1. Field of the Invention
The present invention relates, generally, to medical devices. More particularly, the invention relates to implantable cardioverter defibrillators (ICDs).
2. Background Information
In the past, various ICD devices and methods have been used or proposed. However, these devices and methods have significant limitations and shortcomings. Existing ICD's are primarily designed for chronic applications in that they produce enough shocks to treat a chronic condition wherein the patient is expected to have numerous episodes of sudden cardiac death (SCD) over an extended period. Typical defibrillators are capable of delivering from 150 to 350 full output shocks of 27 to 40 joules each (depending on the model and manufacturer). In order to deliver that many high energy shocks, the device must have sufficient battery capacity to cover the required delivered energy as well as system losses (about 30% is lost in the DC to DC converter). This necessarily adds bulk and weight to the device.
Presently, most patients undergoing ICD implantation have exhibited at least one episode of fibrillation (SCI) and survived due for example to early CPR, trans-chest defibrillation and other care. Since one episode is typically a clear indication of high risk of having another one, an ICD is indicated. Other patients exhibit very early indications for being at high risk for SCD and an ICD is implanted prophylactically. Overall, about 40% of patients who have ICD's implanted do not have another episode during the next four years. However, these patients still need protection and typically another ICD must be implanted after the battery dies in three to five years, even though no shocks are delivered by the device.
Patients who are not shocked by their ICD have unnecessarily had a large device capable of hundreds of shocks implanted. Large devices are uncomfortable and present an increased risk of infections, erosions, and certain psychological problems. A smaller device with a smaller battery (and possibly fewer functions) would serve these patients better. Such a device would have only a sufficient number of shocks available to save the patient from initial SCD episodes, whereupon the patient would immediately have a larger device with more shock capacity implanted. This type of device would be implanted in those patients who were considered at high risk, but have not yet had an episode (and may never have one) and in patients who have had a conventional large device which needs replacement, but who have not had a shock during the last several years. The concept of a prophylactic ICD is disclosed in U.S. Pat. No. 5,439,482.
It may seem obvious to a casual observer that to make a device with fewer shocks, one only need to use a smaller battery. That has not been the case, however, owing to other requirements of the battery. The battery must be capable of charging the output capacitor to its maximum output (27 to 40 joules) in a period of 6 to 10 seconds after detection of fibrillation. This typically requires from 0.7 to 1.0 amp of current during the charging period. With conventional batteries used in ICD's, Lithium Vanadium Pentoxide9 for example, the minimum size battery that meets the charging criteria has sufficient capacity for about 150 or more shocks. Thus, it has not been possible to make a limited shock device with a small battery. Virtually every battery's chemistry has this capacity/power relationship.
Accordingly, it is an object of the present invention to provide an improved ICD which overcomes the limitations and shortcomings of the prior art, particularly those related to the limitations of prior art battery systems.