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Figure 4.10. Detection of double tachycardia, which is ventricular tachycardia (VT) during atrial flutter (AFl). The marker channel demonstrates appropriately detected AFl (1) with a consistent atrioventricular (AV) conduction pattern. A ventricular premature beat (2) initiates VT (3). At the onset of VT, the marker channel demonstrates a change in the AV conduction pattern. During VT, there is AV dissociation. Markers: TD = tachycardia detected; TS = tachycardia sensing; VS = ventricular sensing.

and ventricular tachyarrhythmias, the first step is to analyze the onset of the tachyarrhythmia. Sinus tachycardia is characterized by a gradual onset, whereas ventricular tachycardia has a sudden onset. The application of the blocks "chamber of origin" and "ventricular electrogram morphology" offers additional information for further differentiation between sinus and ventricular tachycardia. The 'chamber of origin' is used to identify the initiating event at the onset of tachycardia. At the onset of ventricular tachycardia, an intrinsic atrial event usually does not occur between the last conducted

Figure 4.11. Stored electrogram showing atrial tachycardia with consistent 1:1 atrioventricular (AV) conduction. Rhythm strip from top to bottom: atrial, ventricular, and shock electrogram. After six normal conducted ventricular events (1), a ventricular premature beat (2) occurs, which is followed by two normal conducted ventricular events. A premature atrial event (3) initiates an atrial tachycardia with stable 1:1 AV conduction (4). The "chamber of origin" is the atrium and there is no change in the "ventricular electrogram morphology". Markers: AS = atrial sensing; VF = ventricular fibrillation window; VS = ventricular sensing; VT = ventricular tachycardia window; Epsd = initial tachycardia detection met; Suddn = sudden onset; - = no annotiation of stored events before detection of tachycardia. (Guidant Prizm DR, model 1861)

Figure 4.11. Stored electrogram showing atrial tachycardia with consistent 1:1 atrioventricular (AV) conduction. Rhythm strip from top to bottom: atrial, ventricular, and shock electrogram. After six normal conducted ventricular events (1), a ventricular premature beat (2) occurs, which is followed by two normal conducted ventricular events. A premature atrial event (3) initiates an atrial tachycardia with stable 1:1 AV conduction (4). The "chamber of origin" is the atrium and there is no change in the "ventricular electrogram morphology". Markers: AS = atrial sensing; VF = ventricular fibrillation window; VS = ventricular sensing; VT = ventricular tachycardia window; Epsd = initial tachycardia detection met; Suddn = sudden onset; - = no annotiation of stored events before detection of tachycardia. (Guidant Prizm DR, model 1861)

sinus beat and the first ventricular ectopic event. On the other hand, an atrial event is present before every ventricular event at the onset of atrial tachycardia (Figure 4.11). It is speculative, but the atrial electrogram should be different when retrograde activation occurs.

The combination of blocks "AV conduction pattern", "chamber of origin", "sudden onset" and "ventricular electrogram morphology" is necessary for challenging tachyarrhythmias with 1:1 AV conduction. Examples are atrial tachycardias with a sudden onset, and sinus or atrial tachycardia with progressive prolongation of the AV conduction.

A Standardized Approach for Electrogram Analysis

For clinical trials, correct electrogram interpretation is important. In a recent study, the overall performance of physicians in electrogram interpretation was similar to the ICD [1, 2, 20]. However, the composition of the misinterpretation was different, which can have a severe impact on the outcome of clinical trials. To improve the accuracy and reproducibility of electrogram analysis, a standardized approach has been developed for a core ICD laboratory (Figure 4.12). We will explain it very briefly here. The electrogram corresponding to the first beat of the tachycardia is labeled "A".

The preceding beat, (usually) the last "normal" conducted sinus beat, is labeled "0". The coupling interval of the premature depolarization initiating the tachycardia (from beat "0" to beat "A") is labeled "V1". The consecutive intervals of the tachyarrhythmia are numerically sequentially labeled Tj through T12. The preceding intervals are labeled S— through S-8. This approach is now under validation and is being used in a large clinical trial [21].

Figure 4.12. Schematic approach of the analysis of ventricular tachyarrhythmias. From top to bottom are displayed the surface electrocardiogram (ECG), the marker channel (M), the atrial electrogram (A), and the ventricular electrogram (V). The premature ventricular depolarization initiating the tachyarrhythmia is labeled "A". The preceding interval, the last normal conducted beat, is labeled '0'. The corresponding intervals for analysis are S_, to S_8, V,, and T, to T12.

Figure 4.12. Schematic approach of the analysis of ventricular tachyarrhythmias. From top to bottom are displayed the surface electrocardiogram (ECG), the marker channel (M), the atrial electrogram (A), and the ventricular electrogram (V). The premature ventricular depolarization initiating the tachyarrhythmia is labeled "A". The preceding interval, the last normal conducted beat, is labeled '0'. The corresponding intervals for analysis are S_, to S_8, V,, and T, to T12.

Preliminary analysis taught us that the most critical step is the definition of the first tachycardia complex. If this is done correctly, further interpetation usually becomes easy when the flow charts as presented in the next chapter are used. It turned out that the core lab was as performant in this way as a team of experienced electrophysiologists.

A Practical Approach to Analyze Electrograms

A first question will always remain whether there is a tachyarrhythmia. Such events can be overlooked as electrograms can be unclear. If so, one should consider that noise (an intracardiac, extracardiac, even extracorporeal source) is interfering with the recording of the electrocardiogram by the device.

If the idea remains present that an arrhythmia is present, one should scan the tracing for the onset. This will reveal whether the arrhythmia starts in the atrium or in the ventricle (if both recordings are available).

The third step is to compare the atrial and ventricular rate, which will reduce the number of possible rhythms.

A fourth step is interpretation of the atrial rhythm (rate, regularity, electrogram), comparing it with the available tracings after conversion, of those available before the arrhythmia. The fifth step does the same with the ventricular rhythm. Critical analysis of the effect of therapy (resetting, changes in rhythm, rate and morphology) will finally elucidate the problem. The next chapter will further describe these practical steps.

It is evident that if all electrogram information is present, a 12-lead electrocardiogram becomes irrelevant for the diagnosis.

References

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