Defibrillator Cutoff Rates – Turn It Up

When taking care of a patient with an implantable defibrillator (ICD), it is important to program the device optimally to minimize the likelihood of an inappropriate shock without reducing the ability of the device to accurately detect and treat a life-threatening episode of ventricular tachycardia (VT) or ventricular fibrillation (VF). Early efforts focused on developing and optimizing discriminating algorithms to withhold therapy for supraventricular tachycardias (SVTs). Device features such as stability-based algorithms were created to distinguish atrial fibrillation (AF) from VT by taking advantage of the fact that the ventricular intervals during AF were variable relative to monomorphic VT. However, they were imperfect during instances where the ventricular rhythm during AF was regular for just enough time to confuse the device, the ventricular rate exceeded the rate cutoff of the VT zone and was detected in the VF zone where the algorithms did not apply, or when a time-out feature such as sustained rate duration was met. Other algorithms tried to withhold shocks for sinus tachycardia based on its gradual onset, but were also imperfect. For a time, there was over-reliance on discrimination algorithms, often linked with promotion of atrial lead implantation for the sole purpose of improving SVT discrimination.

The MADIT-RIT¹ study made it clear that there are better ways to avoid shocks not only for SVTs, but also for VTs that would spontaneously terminate: this includes prolonging the detection time and increasing the cutoff rate. In the randomized, multicenter MADIT-RIT study, device programming using delayed therapy (a 60-sec delay at 170-199 bpm, a 12-second delay at 200-249 bpm, and a 2.5-sec delay at >250 bpm), or high-rate therapy (a 2.5-sec delay at >200 bpm), each reduced the rate of inappropriate shocks from 25% to 6% after two years without any apparent downside. The results of this study have resulted in most centers programming devices using a combination of higher rate cutoffs and longer detection times.

A recently published study has reinforced the benefits and safety of high-rate cutoffs. Using data from the French multicenter DAI-PP registry, Clementy et al² took advantage of heterogeneous programming practices between centers and compared the multicenter outcomes over 3.6 years in 500 patients who underwent ICD implantation (single, dual, and resynchronization devices) and were programmed with a very-high-rate cutoff (VH- RATE group: monitor zone 170-219 bpm, VF zone ≥220 bpm with 13±4 detection intervals) with 1500 matched control patients who were programmed with either one or two therapy zones. Very-high-rate cutoff programming was associated with a 60% reduction of appropriate therapies as well as inappropriate shocks, without affecting mortality. In 2012, Clementy et al also published their own single-center study in which 365 patients were programmed with a cutoff rate of 220 bpm and a monitor-only zone between 170-220.³ Six percent of patients were taking amiodarone. Episodes of  VT, sustained or not, were recorded in the monitor zone in 11.8% of patients, but only 1.9% were symptomatic, without lethal consequence. Only one patient died from unexplained sudden death.

There are a few considerations, however, that one should make before programming a very-high-rate cutoff in all patients who undergo ICD implantation. Clearly, this approach is only applicable to primary prevention devices. For patients who undergo ICD implantation for secondary prevention, one must make sure that the cutoff rate is lower than the rate of any sustained slow VT that the patient has previously experienced. It can be challenging at times, however, to find such documentation many years later when it comes time for ICD generator change, for example. Also, as mentioned by the authors, there is no logic to programming a high-rate cutoff for patients with heart block who cannot have rapid ventricular rates during SVT. An important issue that requires some consideration, though, is whether the patient is at unusually high risk for developing slow VT. Prior studies have shown that risk factors for presenting with VT below the cutoff rate are amiodarone use, inducible monomorphic VT at the time of electrophysiological testing, and NYHA class III heart failure.⁴ It is not mentioned in the DAI-PP paper how many patients were taking amiodarone. However, it is unlikely that there were any patients in the study who underwent device implantation for primary prevention, and by definition had no history of sustained VT or VF, who were also taking high-dose amiodarone. Something else to consider is how generalizable the data are to patients who have not undergone defibrillation testing. It has only recently become common practice to defer VF induction at the time of ICD implantation. There are times when VF is induced, albeit rarely, that the detected rate of the VF is below 200 bpm, particularly in patients with advanced heart failure on amiodarone. Although it is not mentioned by the authors if defibrillation testing was performed, it is presumed that the patients included in the DAI-PP study who underwent implantation between 2005 and 2012 underwent routine defibrillation testing.

The experience with the totally subcutaneous (SQ) defibrillator has also reinforced the approach of waiting before giving a shock. The time to therapy for the SQ ICD is much longer compared to that of transvenous defibrillators, which was initially felt to be a negative. However, data from MADIT-RIT and the favorable outcomes with the SQ ICD published from randomized clinical trials, large registries, and single-center experiences have reinforced the safety of waiting to treat tachycardias that might be about to self-terminate. The UNTOUCHED trial will test the outcomes of programming a discriminating zone at 200-239 bpm and a VF zone at >240 bpm.

Data from several clinical trials should encourage the programming of higher-than-traditional rate cutoffs in most patients who undergo ICD implantation for primary prevention of sudden death to minimize shocks — not just at the time of implantation, but also when patients are seen in follow-up in device clinic. The new information from the DAI-PP registry is encouraging, and should make electrophysiologists even more comfortable when treating patients who have previously had inappropriate shocks for SVT and for patients who are not on high-dose amiodarone, in order to program rate cutoffs as high at 220 bpm without worrying about underdetection of  VT/VF.

References

1. Moss AJ, Schuger C, Beck CA, et al. Reduction in inappropriate therapy and mortality through ICD programming. N Engl J Med. 2012;367:2275-2283.
2. Clementy N, Challal F, Marijon E, et al. Very-high-rate programming in primary prevention patients with reduced ejection fraction implanted with a defibrillator: results from a large multicenter controlled study. Heart Rhythm. 2016 (published online before print) doi: https://dx.doi.org/10.1016/j.hrthm.2016.10.024.
3. Clementy N, Pierre B, Lallemand B, et al. Long-term follow-up on high-rate cutoff programming for implantable cardioverter defibrillators in primary prevention patients with left ventricular systolic dysfunction. Europace. 2012;14(7):968-974. 
4. Bänsch D, Castrucci M, Böcker D, Breithardt G, Block M. Ventricular tachycardias above the initially programmed tachycardia detection interval in patients with implantable cardioverter-defibrillators: incidence, prediction and significance. J Am Coll Cardiol. 2000;36:557-565.