Chapter 3

Discrimination

Telling supraventricular from ventricular: Rhythm ID, onset and stability, the V>A criterion, and the ways discrimination can be fooled.

Basics of Boston Scientific discrimination

Indications

Discrimination for supra-ventricular arrhythmias is only anticipated in patients likely to present with supra-ventricular arrhythmia in the VT detection zone. For patients with complete AV block or a low Wenckebach point, discrimination (rate-only) is not programmed.
Before considering discrimination, the first step is to think carefully about the rate cutoffs for the treatment zones: if the lower limit of the treatment zone is set at a high heart rate, there is a lower risk of interference from supra-ventricular arrhythmias

Special aspects

Discrimination cannot be applied in VF zones.
Discrimination cannot be applied in VT monitor zones.
There is no inhibitory discrimination behind an ATP.
In post-shock mode, the morphology criterion (Rhythm ID) is no longer available.

Algorithms

Comparison of A and V rhythms: comparison of the average A and V rates (calculated over the last 10 beats before the end of the duration). If the V rate is 10 bpm higher than the A rate, VT is diagnosed and treatment is given.
Ventricular signal morphology (Rhythm ID/vector correlation): comparison of the morphology of reference ventricular signals (intrinsic depolarization) with ventricular signals acquired during tachycardia. The analysis is carried out on the shock channel signal (Coil-can, also known as Far-Field). If three of the last ten ventricular complexes immediately preceding the end of the duration are sufficiently similar (percentage match) to the reference template, the algorithm considers the tachycardia to be supra-ventricular. The similarity threshold is nominally set to 94%, and is programmable. The rhythm ID reference can be acquired manually or automatically.
Sudden onset : looks for a jump in rate at the start of the tachycardia. A gradual onset is consistent with sinus tachycardia progressively entering the VT zone.
Stability : tachycardia analysis of R-R interval stability. Stability corresponds to the variance of the RR intervals (between the 5 intervals preceding the V-Episode marker and the intervals following the V-Episode marker until the end of the duration). If this variance is greater than the programmed stability limit, then the rhythm is considered unstable (AF).
AFib rate threshold : search for AF on the atrial channel. When 6/10 A intervals are faster than the AFib rate threshold (170/min nominal), the algorithm validates the presence of AF. To maintain the diagnosis during the analysis, 4/10 intervals must remain faster than the Fib A frequency threshold.

Single chamber algorithms

Boston Scientific VR devices offer two discrimination (enhanced detection) modes: Rhythm ID or Onset/Stability. These two modes cannot be combined.
Rhythm ID: initial discrimination is based solely on morphology. In post-shock, stability alone will be used to discriminate possible AF. NB: post-shock stability cannot be deactivated. See tracings 1 to 5.
Onset/stability this mode is subdivided into discrimination of atrial tachyarrhythmias (based on the stability criterion) and discrimination of sinus tachycardias (based on the onset criterion). When both options are selected, you can choose to associate (AND) or dissociate (OR) the discriminants. When they are combined, the arrhythmia will only be treated in the presence of an abrupt onset AND a stable rhythm. When they are dissociated, the arrhythmia is treated in the presence of an abrupt onset OR a stable rhythm. In post-shock, stability alone will be used to discriminate possible AF. NB: when this mode is selected in 3-zone programming (VT-1/VT/VF), discrimination is only possible in the VT-1 zone.

Dual/triple-chamber algorithms

Boston Scientific DR and CRT devices offer two modes of discrimination (to improve detection): Rhythm ID or Start/Stability. These two options differ from the options offered in single-chamber models, since they incorporate information from the atrial channel. To access the settings, program Detection Enhancement.
Rhythm ID: This dual-chamber algorithm uses a combination of the following parameters: 1) comparison of A and V rates, 2) Rhythm ID morphology (Rhythm ID), 3) ventricular stability (STAB) and 4) Afib rate threshold (AFib). This is the most efficient discrimination algorithm and should therefore be preferentially applied, even for CRT devices (in which case the Rhythm ID reference should be acquired manually).
By going to the Enhanced Detection menu, then Current Rhythm ID settings, you can access the Rhythm ID, Stability and AFib rate threshold settings.
In the Current Rhythm ID settings menu, you can deactivate discrimination of atrial tachyarrhythmias to restore single-chamber Rhythm ID discrimination (useful in the event of atrial lead failure).
This dual-chamber algorithm uses a combination of the following parameters: 1) comparison of A and V rates, 2) Rhythm ID morphology (Rhythm ID), 3) ventricular stability (STAB) and 4) Afib rate threshold (AFib). This is the most efficient discrimination algorithm and should therefore be preferentially applied, even for CRT devices (in which case the Rhythm ID reference should be acquired manually).
Onset/Stability: This dual-chamber algorithm uses a combination of the following parameters: 1) comparison of A and V rates, 2) sudden onset (Onset), 3) ventricular stability (STAB) and 4) AFib rate threshold (AFib). It should only be used in the event of Rhythm ID failure.
By going to the Enhanced Detection menu, then Current Rhythm ID Settings, you can access the V rate > A rate (on/off), onset, stability and AFib rate threshold settings.
To restore single-chamber operation (in the event of failure of A lead and Rhythm ID failure), the V rate > A rate and AFib rate threshold branches must be deactivated. In this case, remember that discrimination will only apply to the first VT-1 zone of a 3-zone configuration (no discrimination in VT and VF).

The cases

From Boston Scientific ICD — clinical cases by P. Bordachar, M. Strik, A. Thiyagarajah, S. Ploux. Published by Cardiocases. Every numbered marker on a recording is explained in the list beneath it; tap a marker on the trace or an entry in the list.

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