Electrocardiographic localization of accessory pathways

 

ECG 1: Left lateral accessory pathway: the delta wave and QRS are positive in V1, hence the accessory pathway is on the left; the delta wave and the QRS are positive inferiorly, hence the accessory pathway is anterior; the delta wave and the QRS are negative in leads 1 and aVL (with a QS pattern, with no R wave), hence the accessory pathway is left lateral;

The left lateral accessory pathways are generally minimally pre-excited since they are located far from the sinus node. The proportion of the ventricular myocardium depolarized by the accessory pathway is therefore very low comparatively to the proportion of myocardium depolarized by the nodo-Hisian pathway. Thus, the delta wave is often barely visible. The left lateral accessory pathway may even be concealed. In this instance, there is an anterograde conduction through the accessory pathway although the latter is not visible on the ECG. Pre-excitation will be visible if nodal conduction is slowed by vagal maneuvers or with adenosine, or if the atrium is paced near the accessory pathway.

 

 

ECG 2A

ECG 2A: Left lateral accessory pathway: the pre-excitation is notably less visible than on the previous ECG; the delta wave and the QRS are positive in V1 and V2, hence the accessory pathway is located on the left; the delta wave and the QRS are positive inferiorly, hence the accessory pathway is anterior; the delta wave is negative in lead I but poorly marked, and the QRS is positive; on the other hand, the delta wave and the QRS are negative in aVL; it is then necessary to imagine what will become of  lead I during maximum pre-excitation; the delta wave and the QRS being negative in aVL, the QRS will also likely be negative in lead I.

 

 

ECG 2B:

ECG 2B: ECG during rapid atrial pacing of the distal coronary sinus (maximum pre-excitation); the QRS is clearly positive from V1 to V6, hence the accessory pathway is on the left; the QRS becomes clearly negative in lead I; the accessory pathway is therefore left lateral; The absence of a Q wave of septal activation in V5-V6 and an early transition of the QRS in V1-V2 are sometimes the only signs indicating the presence of a left lateral accessory pathway.

 

 

ECG 3:

ECG 3: Left anterolateral accessory pathway: the delta wave and the QRS are positive in V1, hence the accessory pathway is located on the left; the delta wave is positive inferiorly, hence the accessory pathway is anterior; the delta wave is positive in lead I and the QRS is positive then negative; it is therefore necessary to imagine that in maximum pre-excitation, the QRS in lead I will become isoelectric (positive then negative) indicating that the impulse is perpendicular to lead I; the delta wave and the QRS are very negative in aVL, which evokes a lateral pathway; by deduction, this accessory pathway is therefore left anterolateral;

Due to the anatomical configuration of the region (presence of the mitral-aortic continuity), both left anteroseptal and left anterior accessory pathways are extremely rare.

 

 

ECG 4A:

ECG 4A: Left posterolateral pathway: the delta wave and the QRS are positive in V1 and V2, hence the accessory pathway is located on the left; the delta wave is positive inferiorly but the first part of the QRS is negative; the second part of the QRS is the result of depolarization via the nodo-Hisian pathway; the delta wave is negative in leads I and aVL and the QRS is isoelectric;

 

 

ECG 4B:

ECG 4B: ECG during rapid atrial pacing of the proximal coronary sinus (maximum pre-excitation): the QRS is negative in lead II and isoelectric in leads II and aVF; the accessory pathway is posterior rather than anterior; the QRS is positive then negative in leads I and aVL; the accessory pathway is not completely left lateral since there is a positive deflection in leads I and aVL (which does not exist on ECGs 1 and 2 of the left lateral pathway). It is therefore a left posterolateral pathway. It is located a little lower and less lateral than the left lateral pathway.

 

 

ECG 5:

ECG 5: Left posterior pathway: the delta wave and the QRS are positive in V1, hence the accessory pathway is located at the left side; the delta wave is positive in leads I and aVL, it is therefore not a left lateral pathway; the delta wave is positive in leads II and aVF but isoelectric in lead III; it is therefore not a posteroseptal pathway since the delta wave should be negative inferiorly; it is thus necessary to imagine the polarity of the QRS in maximum pre-excitation; the QRS in inferior will likely become negative in maximum pre-excitation (since isoelectric in lead III); by deduction, it is a left posterior pathway;

 

 

ECG 6A:

ECG 6A: Left posteroseptal accessory pathway: the delta wave and the QRS are positive in V2, the delta wave is isoelectric in V1, it is therefore a septal pathway; the delta wave is negative in leads III and aVF, positive then negative in lead II; the QRS will likely also be negative in lead II in maximum pre-excitation; the pathway is therefore posterior; the delta wave and the QRS are positive in leads I and aVL, hence it is not a left lateral pathway; the accessory pathway is therefore posteroseptal;

The study of the QRS during maximum pre-excitation allows confirming the localization (in particular for lead II) and determining whether the accessory pathway is rather located in the right or left aspect of the septum.

 

 

ECG 6B:

ECG 6B: ECG during rapid atrial pacing of the proximal coronary sinus (maximum pre-excitation): the delta wave and the QRS are very negative inferiorly as evoked in sinus rhythm which confirms the posterior localization; the QRS is positive in V1, it is therefore a posteroseptal accessory pathway with a connection to the left ventricle.

The classification into right or left posteroseptal accessory pathway is mainly dependent on the ablation site which allowed the disappearance of the accessory pathway. Indeed, according to the literature, most of the posteroseptal accessory pathways connect the right atrium to the left ventricle. Thus, the right or left localization is based on the ventricular insertion of the pathway but does not infer the atrial insertion.

 

 

ECG 7A:

ECG 7A:Right posteroseptal accessory pathway; the delta wave and the QRS are negative in V1 but positive in V2, hence the accessory pathway is septal; the pathway is posterior, the delta wave is negative inferiorly or negative in leads III and aVF and positive or isoelectric in lead II;

 

 

ECG 7B:

ECG 7B: The QRS becomes negative during a maximum pre-excitation; the delta wave and the QRS are positive in leads I and aVL, hence it is not a left lateral pathway; it is therefore a right posteroseptal pathway;

The ablation site is located below the level of the coronary sinus or at its ostium, unlike the mid-septal pathways where ablation is performed above the level of the coronary sinus.

 

 

ECG 8A:

ECG 8A: Right posterior pathway: the delta wave and the QRS are negative in V1, V2 and V4 but positive in V3 achieving a double polarity transition in the precordial leads; this double transition is evocative of an accessory pathway located in the cavo-tricuspid isthmus; the delta wave and the QRS are negative inferiorly, hence the accessory pathway is posterior; the delta wave is positive in leads I and aVL, hence it is not a left lateral pathway; it is therefore a right posterior pathway and the double transition indicates a pathway located in the cavo-tricuspid isthmus.

 

 

ECG 8B:

ECG 8B: ECG during rapid atrial pacing of the proximal coronary sinus (maximum pre-excitation): the polarity of the QRS complexes in the precordial leads shows the same double transition described above; the QRS are negative inferiorly and positive in leads I and aVL confirming the right posterior location;

 

 

ECG 9A:

ECG 9A: Right posterolateral accessory pathway: the delta wave is negative in V1, isoelectric in V2 and V3 and then positive from V4; the QRS complexes are negative from V1 to V3 with a transition in V4; the transition in V4 points to a right lateral pathway; the delta wave and the QRS are negative in leads III and aVF, isoelectric in lead II; since the QRS complexes are completely negative in leads III and aVF, the QRS in lead II will likely also be negative in lead II in maximal pre-excitation; the delta wave and the QRS are positive in leads I and aVL, hence it is not a left lateral pathway; the accessory pathway is therefore right posterolateral.

The right lateral accessory pathways (posterolateral, lateral and anterolateral) generally yield highly pre-excited QRS and therefore broad since these accessory pathways are in proximity to the sinus node compared to the septal or left pathways. Ventricular depolarization is therefore predominantly performed via the accessory pathway.

 

 

ECG 9B:

ECG 9B: ECG during rapid atrial pacing near the sinus node (maximum pre-excitation): the QRS in lead II is negative confirming the right posterolateral location.

 

 

ECG 10A:

ECG 10A: Right lateral accessory pathway: the delta wave is isoelectric in V1 then positive from V2; as in the case of a right posterolateral accessory pathway, the QRS complexes are negative from V1 to V3 and the transition occurs in V4; it is therefore a right lateral pathway; the delta wave and the QRS are positive in leads I and aVL, hence it is not a left lateral pathway; the delta wave and the QRS are positive in lead II, negative in lead III; the delta wave is negative in aVF and the QRS is positive; the accessory pathway is therefore not posterolateral as in the previous example since the delta wave and the QRS are positive in lead II; the pathway is therefore located higher on the tricuspid annulus; it is also not anterolateral since the delta wave and the QRS are negative in lead III; by deduction, the pathway is right lateral;

 

 

ECG 10B:

ECG 10B: ECG during rapid atrial pacing near the sinus node (maximum pre-excitation); the polarity transition of the QRS complexes is identical (in V4); the QRS remains positive in lead II, negative in lead III and becomes isoelectric in aVF confirming the hypothesis of a right lateral pathway.

 

 

ECG 11A:

 

ECG 11A: Right anterolateral pathway: as in the case of the right posterolateral and lateral pathways, the polarity transition of the QRS complexes occurs between V3 and V4 suggesting a right lateral location; the delta wave and the QRS are positive in leads I and aVL, hence it is not a left lateral pathway; the delta wave and the QRS are positive in leads II and aVF; the delta wave is negative in lead III but the QRS is positive; the accessory pathway is therefore located higher than the right lateral pathway on the tricuspid annulus; it is therefore a right anterolateral pathway.

 

 

ECG 11B:

ECG 11B: ECG during rapid atrial pacing near the sinus node (maximum pre-excitation): the QRS complexes remain positive in leads II and aVF and become isoelectric in lead III confirming the anterolateral location;

The Identification of a pathway located on the lateral wall of the tricuspid annulus is important prior to an ablation procedure in order to anticipate potential difficulties. Indeed, ablation is rendered delicate given the difficulty in stabilizing the catheter in this region. The systematic use of a sheath via the femoral vein allows improving the stability of the catheter and thus the effectiveness of the ablation.

 

 

ECG 12:

ECG 12: Anteroseptal pathway: the delta wave is positive in V1 and the QRS is negative; the delta wave is positive from V2, the QRS is isoelectric in V2 and then positive from V3; the QRS polarity transition is in V2-V3 (earlier than for a right lateral pathway); the accessory pathway is therefore right septal; the delta wave and the QRS are positive in leads I and VL, hence the pathway is not left lateral; the delta wave and the QRS are positive inferiorly, hence the pathway is anterior; it is therefore a right anteroseptal pathway.

It is important to identify the location of this type of accessory pathway prior to an ablation procedure because of its proximity to the bundle of His and to the compact portion of the atrioventricular node. There is hence a risk of complete AV block when removing these accessory pathways. The use of cryotherapy may represent an alternative.

The femoral venous approach is rendered difficult by a stability issue of the ablation catheter. A left subclavian or right jugular vein route is preferred. This allows obtaining a greater stability of the ablation catheter which is pressed against the superior portion of the tricuspid annulus.

 

 

ECG 13A:

ECG 13A: Para-Hisian pathway: the delta wave and the QRS are negative in V1 and V2, hence the accessory pathway is to the right; there is no initial R wave and the QRS has a QS morphology in V1 and V2 which is evocative of a para-Hisian pathway; the delta wave and the QRS are positive in lead I, the delta wave is positive in aVL and the QRS is isoelectric, hence the accessory pathway is not left lateral; the delta wave and the QRS are positive inferiorly, hence the pathway is anterior; it is therefore a para-Hisian pathway.

 

 

ECG 13B:

ECG 13B: ECG during rapid atrial pacing (maximum pre-excitation): there is a persisting QS pattern in V1 and V2 that differentiates the para-Hisian accessory pathway from the anteroseptal pathway.

It is important to accurately localize this type of accessory pathway since it is located in the immediate vicinity of the bundle of His. There is therefore a risk of complete AV block when removing these accessory pathways. The use of cryotherapy may represent an alternative.
The ablation is preferably carried out via the femoral vein route with a long sheath to obtain a better stability of the ablation catheter. It is often necessary to ablate this accessory pathway on a suboptimal site, readily on the ventricular side of the annulus in order to stay clear from the compact portion of the atrioventricular node which is more fragile than the bundle of His, the latter being protected by a fibrous sheath.

 

 

ECG 14A:

ECG 14A: Mid-septal pathway: the delta wave and the QRS are negative in V1, but positive in V2, it is thus a septal pathway; the delta wave and the QRS are positive at leads 1 and aVL, hence the accessory pathway is not left lateral; the delta wave and the QRS are positive in lead II, the delta wave is isoelectric in leads III and aVF, the QRS is negative in lead III and isoelectric in aVF.

 

 

ECG 14B:

ECG 14B: ECG during rapid atrial pacing of the proximal coronary sinus (maximum pre-excitation): the QRS remains positive in lead II, negative in lead III and becomes negative in aVF; with the QRS remaining positive in lead II, the accessory pathway is located higher than a posteroseptal pathway; it is therefore a mid-septal pathway.

It is important to identify the location of this type of accessory pathway prior to an ablation procedure since it is located near the atrioventricular node and there is therefore a risk of complete AV block during the ablation.

The ablation is performed via the femoral vein route with the aid of a long sheath in order to obtain better stability of the ablation catheter. The use of cryotherapy may represent an alternative. Unlike the right posteroseptal accessory pathways, the ablation site is above the level of the coronary sinus.

 

 

Comments: When the accessory pathway is identifiable on the surface electrocardiogram, various algorithms have been proposed to determine the anatomical location of the accessory pathway and facilitate a possible ablation procedure. The different classifications proposed are based on the polarity analysis of the delta wave and/or the QRS complex. The precise location of the accessory pathways is particularly useful in the context of an ablation in order to anticipate the access route, the necessary material (sheaths and sheath type; venous or arterial femoral access, venous subclavian or jugular access, transseptal access) and potential complications (atrioventricular block in particular).

The localization algorithm of the accessory pathway presented herein is based on the analysis of the polarity of the delta wave, the polarity of the QRS complex and the anticipated polarity of the delta wave and the QRS during maximal pre-excitation (when ventricular depolarization occurs predominantly or even exclusively via the accessory pathway, during rapid atrial pacing near the accessory pathway during an electrophysiological study, for example). The precise location of the accessory pathway is also dependent on the position of the heart in the chest cavity and the presence of an underlying cardiac disease.

In practice, the search for the location of an accessory pathway begins with the study of leads V1 and V2. In the absence of heart disease, if the delta wave is positive in V1 (right delay pattern), it is a left pre-excitation. If the delta wave is negative in V1 and positive in V2, it is a septal pre-excitation (right or left depending on the ablation site). If the pre-excitation is negative in V1 and V2 (left delay pattern), then the accessory pathway is located to the right. Depending on the polarity transition of the delta wave and the QRS in the precordial leads, it will be possible to define whether the accessory pathway is septal or more lateral. A transition in V1-V2 (negative delta wave and QRS in VI but positive in V2) evokes a septal accessory pathway. If the transition occurs later (V3-V4), then it is a right lateral accessory pathway.

Next, the study of the inferior leads (II, III and aVF) allows establishing whether the accessory pathway is anterior or posterior. If the delta wave is positive inferiorly, the pre-excitation is anterior since ventricular activation travels to the inferior leads. On the other hand, if the delta wave is negative inferiorly, then the accessory pathway is posterior.

Lastly, the study of leads I and aVL allow establishing whether the accessory pathway is left lateral. If the delta wave is negative in leads I and aVL, it is a left lateral pathway since ventricular activation travels away from leads I and aVL. If the delta wave is positive in these leads, the accessory pathway is not left lateral.

Take-home message: When the accessory pathway is identifiable on the surface electrocardiogram, various algorithms have been proposed to determine the anatomical location of the accessory pathway. All of the classifications have their limitations due to the coexistence of QRS abnormalities associated with heart disease, to varying degrees of fusion, to the possible superposition of the P wave in the delta wave, to the presence of a low-voltage delta wave. It is possible to differentiate the accessory pathways between left and right pathways, or be much more accurate and to differentiate between left anterior, left lateral, left posterolateral, left posterior, right and left mid-septal, posteroseptal, right posterolateral, right posterior, right anteroseptal, right anterior and right lateral.