BOLETÍN OFICIAL DEL ESTADO
TÍTULO 12 Derechos sindicales
to see whether this might be due to a recording artifact. The results from the analysis of the combined data available from two previous studies, as well as results from the literature are discussed. The focus here was on the quantification of potential magnitude during the PQ segment of healthy subjects for which some reference data were available from the literature on body surface potential maps and VCGs. No effects of a multitude of various other possible factors or pathology were studied. The derived model is currently used for the analysis of the simultaneous presence and superposition of atrial depolarization and atrial repolarization, or atrial repolarization and ventricular depolarization, both during normal activation and during AF. In addition, it is used to establish which chamber is contributing to which portions of atrial repolarization on the body surface. This can only be accomplished by relating the surface events to what is going on electrophysiologically in the heart at the same time. The model includes sophisticated elements for modeling the myocardial sources, linked to electrophysiology as well as of the biophysics of linking sources to body surface potentials. The results of the application of this model, extending those already shown in98, will be presented at a later stage.
2.7
Conclusion
This study confirms earlier reports that the PQ is not isoelectric. The time course of the potential distribution is very similar to that at the apex of the P wave, but for a reversed polarity and about 3-fold lower magnitude. The local potential extremes during this segment were found at positions not sampled by the standard leads, which implies that the positions of the standard leads are sub-optimal for studying the electric activity of the atria. The results demonstrate a significant involvement of atrial repolarization during the PQ interval, and essentially discordant “atrial T waves”, suggesting a small dispersion of atrial action potential durations. This study stresses the need for an appropriate signal processing of the (low-level) atrial ECG potentials and introduces some new methods for the directional statistics of the directions involved in the analysis of vectorcardiograms.
Adaptation of the standard
12-lead ECG system
dedicated to the analysis of
atrial fibrillation
3
3.1
Introduction
B
EING the most commonly used tool for the non-invasive diagnosis of cardiac diseases, the standard 12-lead ECG measures the body surface potentials (BSP) as an expression of the electric state of the heart. Historically, the interest of electrocardiology has been focused on the electric activity of the ventricles: the nine electrodes of the 12-lead ECG were positioned on the thorax in order to follow the global depolarization and repolarization of the ventricles.1 It is therefore likely that the information content available from these electrodes may not be optimal for studying atrial activity and, in particular, not if the interest is focused on the diagnosis of atrial fibrillation (AF).A clearer view of atrial activity may be gleaned from the method of body surface potential mapping (BSPM). This was in fact performed right after the early introduction of this method.19,89 These studies have not led to any adaptation of the 12-lead system that is used clinically. Moreover, this method so far has not been applied to the analysis of AF. The more recent study of SippensGroenewegen et al.86 reported on the analysis of BSPMs for the localization of the atrial foci responsible for atrial tachycardia, but the general problem of extracting information from the ECG for the classification of different types of AF was not addressed.
The current interest in morbidity and mortality related to AF has prompted the work presented in this chapter, aimed at designing a lead system dedicated to the extraction of information about the atrial electric activity during AF. In view of its ultimate clinical application and the highly limited availability of BSPM equipment in the clinic, the design
34 Chapter 3. OACG lead system
was restricted to the incorporation of just nine electrodes, the number of electrodes in- volved in the recording of the standard 12-lead ECG. In this way, standard, available ECG recording equipment could be used.
Another design constraint formulated in advance was that the electrode positions of the adapted lead system should be anchored as much as possible to those of the standard leads. This would reduce the complexity of lead placement in the clinic and reduce the possibility of lead misplacement, a problem even encountered frequently in the ’standard’ positioning of the nine electrodes.38In an early, heuristic implementation of this constraint, the extremity
electrodes VR, VL and VF were left in place, as were the electrodes V1 and V2.47Following the recording of an ECG at the ’normal’ 12-lead electrode locations, the electrodes sensing V3-V6 were, sequentially, repositioned in a counterclockwise fashion, around those of V1 and V2 (figure 3.1). The positions chosen were inspired by their close proximity to the atria. By including the extremity electrodes, the commonly used Wilson Central Terminal (WCT) as the potential reference for the observed signals could be retained. This lead system dedicated to the recording of atrial signals, denoted in this paper as the ACG (atriocardiogram) lead system, is currently being tested in the clinic. At the present time up to 120 recordings on AF patients have been documented. For each of these, both the 12-lead ECG and the ACG-lead signals have been stored. No difficulties were encountered in the clinical implementation of this procedure.
A preliminary analysis of the clinical data comparing the AF signals derived from both lead systems suggested that, indeed, the adapted lead positions would provide a clearer view on AF. This prompted us to investigate the optimal lead positions for replacing 4 of the 6 precordial electrodes. The method for finding this optimum and the results obtained are discussed in this chapter. This search demands the availability of body surface potentials during AF over the entire thorax. Since no such data are available, we used as an alternative the AF signals generated by a previously developed biophysical model. The model includes the descriptions of the active electric sources during AF, as well as their expression on the thorax. It has recently been shown to generate AF signals that are in full qualitative agreement with those observed clinically.51 An attractive feature of using simulated data is that it permits the analysis of AF signals that are completely free of any ventricular involvement.
After describing the materials and methods employed, the results will be shown of a comparison between the standard 12-lead system, the ACG-lead system and the lead positions found to be optimal. The latter will be referred to as the OACG-lead system.