A potential new technique to estimate the origins of focal atrial tachycardias from 12-lead electrocardiograms

Background: This brief report presents a potential new technique for locating the sites of origin of focal atrial tachycardias from standard 12-lead electrocardiograms. Methods: Estimated P wave vector coordinates are derived from leads I, aVF, V1 and V2, and mapped to a three dimensional coordinate system in space. A simple graphical method is used to find the back projection of the P wave vector onto the surface of atrial muscle to estimate the most likely site of origin of the tachycardia. Results: Graphical back projection correctly identified foci of atrial tachycardias in 19 of 19 cases described in the most complete available published study. Conclusions: Clinical studies are indicated to validate this simple method for initial noninvasive evaluation of candidates for ablation therapy. Freely Available Online www.openaccesspub.org | JHC CC-license DOI : 10.14302/issn.2329-9487.jhc-16-931 Vol 2 Issue 1 Page No18 Introduction Unifocal atrial tachycardia is an uncommon form of supraventricular tachycardia, often resistant to drug therapy, for which percutaneous radiofrequency catheter ablation is an effective treatment option. 2 Anatomic localization of the earliest activation site is important for effective targeting of ablation. Previous workers have proposed that analysis of P wave morphology in the standard 12-lead electrocardiogram may be a useful initial step in predicting the origin of an atrial tachycardia. The present study takes a fresh look at this problem to determine if better localization is possible when information from multiple electrocardiographic leads is combined in a spatial vector analysis. Preliminary studies in computational models of impulse propagation in the atria and the resulting body surface electrocardiographic potentials were done, similar to previous work on ventricular pacing. 9 These studies suggested the following scheme for localizing initial atrial activation sites in a given patient.


Introduction
Unifocal atrial tachycardia is an uncommon form of supraventricular tachycardia, often resistant to drug therapy, for which percutaneous radiofrequency catheter ablation is an effective treatment option. 1,2 natomic localization of the earliest activation site is important for effective targeting of ablation.Previous workers have proposed that analysis of P wave morphology in the standard 12-lead electrocardiogram may be a useful initial step in predicting the origin of an atrial tachycardia [3][4][5][6] .The present study takes a fresh look at this problem to determine if better localization is pos sible when i nformation from mul ti pl e electrocardiographic leads is combined in a spatial vector analysis 7 .Preliminary studies in computational models of impulse propagation in the atria and the resulting body surface electrocardiographic potentials were done, similar to previous work on ventricular pacing. 8,9 hese studies suggested the following scheme for localizing initial atrial activation sites in a given patient.

Methods
A three dimensional, orthogonal (x,y,z) coordinate system is defined with its origin located in the middle of the atrial septum, the x-axis pointing toward the left, y-axis pointing toward the feet (inferiorly), and z-axis pointing toward the sternum (anteriorly).A three dimensional P wave vector is defined by the amplitudes of the P wave at its midpoint in the time domain for standard Leads I, aVF, V 1 , and V 2 .The x coordinate of the P vector is the P wave amplitude in Lead 1.The y coordinate of the P vector is the P wave amplitude in Lead aVF.The z coordinate of the P vector is the average of the P wave amplitudes in Leads V 1 and V 2 .
For easy paper-and-pencil analysis corresponding two dimensional vectors are drawn in three planes: the Lead I vs. aVF (xy or frontal) plane, the Lead I vs. V 12 (xz or horizontal) plane, and the V 12 vs.aVF (zy or sagital) plane, as shown in Figure 1.The legend of Figure 1 describes a detailed example of the method for representative Case 6 in Table 1.In brief, an imaginary line is back-projected in each plane along the axis of each two dimensional vector through the origin to a point intersecting simple anatomic sketches of the left and right atria, which were originally created from inspection of atrial anatomy in CT scans of normal human hearts.The consensus intersection of the back projection with the atrial wall in three dimensions gives the putative origin of the atrial beat.
If a back projection intersects more than one point on an atrial wall in a particular plane, the ambiguity is resolved by finding the one site that best fits all three projections.
Only the angles or directions of the vectors and the relative sizes and shapes of the atrial chambers are critical.Plotting of the vectors can be done in any convenient units, such as millivolts or "boxes" on a standard strip chart record of the electrocardiogram.
The vectors can be drawn to any consistent scale of length, and the atrial sketches can be drawn to any convenient size.

Results
To explore the potential utility of this approach, the published data in Figures 2, 3, and 5 of the study by Kistler et al. 5 were re-analyzed using P vector back projection.This study described 12-lead P wave morphologies in patients undergoing successful radiofrequency ablation.Actual sites of earliest endocardial activation were determined by Kistler et al.
using intracardiac electrodes and point mapping of the times of onset of local electrograms relative to the body surface P wave.Full 12-lead electrocardiograms were provided for 19 representative cases.Figure 1 shows the graphical, back projection analysis of a typical case.
Summary results for all 19 cases are shown in Table 1.
In 19/19 cases the back projection method gave good localization ("hit") within approximately 1 cm of the actual site of first activation.vector is located at a point defined by the P wave amplitudes in Leads I, aVF, and V 1-2 , as listed in Table 1 for Case 6.In this particular example P wave vectors (thick arrows) are constructed from the origin in each plane, as follows.
Frontal plane (top): Lead I, 0; aVF, -4; vector points toward the head.Horizontal plane (middle): Lead I, 0; average of V 1 and V 2 , -2; vector points toward the back.Sagital plane (bottom): average of V 1 and V 2 , -2; aVF, -4; vector points toward the back shoulder.An imaginary line is back projected from the head of each vector, through the tail of the vector at the origin, until it intersects with the superimposed sketch of the atrial wall (intersection point in three dimensions indicated by stars).This point of intersection is the putative electrophysiological origin of the tachycardia.RA right atrium; LA left atrium.In this case the actual source was in the low tricuspid annulus, which is inferior and anterior to the midpoint of the interatrial septum.There are, however, some important assumptions which may prove to be significant limitations.
One assumption is that that impulse propagation from a source is directionally uniform rather than taking preferential intra-and interatrial conduction routes.The effects upon P wave morphology from septal activation are ignored, as is the presence of atrial structural disease.The presented technique applies to the patients with intact atria and may not apply to atrial tachycardias in which atrial activation sequence is altered because of prior atrial ablation or extensive scarring.Further, the present study is retrospective in nature.

Figure 1 .
Figure1.Reconstruction of the atrial activation site (star) by back projection in three planes.The common origin of the Lead I, aVF, V 1-2 (or x,y,z) coordinate system is located in the middle of the interatrial septum.Arrows on axes indicate positive directions from the origin.The tail of each vector is located at the origin.The arrow head of each vector is located at a point defined by the P wave amplitudes in Leads I, aVF, and V 1-2 , as listed in Table1for Case 6.In this particular example P wave vectors (thick arrows) are constructed from the origin in each plane, as follows.Frontal plane (top): Lead I, 0; aVF, -4; vector points toward the head.Horizontal plane (middle): Lead I, 0; average of V 1 and V 2 , -2; vector points toward the back.Sagital plane (bottom): average of V 1 and V 2 , -2; aVF, -4; vector points toward the back shoulder.An imaginary line is back projected from the head of each vector, through the tail of the vector at the origin, until it intersects with the superimposed sketch of the atrial wall (intersection point in three dimensions indicated by stars).This point of intersection is the putative electrophysiological origin of the tachycardia.RA right atrium; LA left atrium.In this case the actual source was in the low tricuspid annulus, which is inferior and anterior to the midpoint of the interatrial septum.

Table 1 :
5pplication of graphical back projection to 12-Lead ECG's in patients with verified sources of atrial tachycardias (Kistler et al.5).