A 9-year-old boy with hypertrophic cardiomyopathy had a single-chamber ICD (Virtuoso VR, Medtronic) implanted for primary prevention. The ICD was programmed with VF detection at 300 ms and a monitoring zone of 400 ms; SVT discriminators were programmed to monitor. The child was seen in the local ER for an episode of ICD shock while playing soccer at school. Device interrogation showed that he had received a shock for sinus tachycardia (CL 290 ms) as well as numerous episodes of sinus tachycardia (CL 380 ms) detected in the monitoring zone. The patient indicated that he had forgotten to take his medication that morning.
While ICD therapy has been shown to be highly successful in the prevention of sudden cardiac death, the incidence of inappropriate shocks in children remains high, ranging from 21%-41%.1,2 Inappropriate shocks may be due to sinus tachycardia, oversensing, or lead failure. An electrogram (EGM) morphology discrimination algorithm such as the Wavelet feature (Medtronic plc) compares a baseline EGM template to the EGM morphology during tachycardia; if at least 3 of the last 8 EGM’s match score is above or equal to the threshold value, it indicates that the tachycardia is supraventricular and therapy is withheld.3 The baseline EGM template can be collected in two possible ways. The first method is automatic, in which the device will perform template confirmation and periodic quality checks to update a new template if the baseline intrinsic match fails. Alternatively, template creation can be manual, in which templates of 6 EGM complexes are collected in clinic and compared; the one that matches the other 5 the most will serve as the temporary template. The template is stored in the device memory, and it will not be updated until another template collection is manually initiated. The Wavelet algorithm is applied within the programmable SVT limit, which may be extended into the VF zone if appropriate. It has been recognized that the Wavelet EGM match scores may vary depending on the patient’s posture or the EGM configuration.4
In this case, the monitor zone counter data showed a poor match of the Wavelet EGM during sinus tachycardia compared to the Wavelet EGM template (Can to RV coil) (Figure 1). This is due to the changes in the EGM amplitude at the faster heart rates. We tested the Wavelet EGM during baseline sinus rhythm with the patient in the upright position, using different EGM sources. This showed that the Can to RV ring gave relatively higher match scores. We proceeded to have the patient run on the treadmill, and used wireless telemetry to manually collect the Wavelet EGM (Can to RV ring); when the patient’s heart rate reached about 150 bpm, this template was stored (Figure 2). Auto collection was turned off and the SVT limit was adjusted to 280 ms. FVT via VF zone was programmed to include ATP therapies. During follow-up, the ICD showed improved discrimination of sinus tachycardia (Figure 3).
Thus, the ability to manually adjust a morphology discriminator (other examples include St. Jude Medical’s Far Field MD™ morphology discrimination algorithm or Boston Scientific’s Rhythm ID®) is helpful, as exercise-induced sinus tachycardia may be associated with changes in EGM amplitude, local ST segment changes, or misalignment of the EGM peaks due to aberration.
Acknowledgement: The authors thank Mr. Saeed Hosseini (Medtronic Canada) for his expert assistance.
Disclosure: The authors have no conflicts of interest to report regarding the content herein.
- Sherrid M, Daubert J. Risks and challenges of implantable cardioverter-defibrillators in young adults. Prog Cardiovasc Dis. 2008;51(3):237-263.
- Maron B, Spirito P, Ackerman MJ, et al. Prevention of sudden cardiac death with implantable cardioverter-defibrillators in children and adolescents with hypertrophic cardiomyopathy. J Am Coll Cardiol. 2013;61(14):1527-1535.
- Klein G, Gillberg JM, Tang A, et al. Improving SVT discrimination in single-chamber ICDs: a new electrogram morphology-based algorithm. J Cardiovasc Electrophysiol. 2006;17(12):1310-1319.
- Wolber T, Binggeli C, Holzmeister J, et al. Wavelet-based tachycardia discrimination in ICDs: impact of posture and electrogram configuration. Pacing Clin Electrophysiol. 2006;29(11):1255-1260.