atrial arrhythmias and supra ventricular tachycardias, such as atrial fibrillation, atrial flutter and atrio-ventricular reentries, are a common postoperative complication among patients who have had heart surgery. See, for example, Cardiac Surg. Kirklin J W, Barrat-Boyes B C (Eds.): NY 1993, pg. 210. During the first 10 days after heart surgery it is estimated postoperative supra ventricular tachycardia occurs in up to 63 percent of patients. See, for example, "The Importance of Age as a Predicator of Atrial Fibrillation and Flutter After Coronary Artery Bypass Grafting", Leitch et al., J. Thorac. Cardiovasc. Surg., 1990:100:338-42; "Atrial Activity During Cardioplegia and Postoperative Arrhythmias", Mullen et al., J. Thorac. Cardiovasc. Surg., 1987:94:558-65.

The presence of these arrhythmias, which in an otherwise healthy patient may not be unduly serious, may be especially harmful to heart surgery patients. The hemodynamic condition of these patients is often already compromised by either the surgery itself or the effects of prolonged anaesthesia or both. Supra ventricular tachycardias may further cause a very irregular ventricular rate which may even further deteriorate their hemodynamic condition. Such further deterioration is especially serious for patients with a compromised left ventricular function. These complications may present a serious impediment to the recovery of the patient. See, for example, "Maintenance of Exercise Stroke Volume During Ventricular Versus Atrial Synchronous Pacing: Role of Contractility", Ausubel et al., Circ., 1985:72(5):1037-43; "Basic Physiological Studies on Cardiac Pacing with Special Reference to the Optimal Mode and Rate After Cardiac Surgery", Baller et al., Thorac. Cardiovasc. Surg., 1981:29:168-73.

Due to the serious and potentially life threatening nature of these conditions, postoperative treatment is often aimed at preventing arrhythmias, such as through drugs. Drugs, however, have been found to not always be effective at preventing arrhythmias. Thus it is often necessary to provide a means for terminating any arrhythmias which may occur. One common method used has been through over-pacing.

For example Waldo et al. in "Use of Temporarily Placed Epicardial Atrial Wire Electrodes For The Diagnosis and Treatment of Cardiac Arrhythmias Following Open-Heart Surgery," J. Thorac. Cardiovasc. Surg., 1978, vol. 76, no. 4, pgs. 558-65 discloses the use of a pair of temporary heart wires placed on the atrium to diagnose and treat arrhythmias by antitachy overdrive pacing. Specifically the temporary heart wires were sutured to the atrial wall at the time of the heart surgery. Once the patient was ready to be released the wires were removed by traction or pulling upon their external end.

Temporary postoperative atrial and ventricular pacing with temporary heart wires has been found to successfully treat many of the potential post-operative arrhythmias. As such the procedure has become widespread at least 100,000 procedures per year. Several problems, however, were encountered with the system disclosed by Waldo et al., referred to above. One problem was the stability of the heart wire within the atrial wall. Because the wall undergoes constant motion, the temporary heart wire lead was found to dislodge an unacceptable amount. Secondly, the relatively thin atrial wall, especially on elderly patients, was sometimes torn by traction upon the lead for removal.

An improved method of temporarily affixing heart wires onto the atrium was achieved with the introduction of the Medtronic Model 6500 Temporary Myocardial Pacing Lead System. That lead system featured a silicone atrial fixation disk to fasten the lead to the atrium. Specifically the silicone atrial fixation disk was permanently sutured to the atrium. The lead was positioned so that it was trapped between the disk and the atrial tissue. The lead could thereby be removed by simply pulling it from between the disk and the tissue. The rubber disk remained in the body after removal of the electrodes. The advantages offered by such a fixation system included more reliable lead fixation along with protecting the relatively thin atrial walls from tearing during lead removal. Thus the Medtronic Model 6500 Temporary Myocardial Pacing Lead permitted post-surgical temporary antitachy over-drive pacing to be performed more safely.

In spite of the improved systems or methods to achieve antitachy overdrive pacing it is not, however, always effective in terminating postoperative atrial arrhythmias or supra ventricular tachycardias. When drugs and over-pacing are not effective in the prevention or termination of postoperative supra ventricular tachycardias, or because of main negative inotropic side effects relatively contraindicated, it may become necessary to perform atrial defibrillation, synchronized to the R-wave of the electrogram, to terminate these potentially life-threatening arrythmia. Because of the large energies involved for defibrillation, however, the temporary heart wires could not be used.

External atrial defibrillation, although an effective treatment, has profound side effects. First it should be noted that in contrast to ventricular defibrillation, where conversion to normal sinus rhythm is required at the first shock, atrial defibrillation may be obtained after several shocks because ventricular contraction continues during supra ventricular tachycardia. In addition, due to the high energy required (40 to 360 Joules), the application of shocks, besides their number, is not tolerated well by a conscious patient. Therefore external defibrillation is preferably performed under general anaesthesia or at least sedation. Of course the use of anesthesia gives rise to another risk to the patient.

External defibrillation requires relatively high energy because the electrical source is not positioned directly upon the cardiac tissue but rather must pass through the thorax, which tends to dissipate the energy. In contrast, internally applied atrial defibrillation, such as may occur during surgery through defibrillation paddles placed directly on the heart, requires considerably less energy because the defibrillation electrical energy is applied only to the tissue that needs to be defibrillated. In fact, direct atrial defibrillation may be accomplished with only 1.0 Joule pulses in contrast to the 40 Joule and greater pulses for external defibrillation. See, for example, Kean D., NASPE abs. 246, PACE, April 1992, pt. II, pg. 570.

It should be understood the defibrillation success rate is dependent on the delivered energy. The lower the energy, the lower the success rate and the higher the number of shocks to be applied to obtain defibrillation success. With direct atrial defibrillation, because the energy may be applied directly to the heart, the energy level can be chosen such that both the shock level as well as the number of shocks required may be tolerated by the patient.

It is thus an object of the present invention to provide a temporary atrial defibrillation lead which is capable of providing electrical stimulation pulses of sufficient energy to result in defibrillation at a tolerable level.

It is a further object of the invention to provide a temporary atrial defibrillation lead which may provide sufficient energy to the atrium so as to be tolerated by the patient and therefore delivered without the necessity of general anaesthesia.

It is a further object of the invention to provide a temporary atrial defibrillation lead which may be reliably fixed to the atrium through a fixation pad.

It is a further object of the invention to provide a temporary atrial defibrillation lead which may be safely and reliably removed from the atrium.

It is a further object of the invention to provide a temporary atrial defibrillation lead which may be safely and reliably removed from the atrium without the necessity of a surgical intervention.

In accordance with the above objects there is provided a temporary atrial defibrillation lead featuring a PTFE felt pad in which three parallel stainless steel defibrillation wire electrodes are mounted. The pad contains holes which expose the electrode wires in a discontinuous fashion. The three electrode wires are merged into one polyurethane insulated lead body, proximal to the pad. At the proximal end of the lead body a stainless steel connector pin with break away needle are mounted for the percutaneous exteriorization of the lead pin in an area separated from the surgical incision. The break away needle can be broken off to make the connector pin suitable for connection to a therapeutic device, such as a defibrillator. The PTFE pad is permanently implanted on the atria and remains implanted after removal of the temporary electrode sections. The temporary electrode sections may be removed by gently pulling them at their proximal end.

Other objects, advantages and features of the present invention will become apparent from the following specification when taken in conjunction with the accompanying drawings in which like elements are commonly enumerated and in which:

FIG. 1 is a plan view of a lead according to the present invention used to connect a pulse generator to a heart.

FIG. 2 details the connector assembly used in a lead according to the present invention having the break-away needle attached.

FIG. 3 details the connector assembly used in a lead according to the present invention having the break-away needle broken away.

FIGS. 4 and 5 detail the lead body used in a lead according to the present invention.

FIG. 6 is a plan view of a lead according to the present invention.

FIG. 7 is a sectional view of the lead shown in FIG. 6 taken along line 7--7.

FIG. 8 is a sectional detail of the stranded conductor and bus within the mounting pad.

FIG. 9 is a graph illustrating the force required to remove a lead according to the present invention.

FIG. 10 is a perspective view of an alternate embodiment of the present invention.

FIG. 11 is a cross sectional view of the pad of FIG. 10 showing a conductor woven through pad.

The drawings are not necessarily to scale.

FIG. 1 is a plan view of a lead 1 according to the present invention used to connect pulse generator 2 to heart 3. As seen lead 1 has essentially three sections: connector assembly 4, lead body 5 and electrode assembly 6.

Connector assembly 4 connects lead 1 to pulse generator 2. Details of connector assembly 4 may be seen in FIGS. 2 and 3. As seen connector assembly 4 features a break-away needle 11 which mates with pin assembly 12. Specifically break-away needle 11 has recess 17 which mates with finger 18 of pin assembly 12. In the preferred embodiment pin assembly 12 is stainless steel. Break-away needle 11 is provided on pin assembly 12 to permit the passage of connector assembly 4 from inside the body, through the skin to outside the body. Break-away needle 11 may thereafter be broken off connector assembly 4 at breakpoint 13 to thereby permit pin assembly 12 to join to a pulse generator 2. As seen in FIG. 3 when break-away needle 11 is broken off it carries with it a portion of finger 18. Pin assembly 12 further features crimp skirt 15 to permit conductors of lead body 5 to be joined thereto. Specifically conductors are crimped within cavity 16 and thereby electrically connected to pin assembly 4.

Lead body 5 consists of an insulative outer sleeve 20 encasing a plurality of conductors 21, 22 and 23 as seen in FIGS. 4 and 5. Gap 26 among inner conductors 21, 22 and 23 is filled by medical adhesive. Outer sleeve 20 may be constructed from any suitable biocompatible material, however in the preferred embodiment outer sleeve 20 is polyurethane.

Inner conductors 21, 22, and 23 are each constructed in a similar fashion and thus only one need be described. Each is constructed from a stranded conductor 30 encased by inner sleeve 31. In the preferred embodiment stranded conductor 30 is a multi-filament stainless steel stranded wire and inner sleeve 31 is PTFE or FEP. It should be understood, of course, that any suitable material or wire could be used for conductor 30 including a coiled wire as well as any type of wire made from an acceptable biocompatible metal including, but not limited to, such materials as platinum, palladium, titanium, tantalum, rhodium, iridium, carbon, vitreous carbon and alloys, oxides and nitrides of such metals or other conductive materials. Of course, some materials are incompatible with others and may not be effectively used together. The limitations of specific materials for use with others is well known in the art. It should also be understood that any other suitable material could also be used for inner sleeve 31 such as silicone, polyurethane, PTFE or FEP, for example.

As best seen in FIG. 6 outer sleeve 20 ends at a point 32 away from the distal end of lead 1. Inner conductors 21, 22, and 23 extend from point 32 to electrode assembly 6. Electrode assembly 6 is formed with inner conductors 21, 22, 23 and mounting pad 33. Specifically distal portion of each inner conductor has each stranded conductor 30 exposed along the length of mounting pad 33. Each of the inner conductors 30 is mounted to mounting pad 33, as best seen in FIGS. 7 and 8. Although the illustrated preferred embodiment features inner conductors 30 mounted within mounting pad 33, it should be understood inner conductors may be mounted to mounting pad 33 in any acceptable manner including, without limiting the variations possible, suturing or gluing all or some of inner conductor 30 to an outer surface of the mounting pad 33. In the preferred embodiment holes 34 within mounting pad 33 are used to provide for intermittent sections of each stranded conductor 30 to be exposed to body tissue. Thus when lead 1, and specifically electrode assembly 6, is mounted to cardiac tissue, intermittent sections of each stranded conductor 30 are directly exposed to cardiac tissue through holes 34. The contour dimensions (length by width of the exposed electrode area) of the conductors is approximately 40 by 30 millimeters in the preferred embodiment. A minimum of two exposed conductors is required to obtain this contour, and by this, a current distribution which results in acceptable defibrillation thresholds (DFT). Application of three conductors is preferred, because it further improves the DFT and the current density at the conductor electrode surface. In the preferred embodiment the conductor electrodes are exposed to both sides of the pad, allowing the current to flow across the front and back side of the pad. This results in a more homogeneous electrical field between the electrodes and usually in a lower DFT.

An alternative embodiment, which yields the same desired characteristics, incorporates a solid pad through which the conductors are threaded or woven, thus being alternatingly exposed to both sides of the pad, as best seen in FIGS. 10 and 11. Specifically FIG. 10 is a perspective view of a lead shown conductors 21, 22, 23 of lead body 5 woven though pad 33. FIG. 11 is a cross sectional view of pad 33 of FIG. 10 showing conductor 21 woven through pad 33. Although as depicted conductors 21, 22, and 23 are exposed equally to each side of pad 33, they may also be woven such that a greater length of each is exposed on one side of pad 33 as compared to another side of pad 33.

Mounting pad 33 further features suture areas 35 (designated by "x"s in the FIGS.) which permit mounting pad 33 to be sutured to the heart, as best seen in FIG. 1. Mounting pad 33 may be fashioned from any biocompatible pliant, material and in the preferred embodiment mounting pad 33 is fashioned from a PTFE felt. Preferably the structure and porosity of the felt should be similar to those which are typically used in reconstructive heart surgery.

In an alternate embodiment, mounting pad 33 may also be fashioned from a bioabsorbable material such as bovine collagen which has been cross-linked. Cross linking may be accomplished in any acceptable manner, including for example, according to the principles set forth in U.S. Pat. No. 5,264,551 entitled "Process for Cross-Linking Collagen by Diphenyl-phosphorylazide the Cross-Linked Collagen Obtained Thereby and Collagen Based Biomaterials Thus Cross-Linked" issued to Petite et al and assigned to Bioetica of Lyon, France, incorporated herein by reference. The particular degree of cross linking used may depend upon the type of collagen used and the amount of time lead 1 will be used in the body. The degree of cross linking should be such that the mechanical characteristics of pad 33 and the holding force of conductors 21, 22, 23 should be maintained and unintended disengagement of conductors is prevented for a period of at least two weeks to a month. Finally, other types of collagen besides bovine may also be used, such as pig or sheep.

Implantation of lead 1 according to the present invention is as follows. Mounting pad 33 is sutured to atrium 8 using suture areas 35. Next connector assembly 4 is exteriorized at a point away from the incision through use of break-away needle 11 and pin assembly 12. Specifically needle 11 is used to pierce the skin from the interior to the exterior so as to expose pin assembly 12. Once lead 1 is satisfactorily sutured to the atrium, pin assembly 12 is exposed and lead 1 is connected to a pulse generator, the patient's incision may be closed. At this point lead 1 may deliver therapeutic electrical pulses, including defibrillating, cardioverting or pacing, to atrium 8.

One important aspect of lead 1 of the present invention is its removability. Inner conductors 21, 22, 23 are mounted within mounting pad 33 so they may be removed, even once implanted, by traction. Specifically the inner conductors may be gently removed from mounting pad 33, and thus body 9, by traction upon proximal end of lead 1.

As seen in FIGS. 7 and 8 inner conductors are positioned within mounting pad 33. Bus 41 (also called a sleeve) is crimped to the conductor. Bus 41 serves to prevent unintended dislodgement of inner conductor 30 out of mounting pad 33. Bus 41 is placed at the proximal end 43 of pad 33, at a point between end 43 of pad 33 and hole 34. As such when inner conductor 30 is removed by traction, bus 41 only needs to pass through a short portion of pad 33 before it is free. Thus only a relatively brief amount of increased force, i.e. a short "jerk" or tug on the distal end of lead body 5 is sufficient to pull bus 41 out of pad 33. Once bus 41 is outside pad 33 the remainder of inner conductor 30 follows easily as there is no other structure along the length of inner conductor 30 which will inhibit the travel of inner conductor 30 through pad 33. This is illustrated in FIG. 9 where it is illustrated that pullout distance initially requires a relatively great pullout force, but which rapidly decreases once bus 41 is withdrawn from mounting pad 33. Thus it may be seen that bus 41 prevents inner conductors 21, 22, 23 from accidentally dislodging from position while also allowing their intended dislodgement and removal without possibly excessive forces from being applied to the atrium 8 during removal. Similar removal properties may be obtained without bus 41. Application of adhesive (i.e. medical adhesive or polyurethane adhesive) to each conductor or each conductor's insulation and pad 33 creates an adhesive bond between each conductor and pad 33. Once, by pulling lead body 5, the adhesive bond is broken, the rest of each conductor may be removed with lower force from pad 33, which results in a similar removal force characteristic as with bus 41, discussed below with reference to FIG. 9. In the preferred embodiment a small amount of medical adhesive 40 (or polyurethane) is applied to the distal end of each conductor 30 in order to cap off the ends of the stranded wire, although other materials, such as polyurethane, may also be used. This is done in order to keep the strands together and to prevent damage to the tissue during the removal procedure or in case the conductor would be forced out of the pad while implanted, as could occur due to heart movement. Mounting pad 33 because it is sutured to the heart, is left in place once conductors and lead body are removed. As discussed above, if mounting pad 33 is fashioned from collagen then if may be absorbed by the body tissues, as is well known in the art. Of course, the time required for absorption depends upon the degree to which the collagen has been cross linked.

Although the invention has been described in detail with particular reference to a preferred embodiment and alternate embodiments thereof, it will be understood variations and modifications can be effected within the scope of the following claims. Such modifications may include substituting elements or components which perform substantially the same function in substantially the same way to achieve substantially the same result for those described herein.