Implantable cardioverter-defibrillator (ICD) art has long distinguished ventricular tachyarrhythmias by rate and type. Ventricular tachycardias (VTs) generally are those arrhythmias with rates between 150 and 250 bpm. These rhythms can be further differentiated by their ECG configuration as either monomorphic or polymorphic. Arrhythmias with rates above the upper VT range, and up to approximately 350 bpm, are often termed flutter waves. Chaotic waveforms at rates higher than 350 bpm are classified as ventricular fibrillation (VF).
To treat each type of arrhythmia with an appropriate therapy, ICDs have been equipped with “tiered therapies”. Such devices are generally referred to as Pacer-Cardioverter-Defibrillators (PCDs). PCDs generally differentiate arrhythmias by rates, with programmable therapies to treat a respective type of detected arrhythmia(s). In such devices, the less-dangerous arrhythmias such as VT are treated by delivering a series of low-power pacing pulses to the heart at a relatively high rate. This therapy is often referred to as anti-tachyarrhythmia pacing therapy (ATP). In contrast, more perilous arrhythmias such as VF are often treated using a more aggressive shock therapy. For example, many PCDs may be programmed to first treat a VT with low-power ATP and then, if the VT progresses to ventricular flutter or fibrillation, deliver one or more high-power cardioversion or defibrillation shocks.
As may be evident from the foregoing discussion, high-voltage shocks are more effective at treating VT than ATP. For this reason, VTs having rates above 200 bpm are often treated by high-energy shocks when, in fact, they are monomorphic VTs that can be treated by low-energy ATP therapy. The more aggressive treatment is selected because most clinicians prefer a fast, immediate solution rather than waiting to determine whether ATP therapy will terminate the rhythm prior to delivering the high-voltage shock, possibly resulting in patient syncope. As a result, some patients must unnecessarily endure the pain of receiving a high-voltage shock delivery when painless ATP could have successfully terminated the rhythm.
Preventing the unnecessary delivery of high-voltage shocks has long been recognized as a very desirable goal. As a result, monitoring the rhythm during the charging of the high-voltage capacitors in preparation for shock delivery has been proposed. For example in U.S. Pat. No. 4,949,719, issued to Pless et al, and U.S. Pat. No. 5,191,884 issued to Gilli et al, the implanted device monitors heart rhythm during charging to determine whether the arrhythmia has spontaneously terminated and aborts the charging of the output capacitors if the rhythm has returned to normal.
Another approach to this issue is found in U.S. Pat. No. 5,318,591, issued to Causey et al., and incorporated herein by reference in its totality. The '591 patent teaches a three-tiered progressive approach using ATP as a first recourse, followed by a cardioversion pulse in the event ATP failed, with a defibrillation shock to be delivered if cardioversion also failed. The ICD begins charging its high-powered capacitors in parallel with the application of the ATP therapy. In addition, this charging may also start in parallel with the verification interval immediately following the previous therapy, during which time the ICD attempts to verify arrhythmia termination.
Numerous other patents describe ATP pacing including U.S. Pat. No. 5,193,536, issued to Mehra, U.S. Pat. No. 5,458,619 issued to Olson, U.S. Pat. No. 6,167,308, issued to DeGroot, and U.S. Pat. No. 6,178,350, issued to Olson, et al. This last patent, although it applies to atrial tachyarrhythmias, is of particular interest because of the manner in which the described system monitors for the continuing presence or absence of an atrial tachycardia (AT).
Other patents describe in more detail systems that involve the analysis of the sequence and timing of atrial and ventricular events prior to the selection of a therapy. Such patents include U.S. Pat. No. 5,205,283 issued to Olson, U.S. Pat. No. 5,193,550 issued to Duffin, U.S. Pat. No. 5,193,535 issued to Bardy et al., U.S. Pat. No. 5,161,527 issued to Nappholz et al., U.S. Pat. No. 5,107,850 issued to Olive and U.S. Pat. No. 5,048,521, issued to Pless et al.
In the patents listed above, one or two basic strategies are generally followed. A first strategy is to associate each type of arrhythmia with a predetermined set of criteria. Next, a patient's heart rhythm is monitored to identify a heart event, including intervals and/or rates associated with the event. This information is then compared against the various criteria sets to analyze the likelihood that the event may be characterized as a specific types of arrhythmia. Monitoring continues until one of the criteria sets is met, resulting in detection and diagnosis of the arrhythmia.
A second strategy used in the identification of a heart rhythm involves defining a set of criteria for events, event intervals and event rates which is generally indicative of a group of arrhythmias. After the criteria is met, the preceding and/or subsequent events are analyzed to determine which specific arrhythmia is present.
Typically and to summarize, many implantable anti-tachycardia pacemakers have the capability of providing a variety of anti-tachycardia pacing regimens. Normally, these regimens are applied according to a pre-programmed sequence, such as burst or ramp therapies among others. Each therapy extends over a series of a predetermined number of pacing pulses. After the series of pacing pulses is delivered, the devices check to determine whether the series of pulses was effective in terminating the detected tachyarrhythmia. Termination is generally confirmed by a return to sinus rhythm, for example, identified by a sequence of a predetermined number of spontaneous depolarizations separated by greater than a defined interval. In the absence of detected termination, the PCD applies more aggressive therapies such as synchronized cardioversion pulses or defibrillation shocks. While the delivery of ATP in some cases makes shock therapy unnecessary, a further reduction in the frequency of shock delivery is still desirable.
Applying an electrical pulse to the heart, whether a pacing pulse or a shock, requires charging of one or more output capacitors. Generally, the amount of energy required to delivery ATP is low. This type of therapy may therefore be delivered by a low-power output circuit relatively quickly. On the other hand, high-power shocks require a set of high-voltage capacitors that may require several seconds to reach a fully-programmed charge. As stated above, when a tiered therapy approach is utilized, both of these therapies may be used to “break” the tachyarrhythmia. That is, first ATP is delivered. During this time, the high-voltage capacitors may be charged so that if ATP fails to break the VT, a high-voltage shock may be delivered soon thereafter. If the VT is terminated by ATP, the charged high-voltage capacitors must abort delivery and internally “leak off” the stored energy in the capacitors, which depletes battery power. This can significantly shorten the life of the implanted device.
What is needed, therefore, is a method and apparatus to deliver successful ATP therapy without needlessly depleting battery resources.