A step-wise logical approach is presented both for devices with and without atrial information. The presented flow charts can help to interpret rhythm strips, but keep their limitations, certainly for single chamber devices.
Rhythms detected by the ICD may represent either a tachyarrhythmia, which can be atrial or ventricular, or oversensing of electrical signals (Figure 5.1). The first step in the analysis of stored electrograms is to determine whether a true tachyarrhythmia is present (Figure 5.2).
In the absence of a true tachyarrhythmia, inappropriate therapy is mainly caused by oversensing of intracardiac or extracardiac signals. Intracardiac signals that can cause oversensing include P, R, or T waves (Figure 5.3). Ventricular oversensing of intracardiac signals results in more than one detected ventricular activitiy per cardiac cycle. Dependent on the cardiac cycle, ventricular oversensing can result in inappropriate detection of ventricular tachyarrhythmias either in the tachycardia detection zone or in the fibrillation detection zone. Oversensing of intracardiac signals can be recognized by characteristics alternation of intervals and electrogram morphology separated by isoelectric baseline. Extracardiac signals include electromagnetic interference, signals generated by lead or connector problems, and myopotentials. The hallmark of oversensing of extracardiac signals is the replacement of the isoelectric baseline with high-frequency noise.
If the device is triggered by a true tachyarrhythmia, the investigator has to determine if the initial detected rhythm is a ventricular tachyarrhythmia or an atrial tachyarrhythmia. The second step in the analysis of stored electrograms is to determine the onset of the tachyarrhythmia. Figure 5.4 shows the second step of the analysis for single-chamber and dual-chamber electrograms.
The ventricular electrogram and the marker channel are used to determine the onset of the tachyarrhythmia. The first step is to determine the last normal conducted beat followed by the first beat of the tachyarrhythmia. In the marker channel, the first beat of the tachyarrhythmia is often labeled as "TS", preceded by a normal beat labeled as "VS". The ventricular electrogram can be used to confirm the onset of a ventricular tachyarrhythmia. The tachyarrhythmia is classified as ventricular if the morphology during tachyarrhythmia is distinctly different from the baseline rhythm (Figures 5.5 and 5.6).
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