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Optimal energy steering for an implantable defibrillator
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Inventors: Kroll, Mark W.; Adams, Theodore P.; Supino, Charles G.;
Assignee: Angeion Corporation (Plymouth, MN)
Primary Examiner: Kamm; William E.
Assistant Examiner: Getzow; Scott M.
Attorney, Agent or Firm: Patterson and Keough
Epicardial-patch electrodes for defibrillation are efficient in deliverying electrical energy accurately to the necessary tissue and in minimizing electrical losses, but they are risky and costly because their implantation requires major surgery. Intravenous implantation of endocardial-coil electrodes, such as right-ventricular-apex (RVA) and superior-vena-cava (SVC) electrodes, by means of cardiac catheters, on the other hand, involves simpler procedures. Also, implantation of a subcutaneous-patch (SUB) electrode or of a pulse-generator housing (CAN, subject to co-pending application) electrode requires comparatively minor surgery. Using these last four electrodes, however, involves accepting electrical losses in intervening tissue, relatively inefficient current directions in some cases, and unfavorable energy distributions among paralleled paths. The present invention achieves more favorable energy and current distribution by introducing lossy elements in one or more paths, or by capacitor-switching methods, where RVA is given one polarity, and at least two of the other electrodes are given opposite polarity, yielding at least two pathways. The present invention also achieves current and energy steering by means of differing pulse durations in different paths. |
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DETAILED DESCRIPTION Three prior-art electrode options and one that is the subject of a co-pending application have just been described, all of them requiring substantially less hazard and trauma for the patient than do epicardial-patch electrodes.
In a schematic way, FIG.
3 shows the relationships of all four of these electrodes that are comparatively easy to implant.
It is evident, now, that several choices of electrode interconnections and polarities exist for applying the defibrillation waveform.
In a common and typical defibrillation method, only the RVA, SVC, and SUB electrodes are present.
Similarly typical is the "(as) laser cathode" discharge.
In the monophasic case, the positive potential is usually applied to the SVC and SUB electrodes in common.
Having the SVC and RVA electrodes common and negative has been found ineffective, and is usually avoided.
Also, when the fourth or CAN electrode is present in addition to the other three, it is standard practice to let SUB and CAN be common; to do otherwise would cause a local current in the left chest that would scarcely intersect the heart.
Thus, it is that the path and polarity options that at first glance appear to be numerous are reduced by practical and elementary considerations to a considerably smaller number.
Return now to the case of three electrodes, for which relevant data can be drawn from the literature.
It has been reported that in the canine heart, the SVC-RVA pathway presents a resistance of 75 ohms, and the RVA-SUB pathway, 103 ohms.
(J.
N.
Wetherbee, et al.
, "Sequential Shocks are Comparable to Single Shocks Employing Two Current Pathways for Internal Defibrillation in Dogs", PACE, Vol.
11, p.
696, June, 1988.
) Thus, when the SVC and SUB electrodes are made common, the first-named pathway will carry the larger current.
This causes problems: First, the top-down current is not as effective as an equal "horizontal" current would be, one passing through much of the ventricular myocardium.
Second, the SVC electrode typically makes a small-area contact with the cardiac tissue, and the resulting locally high value of current density can cause tissue damage
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