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International Journal of Arrhythmia 2011;12(4): 16-23.
Zen and the Art of ICD Rhythm Discrimination

세인트쥬드메디칼 Eliot L. Ostrow / 류 경 무
Timothy A Fayram MS and Kyungmoo Ryu PhD
Research, Cardiac Rhythm Management Division, St. Jude Medical, InC., Sylmar, CA, USA

What is rhythm discrimination?

   The term rhythm discrimination is generally used to describe the ability of an implantable cardioverter-defibrillator (ICD) to distinguish between tachyarrhythmias that the device should treat and other detected rhythms for which therapy should not be delivered. In practice, defining what should and should not be treated is as much a matter of philosophy as it is a technological issue. The transition from secondary prevention indications to primary prevention implants, and the related changes in clinicians’ and patients’ attitudes over the years have further complicated the matter. In the discussion that follows, the authors review the discrimination features and algorithms available in current ICD systems, and highlight the choices faced by the practitioner when deciding what to treat and what to consider when programming an ICD system to achieve the desired results. The potential for future enhancements will also be discussed.

Basic building blocks of ICD rhythm discrimination

   When a clinician looks at an ECG or intracardiac electrogram (IEGM) to evaluate a rhythm, he or she usually considers a variety of factors, such as:

  • How fast is the rhythm?

  • Is the rhythm sustained?

  • What is the relationship between atrial and ventricular events? Is there synchrony or dyssynchrony? Is every P-wave associated with an R-wave? Is the timing between P-waves and R-waves relatively constant? Is there a consistent pattern?

  • How did it start? Was the onset abrupt? Was it initiated by a premature ventricular contraction (PVC) or a premature atrial contraction (PAC)?

  • Is the ventricular rate regular or irregular?

  • How do the R-waves look? Do they look like Rwaves during sinus rhythm, or different?

  • Today’s ICDs use features and algorithms that attempt to mimic the clinicians’analytic approach by attempting to answer these same questions. These features and algorithms, which are summarized in Table 1, are well known to practitioners who regularly program ICDs, and will not be discussed in detail here. Dual-chamber ICDs, because they allow for the assessment of atrial activity and the atrioventricular (AV) relationship, provide an enhanced ability to distinguish supraventricular tachycardia (SVT) from ventricular tachycardia (VT). A schematic example of how atrial activity and the AV relationship could be integrated with the earlier discriminators is shown in Figure 1. Some ICD systems provide nearly infinite programmability of each of the component features of their discrimination algorithms. This enables the system to be customized to the specific needs of an individual patient, which may be particularly useful when inappropriate therapies are delivered because of atypical combinations of events. The system can be confusing to program, however, particularly for primary prevention patients, where nothing is known a priori about the characteristics of the patient’s tachyarrhythmias. Other ICD systems seek to minimize the programming complexity by creating ‘black box’algorithms. As the term implies, the workings of these algorithms are largely hidden from the user, and there may be little programmability beyond the ability to turn them on or off. They are simple to use, and they work well most of the time, but there is little recourse when they do not. ICD manufacturers are continually seeking to improve the user interfaces on their programmers in order to reach an ideal compromise: an on/off feature that works well most of the time, combined with the ability to troubleshoot and fine-tune the algorithm when patient conditions dictate that adjustments be made. What rhythms are we trying to discriminate between? When the earliest ICDs were implanted, the philosophy implicit in the approach to detection and discrimination was simple: when in doubt, shock. Because these were patients who had already experienced at least one episode of arrhythmic sudden cardiac death (SCD) or had documented VT or ventricular fibrillation (VF), every instance in which a shock was withheld in the face of a fast rhythm presented the risk of failing to treat a potentially lethal arrhythmia. For this reason, these devices required only simple detection and discrimination strategies and were designed with

    What rhythms are we trying to discriminate between?

       When the earliest ICDs were implanted, the philosophy implicit in the approach to detection and discrimination was simple: when in doubt, shock. Because these were patients who had already experienced at least one episode of arrhythmic sudden cardiac death (SCD) or had documented VT or ventricular fibrillation (VF), every instance in which a shock was withheld in the face of a fast rhythm presented the risk of failing to treat a potentially lethal arrhythmia. For this reason, these devices required only simple detection and discrimination strategies and were designed with the philosophy of ‘better a few shocks too many than one shock too few’. With the advent of tieredtherapy devices, which delivered antitachycardia pacing (ATP) and cardioversion shocks to treat slower, potentially better-tolerated VTs in addition to hemodynamically unstable VT and VF, it became desirable to be able to differentiate between VT and VF, and also between rapid ventricular rates due to ventricular arrhythmias and those associated with SVT. Today, most ICDs are implanted for primary prevention (i.e., prophylactically). Therefore, in patients who are not as keenly aware of the need for their ICD implantation, the tolerance for inappropriate or unnecessary shocks has diminished. The result has been a renewed emphasis on the development of algorithms and programming strategies that will result in the delivery of ICD therapy, and shocks in particular, only when absolutely necessary. Thus, for today’s primary prevention patient, the implicit philosophy can be summarized as ‘better one shock too few rather than a few too many’. The challenge in designing and programming ICD discrimination algorithms is that it is not always clear which rhythms are appropriate to treat, and with what therapy. Many different factors must be considered when deciding what device behavior is appropriate and necessary, and this decision may differ from patient to patient, and from physician to physician. Some key considerations in the decision-making process are as follows:

    Future Generations of the ICD

       The current state of the art in ICD packaging has reached technological maturity. Increased miniaturization will probably yield just a marginal decrease in the overall displacement volume of the device, which will be accompanied by a significant increase in manufacturing cost. The cost of the precious metals used in the device continues to rise. Because the ICD has a number of components that are unique to its application, it is doubtful that the current advances being made in the high volume personal electronics revolution would be of use in reducing its size and cost. If the clinical requirements for an ICD could be modified or relaxed, it would be possible to conceive of a new generation of devices that are smaller and that cost less. Relaxation of the design constraints could lead to the modification or complete elimination of the components within the ICD. Special batteries, high voltage capacitors, and high voltage output circuits could become smaller and less costly to manufacture. The first example of a future generation ICD is one that allows for a significant increase in its charge time in preparation for the delivery of therapy to the patient. The device would charge at a lower input power. A reduction in the maximum target charging voltage would make possible a significant reduction in the size of the device circuits and their respective components. In this case, alternative battery chemistries could be employed, such as Carbon Monofluoride (CFx). The size of the battery, the high voltage capacitor set, and the high voltage output circuit could also be reduced, bringing about a significant cost reduction as well. It is recognized that this new ICD may not be suitable for all patients. The second example of a future generation ICD is one that is more disruptive and unconventional in its design approach. This device is able to sense and detect tachyarrhythmias just before their onset. One class of therapy regime would consist of low voltage pace trains that would decelerate the arrhythmia into a normal sinus rhythm.2 Another class of therapy regime would stimulate the autonomic nervous system via thoracic spinal cord stimulation in a way that would prevent the onset of an arrhythmia.4 This new generation of devices has the function of a low voltage implantable pulse generator, and it looks more like a pacemaker or a spinal cord stimulator. It would make possible a significant size and cost reduction relative to the design of the conventional ICD.

    Is it SVT or VT?

       In the era when ICDs were predominantly implanted for secondary prevention, there was general agreement in the device community that every episode of VT or VF should be treated, and that every shock delivered for anything other than VT or VF was inappropriate. The discrimination algorithms of this era, then, which were largely based on the features listed in Table 1, were generally effective in making these distinctions, but were known and are known to be less than perfect. Because the predominant philosophy was that it was unacceptable to miss even a single episode of VT, when the device was unsure whether a rhythm was SVT or VT (for example, when different discriminators reached different conclusions), the algorithms were biased towards overtreatment, with the result that patients received inappropriate shocks (i.e., shocks for rhythms other than VT or VF). Recent studies1,2 estimate that approximately 20% of ICD patients receive inappropriate shocks within three years of receiving an implant.

    Is a Shock Necessary?

       As more patients receive their ICDs for primary prevention, and because of concerns that the shocks themselves may negatively affect their survival, the delivery of shocks has become less acceptable to patients and clinicians. This has led to a distinction between appropriate shocks (i.e., shock delivered as a result of a correct device diagnosis of VT or VF) and necessary shocks (i.e., shocks delivered only when shocks are the best alternative). Until recently, most ICDs were programmed with relatively low VT detection rates (often in the range of 150-160 bpm) and relatively short detection durations (typically 10-15 intervals) Any VT that remained above the programmed detection rate for the programmed duration and met any other programmed discrimination criteria would cause the device to deliver therapy, most often in the form of a shock. Several studies have shown, however, that for primary prevention patients, many shocks can be avoided by employing alternative programming strategies.3,4,5,6 First, these studies demonstrated that many VT episodes would self-terminate (i.e., without any device intervention) if they were allowed more time (up to 30-40 intervals) to run their course. Second, the studies showed that the VT detection rate could be programmed significantly faster (up to a range of 180-190 bpm) without compromising the patient’s safety. Finally, the studies demonstrated that a single burst of ATP could be very effective in terminating even fast VTs. This combination of higher detection rates, longer detection times, and at least one ATP burst has been shown to reduce the number of delivered shocks by as much as 50%. As a result, many experts are now advocating that clinicians adopt a default programming strategy based on the parameters that were shown to be effective in these studies. Thus, the philosophy has begun to evolve from, ‘The ICD must terminate every episode of VT’, to ‘The ICD is meant to prevent sudden death, not to treat non-lethal rhythms’. Several manufacturers have recently revised their default parameters to better reflect this change in indications and attitudes, and the analyses7,8 indicate that these revised parameters may dramatically reduce the number of inappropriate and unnecessary shocks.

    Is the Device Correctly Detecting R-Waves and P-Waves?

       ICD rhythm discrimination is predicated on the assumption that the device accurately senses Rwaves, and that in the case of dual-chamber ICDs, it detects P-waves as well, despite the fact that the signal amplitudes may vary by more than one order of magnitude as the patient’s rhythm transitions between sinus rhythm, tachycardia, and fibrillation. In addition, the device must not be prone to oversensing anything other than R-waves on the ventricular lead, and P-waves on the atrial lead, whether they are of physiological or nonphysiological origin. To ensure proper sensing, all modern ICDs employ bipolar sensing, either between closely-spaced tip and ring electrodes (standard bipolar configuration) or between a tip electrode and the right ventricular shock coil (integrated bipolar configuration). They also employ sophisticated automatic gain control or automatic sensitivity control algorithms in the ventricle (and, in some cases, in the atrium) that rapidly adapt to changes in signal amplitude, and frequency filters designed to optimize the sensing of R-waves, Pwaves, and fibrillation, and to minimize the sensing of physiological signals such as T-waves and myopotentials, and the sensing of nonphysiological signals from various external electromagnetic sources. Noise generated by failing leads (either due to breaches of a lead’s insulation or breaks in the conductor wires) remains a troubling complication of the sensing function. This often manifests as oversensing, which results in the delivery of inappropriate shocks, which, in the worst case, may occur in clusters.9 While this was once considered an unfortunate but unavoidable side effect of lead failure, recent attention to this problem from ICD manufacturers has led to advances that promise to aid in the earlier detection of such lead-related oversensing, thereby reducing the number of inappropriate shocks that occur.

    What Does the Future Hold?

       As in every other aspect of ICD technology, ICD rhythm discrimination will continue to evolve. While it is impossible to predict exactly what changes will be successfully implemented, certain technological trends seem likely to have a positive impact on the ICD’s ability to discriminate. Three of the most promising trends are as follows:

  • More processing power coupled with more memory: This will result in smarter, more powerful systems that can incorporate more sophisticated signal processing, which could facilitate the development of advanced algorithms that could self-learn and adapt to changes in a patient’s condition or physiology, or could better deal with the incomplete or conflicting data that the ICD must use to make its decisions.

  • Hemodynamic sensing: An ideal discrimination system would be able to distinguish between hemodynamically stable and unstable rhythms, and make treatment decisions accordingly. Today’s devices use the rate as a surrogate for hemodynamic tolerance, which allows discrimination and therapy strategies to be applied differently to different rate zones, based on the assumption that arrhythmias in slower zones will be better tolerated and will present less of a risk if they go untreated. Unfortunately, the rate is a far-from-perfect indicator of hemodynamic tolerance, both among multiple patients and in the same patient at various times and under various conditions. Today, hemodynamic sensors are being integrated into ICD systems for non-discrimination purposes, such as for the detection of the onset of acute heart failure decompensation. It is thus entirely possible, and even likely, that they will eventually be evaluated for their ability to determine hemodynamic tolerance.

  • Integration of relevant patient data into device operations: As the data from devices is merged into electronic health records, the relationship between changes in a patient’s condition and treatment and their effect(s) on the device data could be analyzed. This would result in suggestions for programming changes that would optimize the device performance as well as patient outcomes.

  • Summary

       Today ’s ICDs provide a wide range of programmable parameters and sophisticated algorithms that effectively discriminate between VT/VF and SVT, and also prevent inappropriate detection and therapy delivery due to the oversensing of physiological and non-physiological signals. Despite the availability of this advanced technology, or perhaps because of it, clinicians are faced with the daunting task of programming these systems to achieve goals that are sometimes unclear. The ability to distinguish VT and VF from every other manifestation, once considered the holy grail of rhythm discrimination, is no longer sufficient, particularly for patients who receive their implants for primary prevention. Philosophical decisions, including whether the ultimate goal of ICD therapy is to treat every ventricular arrhythmia or only to prevent sudden death (or, stated another way, whether it is ever acceptable to not detect and treat a true VT), and when shocks are not only appropriate but are necessary, must be decided for each patient, and the devices must be programmed accordingly. While it is likely that future developments will enhance these systems’technological capability to detect and distinguish between various rhythms, these advances are equally likely to raise new philosophical issues that the clinician must wrestle with.

       Employee: Dr. Ostrow is a board member of Clinical Advisory Group of St. Jude Medical International.


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