Figure 4.2. Examples of near-field (A) and far-field electrograms (B). The terms "rate" and "shock" electrogram are commonly used to describe each respective type.
In general, tachyarrhythmia detection algorithms use combinations of information derived from atrial and ventricular data. This applies for tachycardia differentiation in general electrocardiography, but is improved for antitachycardia devices which have more precise diagnostic information from intracardiac signals. Tachyarrhythmia discrimination in devices is performed in a stepwise process using sets or "blocks" of physiologically relevant information. Each set describes a specific timing or morphological aspect which contains characteristics of tachyarrhythmias. This results in a device diagnosis which is not always accurate, but is aimed at detecting all ventricular tachyarry-thmias, and therefore accepts to a certain level inappropriate therapies for supraventricular rhythms. These blocks with physiological information can also be applied in visual analysis of stored electrograms, derived from intrac-ardiac or thoracic leads and devices. This approach can be considered as a tool to improve the device diagnostics.
In single chamber devices, information "blocks" are derived from the sensed ventricular activity (Table 4.1). Each block contains information about the tachyarrhythmia, but it is evident that limitations are inherent to single chamber devices, certainly for discrimination between atrial and ventricular tachyarrhythmias.
VV Intervals (or Ventricular Rate)
Ventricular rate (the inverse of VV interval or ventricular cycle length) is a very sensitive parameter to detect all ventricular tachyarrhythmias. However it has a low specificity as it overlaps with all other tachycardias (sinus, atrial, and supraventricular). Nevertheless, rate-only detection was associated with a better outcome than detection based on early morphology criteria . Rate zones are now increasingly used, and divided into slow, normal, fast (or VT), very fast VT, and ventricular fibrillation. Evidently, this is not based on electrophysiology, but just on arbitrary definitions.
Table 4.1. Available information blocks in single chamber devices.
• VV intervals (or ventricular rate)
• electrogram width
• electrogram morphology
• VV interval stability
42 4. Understanding Stored Electrograms Electrogram Width
Far-field ventricular electrograms are used to measure the electrogram width. This interesting feature, with a link to the 12-lead criteria, has limitations, as the electrogram width should be compared to width during normal baseline or sinus rhythm. This criterion is not applicable in patients with complete bundle branch block . Another shortcoming of this criterion is the change in electrogram width due to additional antiarrhythmic drug therapy . In devices this criterion is linked to slew rate, which might indeed give a clue to the diagnosis of VT, as it is assumed that a long intrinsic deflection is associated with structural heart disease .
The widely used block "ventricular electrogram morphology", which is related to QRS width, or includes QRS width, is mainly based on the premise that the electrogram morphology will change during ventricular tachyarrhythmias as compared to a supraventricular baseline rhythm (Figure 4.3).
A distinct change in the electrogram morphology was identified in 93% of induced ventricular tachycardias . The analysis of far-field electrograms permits a more accurate arrhythmia classification. However, the development of rate-dependent aberrancy during supraventricular tachycardia alters the electrogram morphology as compared to baseline sinus rhythm . Specifically, a change in electrogram morphology was predominantly observed at recording sites ipsilateral to the bundle branch block .
The information block "VV interval stability" is used to discriminate between monomorphic ventricular tachycardia, characterized by regular ventricular intervals, and atrial fibrillation, characterized by irregular ventricular intervals . The limitation of this block is the regular ventricular response during atrial tachyarrhythmias with fixed N:1 atrioventricular conduction, as supraventricular and atrial tachycardia, and typically, 2:1 atrial flutter. Another limitation is the increased stability of VV intervals during atrial fibrillation with very fast ventricular response .
The block "sudden onset" can be used to discriminate sudden onset ventricular tachyarrhythmias from sinus tachycardia which is characterized by a gradual onset. However, sudden onset may not be specific for ventricular tachyarrhythmias as atrial or supraventricular tachycardia can be initiated by a premature beat and show a sudden onset pattern. This feature has been studied extensively [10, 12, 13].
Appropriate interpretation of stored electrograms in single chamber devices is not only based on changes in one information block. The information of the electrogram morphology is combined with information on the rate of the arrhythmia, the onset, and the stability of the arrhythmia. The recording of device activity markers (marker channel) provides additional information and requires little memory capacity (Figure 4.4).
It has been demonstrated that the combined use of electrogram morphology, rate characteristics, and VV interval stability allowed a correct diagnosis to be made in 97% of the events . A flow chart displaying a way of analyzing arrhythmias is shown in Figure 4.5. It also suggests that limitations remain present even with the combined information.
The visual analysis of stored electrograms from dual chamber devices applies information blocks based on atrial and ventricular physiological data (Table 4.2). Blocks derived from the ventricular activity are similar to those used in single chamber devices. Information blocks with clinical data of atrial activity are "AA stability", "atrial cycle length" and "atrial electrogram
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