Rhythm Classification by Arrhythmia Management Devices

Summary

A reliable ICD needs both sensing of the endocardial signal and detection algorithms for arrhythmia diagnosis. Initially, the only target was ventricular fibrillation, but with the possibility of rate detection ventricular tachycardia could be detected as well. This led to the development of tiered-therapy devices with arrhythmia zones. Simple arrhythmia discriminators as stability and sudden onset became available, and proved to be very useful. The place of more complex algorithms is still unclear.

The ability of an ICD to reliably detect life-threatening ventricular arrhythmias is one of the most essential features of a device. It involves both sensing of the endocardial signal and the application of detection algorithms for arrhythmia diagnosis [1, 2]. The ICD must consistently sense all ventricular depolarizations to accurately determine the heart rate during sinus rhythm and tachyarrhythmias (Figure 2.1). The challenge is to reliably sense low amplitude signals during ventricular fibrillation and to avoid sensing of T waves or extracardiac signals (Figure 2.2).

Actually, two different endocardiac lead designs for sensing, either dedicated bipolar or integrated bipolar, are used. Bipolar epicardiac signals are only exceptionally necessary (pediatric patients, congenital heart disease) and pose specific problems (Figure 2.3). Sensing with a dedicated endocardiac bipolar system is accomplished between the tip electrode and a second ring electrode, approximately 10 mm from the tip. With the integrated bipolar configuration, sensing is accomplished between the tip electrode and the right ventricular shocking coil. Both dedicated and integrated bipolar lead sensing concepts are effective for sensing low amplitude signals during ventricular fibrillation.

Other sensing configurations have been used in different conditions or for specific devices, as a unipolar lead with a patch, or a can. Sensing of the left ventricular signal is now introduced for biventricular approaches (resynchronization).

Figure 2.1. Different levels of sensitivity result in recognition (black vertical bars) of only the normal complexes (low sensitivity); recognition of normal complexes and ventricular premature beats (normal sensitivity); recognition of normal complexes, ventricular premature beats, and ventricular fibrillation (high sensitivity). The drawback in the last situation is that T-waves are sensed as well (grey bars).

Figure 2.1. Different levels of sensitivity result in recognition (black vertical bars) of only the normal complexes (low sensitivity); recognition of normal complexes and ventricular premature beats (normal sensitivity); recognition of normal complexes, ventricular premature beats, and ventricular fibrillation (high sensitivity). The drawback in the last situation is that T-waves are sensed as well (grey bars).

Figure 2.2. Current ICDs utilize either automatic gain control or auto-adjusting threshold to ensure reliable sensing. In automatic gain control, the sensing threshold is fixed while continuous adjustment of the gain is performed to ensure maximum sensing. With this method, the gain is increased when the amplitudes of the R wave decrease from large to small. In auto-adjusting threshold, the gain is fixed and the threshold is adjusted. Auto-adjusting threshold uses a constant amplification of the amplitude of the R wave, which becomes the starting amplitude of the time-decay threshold. The figure shows how the ventricular arrhythmia is correctly recognised because the threshold (dotted line) is adjusted after the QRS complexes, while the T-wave is not sensed.

Figure 2.2. Current ICDs utilize either automatic gain control or auto-adjusting threshold to ensure reliable sensing. In automatic gain control, the sensing threshold is fixed while continuous adjustment of the gain is performed to ensure maximum sensing. With this method, the gain is increased when the amplitudes of the R wave decrease from large to small. In auto-adjusting threshold, the gain is fixed and the threshold is adjusted. Auto-adjusting threshold uses a constant amplification of the amplitude of the R wave, which becomes the starting amplitude of the time-decay threshold. The figure shows how the ventricular arrhythmia is correctly recognised because the threshold (dotted line) is adjusted after the QRS complexes, while the T-wave is not sensed.

Verification of Signals During Implantation

The lead electrogram and markers should be checked during implantation for evidence of correct sensing. Loose connections and oversensing of intracardiac or extracardiac signals should be recognized in this stage (Figure 2.4). If the patient had a previously implanted or abandoned ventricular lead in place, it is important to check for mechanical lead 'chatter' that can generate signals mimicking ventricular tachyarrhythmias. If lead 'chatter' is observed on the electrograms, a reposition of the lead is necessary. It seems wise to remove redundant leads. Connectors and adaptors are predisposed to additional noise, and should be avoided.

Early Devices: Detection of Ventricular Fibrillation 9

Early Devices: Detection of Ventricular Fibrillation 9

Rhythm Classification

Figure 2.3. Ventricular fibrillation with conversion to sinus rhythm after a short irregular episode with wide QRS complexes (ventricular wide band and bipolar electrograms are displayed). The first electrogram is taken between epicardiac patches and shows impressive ST segment elevation, also after normalisation of the QRS width. This will certainly affect sensing properties directly after a shock.

Figure 2.3. Ventricular fibrillation with conversion to sinus rhythm after a short irregular episode with wide QRS complexes (ventricular wide band and bipolar electrograms are displayed). The first electrogram is taken between epicardiac patches and shows impressive ST segment elevation, also after normalisation of the QRS width. This will certainly affect sensing properties directly after a shock.

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