Atrial tachycardia (atrial tachycardia) is defined as tachycardia of atrial origin with a regular atrial rhythm and can be classified as focal atrial tachycardia or macroreentrant atrial tachycardia. Focal atrial tachycardia may be due to autoregulatory mechanisms, triggering mechanisms, and microfold mechanisms.
Because focal atrial tachycardia is a condition in which excitation propagates radially, circularly, or centripetally outward from a single focal point of excitation, and there is no electrical activity spanning the entire foldback loop, focal radiofrequency ablation at the local site of earliest atrial excitation can successfully eliminate atrial tachycardia. The points of origin of focal atrial tachycardia are mainly located at specific anatomic sites within the atria, such as the terminal crest, the atria near the tricuspid and mitral annuli, the coronary sinus orifice, the pulmonary vein orifice, the junction of the superior and inferior vena cava with the right atrium, and the right and left heart ears.
In general, focal atrial tachycardia can be seen as P waves separated by isotropic lines on all ECG leads. However, differences in atrial origin and overall depolarization vectors of the atria in focal atrial tachycardia lead to differences in body surface ECG P-wave morphology during atrial tachycardia. The analysis of P-wave morphology on the surface ECG can roughly locate the origin of focal atrial tachycardia, which is helpful for the preoperative preparation and rapid target marking during RF ablation.
The literature on locating the site of origin of atrial tachycardia based on P-wave morphology and a flow chart for rapid determination of the origin of atrial tachycardia based on body surface ECG P-wave morphology is presented as follows.
1. Judgment of atrial tachycardia in the left atrium (left atrium) and right atrium (right atrium)
The difference in P-wave morphology between left and right atrial tachycardia is determined by the relative position of the left and right atria. Tang et al. have proposed a flow chart to identify left atrial tachycardia and right atrial tachycardia based on body surface ECG P-wave morphology (Figure 1).
The P-wave morphology in leads V1 and aVL is most helpful in differentiating left atrial tachycardia from right atrial tachycardia. v1 is a right thoracic lead positioned in the right anterior wall of the atrium, and the left atrium is anatomically positioned in the posterior center of the heart, where the excitation of left atrial tachycardia produces a forward depolarizing vector, i.e., a positive P-wave in lead V1. The study confirmed that the positive P wave in the V1 lead has a high specificity and sensitivity in predicting left atrial tachycardia. aVL lead is located in the high lateral wall of the left atrium and deviates from the depolarizing vector generated by left atrial tachycardia, so a negative P wave can be observed in the aVL lead.
Studies have shown that negative P waves in the aVL leads have high specificity but low sensitivity in predicting left atrial tachycardia; positive or bidirectional P waves in the aVL leads have higher specificity and sensitivity in predicting right atrial tachycardia.
In addition, the specificity of forward P waves in lead I for predicting left atrial tachycardia was high, but extremely insensitive.
Determination of upper and lower atrial tachycardia
Atrial tachycardia of superior and inferior atrial origin can be distinguished according to the morphology of P waves in the inferior wall leads. If the P waves in the II, III and aVF leads are positive, it indicates that the atrial tachycardia originates from the upper part of the atrium, such as the right auricle, the high lateral wall of the right atrium, the superior vena cava, the superior pulmonary vein of the left atrium or the left auricle; conversely, if the P waves are negative, it indicates that the atrial tachycardia originates from the lower part of the atrium, such as the coronary sinus orifice, the posterior septum of the right atrium or the inferior lateral wall of the left atrium.
3.Determination of right atrial tachycardia
Tada et al. divided the right atrium into 4 zones according to the 45° image of the right atrium in the left anterior oblique position, using the horizontal line passing through the Hirschsprung bundle and the line connecting the superior and inferior vena cava orifices as the horizontal and vertical axes, respectively (Figure 2), and proposed the identification process of right atrial tachycardia (Figure 3).
3.1 Atrial tachycardia of terminal crest origin
The terminal ridge is a longitudinal bulge on the endothelial surface of the right atrium that begins in the upper part of the interatrial septum, passes anterior to the superior vena cava opening, continues down and across the entire posterior free wall of the right atrium, and forms the Euclidean flap and Euclidean ridge at the anterior edge of the inferior vena cava opening.
The physiological reason for the high incidence of atrial tachycardia in this region is the poor lateral cell-to-cell coupling within the terminal cristae tissue, which results in significant anisotropy and the possible formation of slow conduction and microfold;
Another reason may be the presence of autoregulatory cell clusters. The terminal ridge is the site of the right atrial tachycardia, especially the high terminal ridge, and the site of origin decreases from top to bottom along the long axis of the terminal ridge.
P waves in leads I and II are positive, and V1 leads are positive and negative, or V1 leads are positive in both subsinus rhythm and atrial tachycardia to predict the origin of atrial tachycardia in the terminal ridge with high specificity and sensitivity (93%, 95%). Positive P waves in the inferior wall leads can further distinguish between high and low terminal crests.
Since the right superior pulmonary vein is anatomically closer to the high end crest, it is more difficult to distinguish between the two, and changes in P-wave morphology in the V1 leads can help to differentiate them. If the P-wave in V1 leads is bidirectional during sinus rhythm and becomes positive during atrial tachycardia, it can be identified as atrial tachycardia of right superior pulmonary vein orifice origin, while atrial tachycardia of terminal crest origin does not have this change.
3.2 Atrial tachycardia of tricuspid annular origin
MORTON et al. subdivided the tricuspid annulus into 4 regions: superior, inferior, anterior, and septal parts of the tricuspid annulus. The origin of atrial tachycardia is mostly found in the anterior-inferior part of the annulus (7 of 9 cases originated in the anterior-inferior part of the tricuspid annulus). v1 lead P-wave negative with tangential traces and aVL lead P-wave positive, or located in the isotropic line, has a high specificity and sensitivity in predicting the origin of atrial tachycardia in the tricuspid annulus (97%, 83%). Because of the relatively inferior position of the tricuspid annulus, the P wave in atrial tachycardia originating from the tricuspid annulus is negative in at least one inferior wall lead, especially in lead III. In addition, KISTLER et al. suggested that the P-wave morphology of atrial tachycardia of right auricular origin is similar to that of tricuspid annulus origin.
3.3 Atrial tachycardia of septal and coronary sinus orifice origin
After identification of right atrial tachycardia, negative P waves in leads V5 and V6 suggest that atrial tachycardia originates in the septal and coronary sinus orifice sites. The septal atrial tachycardia was seen in the anterior septum, middle septum and posterior septum. With the transition from the anterior septum to the posterior septum, the P waves in the inferior wall leads changed from positive to negative, and the P waves in lead V1 changed from negative to positive. Because of the low position of the coronary sinus orifice, the P waves of atrial tachycardia originating from the coronary sinus orifice were deeply inverted in leads II, III, and aVF, and the degree of P wave inversion in leads II and III was significantly deeper than that in lead aVF, and the P waves in leads aVL and aVR were positive, and the low voltage of P waves in lead I was <0.05 mV. For atrial tachycardia originating from Koch's triangle, the P wave time frame in the inferior wall leads was narrower than that in sinus rhythm because of the simultaneous excitation of the right and left atria. The P-wave time frame in the inferior wall leads is narrower than that in sinus rhythm: atrial tachycardia/sinus rhythm <0.85.
3.4 Atrial tachycardia of superior vena cava origin
The superior vena cava is located at the base of the heart and is connected to the high right atrium, similar to the pulmonary veins, and also has a myocuff-like structure, which is an important site of origin for myocuff arrhythmias. Due to its anatomical location close to the sinus node, the P-wave pattern during atrial tachycardia is similar to that of sinus rhythm.
In our center, we have analyzed 120 cases of myocuffal atrial arrhythmias treated by radiofrequency ablation, and 8 of them originated from the superior vena cava, with the following common features: the P-wave amplitude in the inferior wall leads was higher than that in sinus rhythm, especially in lead II; the P-wave in the aVL leads was negative with low amplitude; the P-wave in lead I was positive but low and flat. Due to the close anatomical location of superior vena cava and right superior pulmonary vein, the variable geometry of superior vena cava, right superior pulmonary vein and left atrium, the anisotropic conduction between myocardial cuff tissue and atrial tissue, and the variable electrical connections between left and right atria, it is difficult to identify atrial tachycardia of superior vena cava and right superior pulmonary vein origin based on body ECG P-wave morphology.
The common features of both are: positive P waves in the inferior wall leads, negative P waves in the aVR leads, positive P waves in most of the I leads, and indeterminate P wave polarity in the aVL leads. However, V1 lead P waves are positive in the right superior pulmonary vein origin and can be positive or negative in both directions or located in the isoelectric line in the superior vena cava origin.
In summary, when identifying atrial tachycardia of superior vena cava and right superior pulmonary vein origin, the sensitivity of P-wave morphology in inferior wall leads, lead I, aVR leads and aVL leads is high but specificity is low; the specificity of P-wave in V1 leads is higher. However, it is less meaningful to distinguish superior vena cava from right superior pulmonary vein origin by P-wave morphology in individual leads of the body ECG. The sensitivity and specificity of predicting the origin of superior vena cava can be improved when the P waves in V1 leads are positive or negative in both directions or located in the isotonic line, and when the P waves in aVL leads are also bidirectional.
4.Determination of left atrial tachycardia
Thirty-seven percent of focal atrial tachycardias originate in the left atrium. Because of the variable geometry of the pulmonary veins and left atrium, the complex anatomy of the pulmonary vein orifice and mitral annulus, the variety of myocardial fibers, the ease of slow conduction, and the increased anisotropy, focal atrial tachycardia in the left atrium is mostly seen in the pulmonary vein orifice and mitral annulus.
4.1 Atrial tachycardia of pulmonary vein origin
The occurrence of atrial tachycardia at the pulmonary vein site is related to the embryonic developmental process and cellular organization of the pulmonary veins. During the embryonic period, the posterior lateral wall of the left atrium differentiates into the primitive pulmonary veins. With growth and development, the atrial muscle at the pulmonary vein should gradually degenerate and disappear. However, autopsies have revealed that in some patients, atrial muscle extending from the left atrium is still present in the pulmonary veins, sometimes in the form of “cuffs” within the pulmonary veins or deeper into the pulmonary vein segments at the hilum.
These residual myocardial tissues in the pulmonary veins deliver single or continuous, sequential or disordered, rapid electrical excitations that trigger or drive the atrial myocardium and can lead to atrial arrhythmias – myocuff atrial arrhythmias, including myocuff atrial tachycardia.
Myocuffal atrial tachycardia can originate in the superior, inferior, left, and right pulmonary veins, mostly in the superior pulmonary veins, especially the left superior pulmonary vein. The morphology of P waves produced by different pulmonary vein origins varies, and the site of origin can be initially inferred from the morphological characteristics of P waves in each lead of the body ECG.
Ohkubo et al. concluded that the right and left pulmonary veins should be distinguished by P-wave positivity in lead Ⅰ and P-wave positivity in right pulmonary vein origin. Yamane et al. found that the specificity of P-wave positivity in aVL leads and P-wave amplitude ≥0.05 mV in lead Ⅰ to predict the right pulmonary vein were higher (100%, 99%); P-wave tangent in lead Ⅱ, P-wave positivity in lead V1 with time limit ≥80 mS or P-wave amplitude in Ⅲ/II The specificity of ≥0.8 for predicting left pulmonary veins was 95% and 75%.
Ahar et al. concluded that a P-wave time frame <120 mS also predicted right pulmonary vein origin. kistler et al. found that the specificity of positive P-wave in leads V1 to V6 in I predicted right pulmonary vein origin was 94%, and if the P-wave in lead V1 was bidirectional during sinus rhythm and became positive during atrial tachycardia, the specificity was 100%, but the sensitivity decreased; negative P-wave in lead I or isotropic line. And the specificity of positive P waves in lead II and/or V1 with tangential traces to predict the left pulmonary veins was 98%. Since the distance between the ipsilateral superior and inferior pulmonary veins is closer than that between the left and right pulmonary veins, and there may be electrical connections between the ipsilateral superior and inferior pulmonary veins, it is more difficult to identify the superior and inferior pulmonary veins than the left and right pulmonary veins on the basis of the ECG P-wave pattern. The P-wave amplitude of lead II ≥0.1 mV predicts the origin of upper pulmonary veins, and Ahar et al. suggested that the P-wave amplitude of leads II, III and aVF and >0.3 mV suggest the origin of upper pulmonary veins, while the P-wave in leads II, III and aVF has tangential traces suggesting the origin of lower pulmonary veins.
In summary, the P wave morphology of V1, aVL, Ⅰ and Ⅱ leads is more significant for identifying atrial tachycardia of pulmonary vein origin. aVL lead P wave positive, Ⅰ lead P wave positive amplitude ≥0.05 mV, V1 lead P wave biphasic in sinus rhythm and becomes positive in atrial tachycardia are suggestive of right pulmonary vein origin; Ⅰ lead P wave negative or isotropic line, Ⅱ lead P wave tangential, V1 lead P wave positive time limit ≥80mS or P-wave amplitude in III/II ≥0.8 predicts left pulmonary vein origin; P-wave tangent in inferior wall leads suggests inferior pulmonary vein origin, and P-wave amplitude in II leads ≥0.1mV suggests superior pulmonary vein origin.
4.2 Atrial tachycardia of left auricular origin
The incidence of atrial tachycardia of left auricular origin is low, accounting for 3% of all focal atrial tachycardia. Since the left auricle is located in the upper part of the left atrium, the specificity of localizing the site of origin solely by the P-wave morphology in V1 leads and inferior wall leads is low. The left auricle is closer to the anterior wall of the left atrium than the pulmonary vein, and the depolarization vector generated during excitation deviates from the anterior thoracic leads (V2-V6 leads), resulting in the P wave in V2 and V6 leads being located at the isoelectric line during atrial tachycardia, which can be used to identify atrial tachycardia of left auricle and pulmonary vein origin. In addition, because of the proximity of the left auricle and the left pulmonary vein, the P-wave morphology is similar, but the deep inversion of the P-wave in lead I of atrial tachycardia of left auricle origin is obvious, which helps to identify the origin of the left auricle and the left pulmonary vein.
4.3 Atrial tachycardia of mitral annular origin
KISTLER et al. studied 7 cases of atrial tachycardia of mitral annular origin, all of which originated in the upper part of the mitral annulus, near the left fibrous triangle and the mitral-aortic junction. The P waves of atrial tachycardia of mitral annular origin are low voltage in the limb leads and show a typical negative-positive bidirectional wave in the thoracic leads. v1 lead and aVL lead P wave patterns are more significant in guiding the localization of the mitral annulus, with a significant positive component in the V1 leads and aVL lead P waves located at the isovolumic line or in a negative direction. v1 lead P wave patterns are useful in identifying atrial tachycardia of mitral annular and pulmonary venous origin.
Because the mitral annulus is located anteriorly with respect to the pulmonary veins, the initial vector of excitation from the upper mitral annulus points posteriorly away from the anterior thoracic leads, while the posterior left atrial excitation vector points toward the anterior thoracic leads, so the anterior thoracic lead P waves are negative and positive in both directions. In contrast, in the pulmonary venous origin, a positive wave can be recorded in the V1 lead, so it is possible to identify the mitral annulus origin and the pulmonary venous origin of atrial velocity.
In addition, the anatomic position of the mitral annulus is lower than that of the left auricle, so the atrial velocity of the mitral annulus and that of the left auricle can be identified based on the morphology of the P waves in the inferior wall leads: the atrial velocity of the P waves in the inferior wall leads in the mitral annulus origin is mostly located in the isotropic line or positive direction, whereas the atrial velocity in the left auricle origin is typically positive and has a higher amplitude.
Reviewing the above literature, the author proposes a procedure to localize the site of atrial tachycardia origin according to body surface ECG P-wave morphology (Figure 4) to guide atrial tachycardia localization and radiofrequency ablation, which can lead to more adequate preoperative preparation and shorten the marking time and x-ray exposure time. Since various localization methods are performed under spontaneous atrial tachycardia, evoked atrial tachycardia and pacing markings, the findings vary among scholars, there is some overlap in P-wave morphology, and the variable electrical connections between the left and right atria and the variable and complex left atrial structures all affect the accuracy of localization. During the onset of atrial tachycardia, it is also difficult to accurately determine the P waves because the atrial tachycardia is accompanied by 1:1 atrial conduction or the P waves are masked by the T waves and QRS wave groups, and in this case, compression of the carotid sinus, intravenous adenosine triphosphate or verapamil can be used to cause atrioventricular block, so that some of the P waves are separated from the T waves or QRS wave groups.