Optimal Strategies to Reduce Inappropriate Implantable Cardioverter-defibrillator Shocks

Figure 1:. An example of inappropriate shock due to AF with a VT zone programmed at 370 ms due to previous monomorphic VT at this rate. In the top panel, PR Logic™ (Medtronic, Minneapolis, MN, USA) was used and AV dissociation was deemed to be present, likely due to the changing relationship between the atrial- and ventricular-sensed activities. The bottom panel shows that the morphology discriminator (Wavelet) detects that the QRS morphology is on average less than a 70% match for eight QRS complexes as compared with the template that was created. The rhythm is then deemed to be a double tachycardia with AF and VT as noted and antitachycardia pacing is subsequently delivered. Options to avoid repeat inappropriate therapy include decreasing the Match threshold for Wavelet. The VT zone could also be changed and the tachycardia onset discriminator could be turned off.

Figure 2:. In this example, instability of the ventricular-sensed intervals is noted (red boxes) and therapy is appropriately withheld.

Figure 3:. A simplified overview of the PR Logic™ algorithm (Medtronic, Minneapolis, MN, USA).

Figure 4:. Following the PR Logic™ algorithm (Medtronic, Minneapolis, MN, USA), the atrial rate and ventricular rate are compared. Here, due to the fact that the atrial rate exceeds the ventricular rate and as there appears to be some degree of AV dissociation (red box) and also the morphology matches the template of sinus rhythm (blue box), therapy is appropriately withheld during AF.

Figure 5:. A simplified overview of the Rhythm ID algorithm (Boston Scientific, Natick, MA, USA).

Figure 6:. Example of Rhythm ID (Boston Scientific, Natick, MA, USA). In a typical case, first, there is a determination made regarding whether or not the ventricular rate is greater than the atrial rate; in this example, it is not. The next step is to compare the timing of the ventricular electrogram with the shock electrocardiogram and the morphology of the shock electrogram in tachycardia as compared with a baseline template to determine whether or not there is a sufficient match (the morphology match can be programmed between 70% and 96%). Finally, the algorithm determines if the atrial rate is greater than 170 bpm and if the ventricular rhythm is unstable (> 20-ms variability) in order to make the final determination of categorizing a rhythm as SVT or VT.

Figure 7:. A simplified overview of the Rate Branch algorithm (Abbott Laboratories, Chicago, IL, USA).

Figure 8:. A simplified overview of the SMART algorithm (Biotronik, Berlin, Germany).