Device: ICD Field: Discrimination

1. Basic concepts


ICD Discrimination ALGORITHMS

When programming an ICD, particular attention should be paid to the risk of inappropriate therapies. Modern ICDs offer several advanced criteria, which, alone or combined, can discriminate VT from SVT. The “heart rate” criterion cannot safely be used alone, as its specificity for the diagnosis of ventricular tachyarrhythmias is only 60%, while its sensitivity can be easily increased to 100%. Moreover, the lower the programmed VT zone, the lower its specificity. Without other discrimination criteria, the therapy will be delivered irrespective of the origin of the tachycardia. The VF zone is typically reserved for very high rate and left without other intervening criteria, which are used only in the VT zone for patients susceptible of developing very rapid ventricular rates during SVT. These criteria should be deactivated in patients with fixed complete AV block.

 Discrimination IN SINGLE CHAMBER ICDs

The discriminating criteria offered by single chamber ICD include sudden onset, stability of the tachycardia cycle length and morphology of the ventricular EGM.

The « sudden onset » criterion withholds therapies for tachycardia of gradual onset, and is typically used to prevent inappropriate therapies for sinus tachycardia. The « stability » criterion limits the delivery of therapy for irregular tachyarrhythmias in the VT zone and is often used to prevent inappropriate therapy for atrial fibrillation with a rapid ventricular rate, which is usually irregular, whereas it is usually regular during VT. “Morphology” is a matching criterion that compares the EGM during tachycardia with that previously recorded during intrinsic rhythm. It is based on the principle that the ventricular EGM should be identical during a rapidly conducted SVT and during sinus rhythm. It dynamically compares the ventricular EGM recorded during all episodes of tachycardia in the VT zone with a previously recorded template. If both EGMs are classified as similar, the arrhythmia is diagnosed as “supraventricular”.

 Discrimination IN dual chambER ICDs  

The atrial lead offers the opportunity to compare atrial rates to ventricular rates and to analyze the potential association between them.

Given the different methods of discrimination between manufacturers, caregivers should become familiar with the specificities of each manufacturer.

3. Take home messages

1. In patients with preserved AV conduction, the single criterion of heart rate is not sufficient to discriminate ventricular arrhythmias and supraventricular arrhythmias. In contrast, in a pacemaker dependent patient, any rapid ventricular rhythm is of ventricular origin and no further discrimination is necessary.

2. Traditionally, discrimination applies in the VT zone, but not in the VF zone.

3. In a single-chamber defibrillator, three discrimination criteria may be available: sudden onset, stability and morphology. The sudden onset criterion discriminates sinus tachycardia (gradual onset) from VT (sudden onset). This parameter, however, does not discriminate atrial fibrillation or flutter from VT. The stability criterion discriminates AF (usually irregular) from VT (usually regular). It does not, however, discriminate sinus tachycardia, atrial tachycardia or atrial flutter from VT. Morphology is a matching criterion that compares the EGM during tachycardia with that previously recorded during intrinsic rhythm. It is based on the principle that the ventricular EGM should be identical during a rapidly conducted SVT and during sinus rhythm.

4. In a dual chamber defibrillator, “single chamber” parameters (sudden onset, stability, morphology) can be programmed. However, the atrial lead enables a continuous comparison of the ventricular with the atrial rate in order to identify the origin of the tachycardia. This comparison is the first discriminator in most dual-leads algorithms.