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" 1 ,V',T T~1 ' 1 1 1 i i J ' 1" ' i "'i' i 1 i i i i i J i i i i i J i I I I j l I I l ¡ I 1 . t ,|

Figure 2.7. Panel A, interval plot showing a tachyarrhythmia with a gradual onset. Panel B, interval plot showing a tachyarrhythmia with a sudden onset. (Medtronic, model GEM 7271 DR). □ A-A: AA intervals; • V-V: VV intervals; FVT = fast ventricular tachycardia; VF = ventricular fibrillation; VT = ventricular tachycardia.

Figure 2.7. Panel A, interval plot showing a tachyarrhythmia with a gradual onset. Panel B, interval plot showing a tachyarrhythmia with a sudden onset. (Medtronic, model GEM 7271 DR). □ A-A: AA intervals; • V-V: VV intervals; FVT = fast ventricular tachycardia; VF = ventricular fibrillation; VT = ventricular tachycardia.

Initially, both discriminators have been used infrequently because physicians were concerned about underdetection of VTs. Serious underdetection was observed in only a minor proportion of the episodes.

The addition of a safety timer (duration) may prevent underdetection of VT. If a tachyarrhythmia satisfies the ventricular rate criterion and the discriminators indicate SVT, the safety timer will override the discriminators and therapy will be delivered. However, this feature ensured 100% sensitivity for VT but at the price of decreased specificity for rejection of SVT [18]. In a case control study, the interval-based discriminators 'sudden onset' and 'stability' reduce inappropriate therapies due to atrial fibrillation and sinus tachycardia [17]. The major limitation of'sudden onset' and 'stability' is the inefficiency to reject sudden onset atrial tachyarrhythmias with a consistent atrioventricular (AV) relation, e.g. atrial tachycardia or atrial flutter. Complex algorithms with the addition of atrial information might improve the specificity of arrhythmia discrimination.

Dual-chamber Discrimination

An early plea for the addition of atrial sensing to improve arrhythmia discrimination was proposed by Furman as early as in 1982 [19]. The comparison between atrial and ventricular rates is a simple and effective arrhythmia discriminator. In the majority of VTs, the ventricular rate is faster than the atrial rate. Limitations of this simple criterion are the underdetection of VTs with 1:1 VA conduction and ventricular tachyarrhythmias during atrial fibrillation. To address this limitation, the analysis of the AV relationship was postulated as a feature of interest to discriminate sinus tachycardia from VT [20]. Measurement of the AV relationship provides a reliable diagnostic tool for AV association. Further, timing relationships between atrial and ventricular electrograms can be used to identify atrial tachyarrhythmias with stable AV conduction.

All dual-chamber algorithms comprise both single- and dual-chamber discriminators (Table 2.1). Dual-chamber discrimination algorithms include comparison of atrial and ventricular rates and/or measures of the AV relationship. The algorithms in dual-chamber devices can be roughly divided into two groups: (1) Comparison of atrial and ventricular rates (rate branches) and (2) Hierarchical analysis of the atrioventricular relationship.

Dual-chamber Algorithms Based on Analysis of the Atrioventricular Relationship

A hierarchical structure of single- and dual-chamber arrhythmia discriminators is applied in the algorithms PARAD, PARAD+ (ELA Medical), and PR Logic (Medtronic). The PARAD algorithm (P And R Based

Table 2.1. Arrhythmia discrimination components in dual-chamber algorithms.

Biotronik ELA Medical Guidant Medtronic St. Jude Medical

Biotronik ELA Medical Guidant Medtronic St. Jude Medical

Table 2.1. Arrhythmia discrimination components in dual-chamber algorithms.

Single-chamber detection

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