What Are the Benefits of Morphological Signal Analysis Using Digital Technology?

What Are the Benefits of Morphological Signal Analysis Using Digital Technology?

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Digital technology is all around us. However only since the late 1970s has digital technology been incorporated into cardiac pacemakers. Memory, cir- cuits, and counters became larger, faster, and more accurate. Improvements in sensor-driven pacing, dual-chamber pacing, and specific algorithms such as mode switching and overdrive pacing were made possible because of the advances in digital technology. The first microprocessors in cardiac pacing were introduced in the DGP-1 platform made by Vitatron b.v. (Arnhem, The Netherlands), the same company who recently introduced digital signal pro- cessing (DSP).

Standard pacemakers will amplify the intracardiac signal, then filter the signal using high- and low-pass filters, after which the signal is sensed by the pacemaker if it exceeds a certain programmable threshold. However, the true morphology of the signal is no longer present after this process and a certain amount of information is lost. Classification of intracardiac events and events of extracardiac origin based on timing and morphology is not possi- ble in these analogue devices. Filters and blanking periods are a suboptimal solution to this problem. This may result in serious adverse events for the pacemaker patients such as undersensing of atrial arrhythmias.

In the new digital pacemakers, DSP will process the analogue electrical signals coming from the heart (atrium or ventricle) into a digital format.

After digitising the intracardiac signal, the pacemaker will use specially designed algorithms to make certain decisions or discriminate between sig- nals from different origins. To sample a signal reliably, a very high sampling rate has to be achieved. The current DSP platform samples at a rate of 800

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samples per second, allowing reliable description of the signal in which all relevant information is contained. After the signal is digitised by the ana- logue-to-digital converter, flexible filters are used to yield the components of the intracardiac signal, which are of clinical interest. The main engineering challenge consisted of designing a DSP circuit without increasing the power consumption. This has been achieved in the currently available commercial digital pacemakers.

Now that we have DSP, numerous opportunities arise which will make pacing more physiological and reliable. The first application, which is cur- rently being evaluated, is discriminating far-field R waves (FFRW) from true atrial signals. FFRW are usually managed by extending the post-ventricular atrial blanking period. However, by doing so, we increase the risk of 2:1 sens- ing and tracking of atrial tachyarrhythmias. In addition, FFRW sensing may result in false detection of atrial tachyarrhythmias, leading to unnecessary mode switches with loss of atrioventricular synchrony and incorrect diag- nostic counters for atrial tachycardias and atrial fibrillation. A recent study by Van Hemel et al. demonstrated that a novel DSP algorithm reliably sepa- rated FFRW from P waves [1]. Data from 100 bipolar and unipolar intracar- diac atrial recordings from 31 patients were collected during pacemaker replacement and analysed off line. Sensitivity and specificity of FFRW detec- tion were 99.3% and 100%. Moreover, no P waves were falsely classified. The DSP algorithm is therefore capable of preventing false mode switches.

Further studies are on their way using data collected from implanted pace- makers during 24-h external Holter recordings and bicycle exercise. These results will provide further information on the accuracy of this specific DSP algorithm, and ultimately this algorithm will be incorporated in future devices.

Looking to the future, it seems that the future is digital. Numerous oppor- tunities arise from digital signal processing, not only in pacemakers but also in implantable cardioverter–defibrillators (ICDs). New applications of digital morphology discrimination could be the discrimination of fusion beats, especially in auto-capture devices. Improved sensing of supraventricular tachycardias in ICDs could reduce the incidence of inappropriate therapies, thereby greatly improving patient quality of life and the acceptance of ICDs.

In the coming years many of these new applications will find their way into clinical practice.

Reference

1. Van Hemel NM, Wohlgemuth P, Engbers J et al (2004) Form analysis using digital signal processing reliably discriminates far-field R waves from P waves. Pacing Clin Electrophysiol 27:1615–1624

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