The Wavelet feature is designed to withhold inappropriate ventricular detection if the rhythm displays characteristics of an SVT origin. The Wavelet feature compares the patient’s current QRS waveform to a collected and stored template of the patient’s QRS waveform during sinus rhythm. VT/VF detection is withheld if the current waveform sufficiently matches the template. Wavelet works in conjunction with PR Logic™ to determine whether a rhythm is an SVT or a VT.
This feature can be found in some Medtronic ICD and CRT-D devices. Please go to manuals.medtronic.com or consult with your local Medtronic representative regarding device models available in your geography.
Wavelet is automatically enabled when VF Detection is programmed On with an SVT V. Limit programmed to 260 ms (Cobalt™/Crome™/Protecta™ devices) and 320 ms (all other devices). The Match Threshold value is nominally set to 70%. Auto Collection is nominally programmed On for all devices, except Claria™/Amplia™/Compia™/Viva™/Protecta™ CRT-D.
To program this feature, go to Params -> Detection(V.)… -> Wavelet…
High Rate Timeout can be programmed On or Off for this feature.Note: The SVT V. Limit cannot be programmed longer than the VT detection interval.
When Wavelet is programmed On, the device collects EGM data for sensed ventricular events up to VF detection and compares each arrhythmic waveform to a stored template using the programmed Match Threshold value. A QRS complex is classified as a “Match” if its match score is greater than or equal to the programmed Match Threshold value. If at least 3 of the last 8 QRS complexes match the stored template, the device withholds detection. After VT/VF detection is withheld, the ventricular rhythm is continuously evaluated until VT/VF detection occurs or the fast ventricular rate ends. If fewer than 3 of the last 8 QRS complexes match the stored template, the device allows detection and therapy. Wavelet analyzes rhythms up to the programmed SVT limit. Wavelet applies to initial detection only.
Using Wavelet with PR Logic – When both Wavelet and PR Logic features are programmed to On, PR Logic will first review the rhythm and if PR Logic does not indicate that the arrhythmia is an SVT, the following conditions are checked:
- The programmed Number of Intervals to Detect value is reached.
- The atrial rate is faster than or equal to the ventricular rate.
If these conditions are true, Wavelet assesses the rhythm and makes a determination on whether to withhold detection. If Wavelet is programmed On and PR Logic is programmed Off, Wavelet analyzes the rhythm when the Number of Intervals to Detect is reached.
Collecting the Wavelet template – The template can be collected automatically using the Auto Collection feature or manually. In most cases, Auto Collection is appropriate. Certain factors such as a change in the patient’s medication or disease progression may affect the appropriateness of the currently stored template, in which case a manual template collection would be desirable at follow-up. Note that for CRT patients who do not have devices set to Auto Collection, the template will need to be manually collected. Cobalt and Crome devices can promote sensing by either increasing the AV delay (DDI/DDIR and DDD/DDDR modes) or decreasing by 10 pulses per minute (in VVI/VVIR and DDI/DDIR modes).
When Auto Collection is programmed to On and the patient is not being paced, the device regularly collects and confirms a Wavelet template and monitors it for consistency with the patient’s current QRS complexes in sinus rhythm. When the stored template is no longer consistent with the patient’s current QRS complexes (for example, due to lead maturation or changes in drug therapy), the device recollects and confirms a new template. After a template is calculated, the device performs a confirmation process before using the template for detection operations. Template confirmation takes approximately 12 minutes, but could take longer if the intrinsic rhythm changes after the template is collected.
Collecting the Wavelet template for Cobalt and Crome ICD and CRT-D devices: When Auto Collection is programmed to On, the device attempts to collect a new Wavelet template and evaluate it against the stored template on a weekly basis (or hourly in the first 24 hours). The device confirms a new template and uses it for detection operations if it provides a better match to the patient’s QRS complexes than the stored template (for example, due to lead maturation or changes in drug therapy). The new template becomes the stored template and is used until it is replaced by the device or manual template collection.
EGM2 Source and Range – Wavelet uses EGM2 as the source for template collection and comparison. It may be necessary to adjust the EGM2 Source and EGM2 Range to optimize Wavelet performance. Wavelet is less effective at identifying SVTs and withholding detection in the following situations:
- R wave amplitudes on the EGM2 signal are too small relative to myopotential interference.
- R wave amplitudes on the EGM2 signal are so large during intrinsic rhythm or SVT that they exceed the maximum EGM range and as a result are clipped.
You can assess the EGM2 signal using the programmer strip chart recorder. If the peak-to-peak R wave amplitudes on the EGM2 trace are less than 3 mV, consider selecting a different EGM2 source. If the R wave amplitudes are too large (either clipped or within 1 mV of the EGM2 Range), consider selecting a larger EGM2 Range value. If the R wave amplitudes are too large at any EGM2 Range, try a different EGM2 Source and assess the R wave amplitudes starting with an EGM2 Range value of ±8 mV. Cobalt and Crome ICD and CRT-D devices will display an observation if the EGM signal is too small or too large.
Concurrent pacemaker – Use caution when programming Wavelet for patients who have a pacemaker concurrently implanted because the ICD cannot distinguish between intrinsic events and paced events from the pacemaker. Program Wavelet to Monitor and evaluate its effectiveness before enabling it for detection. In addition, it is strongly recommended that you disable Auto Collection and collect a template manually with the Wavelet test, making sure that the pacemaker is not pacing the heart during the collection process.
In the example below, the patient has AF that conducts into the VF zone. The device first uses PR Logic and then Wavelet. The EGM shows a WV annotation on the decision channel, indicating Wavelet overrule of VT detection. The episode text and QRS detail show that all 8 QRS complexes measured exceed the programmed match threshold (>70%). Therefore, therapy is withheld appropriately.
Using the Protecta Performance Model, Wavelet, combined with ATP, TWave Discrimination, and other SVT discriminators applied in the VF zone, significantly reduced shock frequency when tested on stored episodes from the SCD-HeFT Trial. This model, which combined existing and new shock reduction features, predicted 98% of patients would be free of inappropriate shocks 1 year after implant, and 92% at 5 years post-implant.1
Wavelet substantially reduced inappropriate therapy (78.2%) while maintaining sensitivity (99.2%) in single chamber ICDs.2 Specificity of Wavelet for atrial tachyarrhythmias compared to Rhythm ID and MD morphology discriminators was significantly higher (87% versus 56% and 53%, respectively).3
- Volosin K, Exner DV, Wathen MS et al., Virtual ICD: A Model to Evaluate Shock Reduction Strategies, HRS Conference, May 2010.
- Klein GJ, Gillberg JM, Tang A, et al, for the Worldwide Wave Investigators. Improving SVT Discrimination in Single-Chamber ICDs :A New Electrogram Morphology-Based Algorithm. J Cardiovasc Electrophysiol. December 2006;17(12):1310-1319.
- Theuns DAMJ, Burke MC, Leman RB, et al. Head-to-head comparison of three morphology discrimination algorithms in implantable cardioverter-defibrillators: Arrhythmia discrimination of atrial and ventricular arrhythmia. HR 2009 Europace - Abstract.
Sources: Medtronic Cobalt and Crome ICD and HR CRT-D Reference Guides, Medtronic Protecta™ XT DR Clinician Manual; Medtronic Protecta™ XT CRT-D Clinician Manual.