ECG Blog #76 — Localization of the Problem?

Does the ECG in Figure-1 show evidence of anterior infarction or WPW?

• IF your answer is WPW — Where is the AP (Accessory Pathway) likely to be?
• Is AP localization on ECG clinically important?

Figure-1: 12-lead ECG. Is there evidence of prior anterior infarction? 

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Interpretation of Figure-1:

     The ECG in Figure-1 shows sinus rhythm (upright P wave in lead II ) — but the PR interval is short. Closer inspection reveals initial slurring of the R wave upstroke consistent with delta waves in many leads (red arrows in Figure-2). The patient has WPW (Wolff-Parkinson-White). We highlight the following points:

• All 3 of the features of WPW are present in Figure-2. These include: i) short PR interval; ii) QRS widening; and iii) delta waves.
• Not all leads show delta waves. For example — there is no delta wave in lead aVL. It is not uncommon in patients with WPW for delta waves to only be seen in a few of the 12 leads. Furthermore, because conduction may be divided between impulses passing over the AP (Accessory Pathway) and the normal AV nodal pathway — the QRS will not always be as wide as it is in Figure-2.
• In addition to positive delta waves — there are also negative delta waves in Figure-2. These negative delta waves in leads V1,V2,V3 simulate anterior infarction.
• The ECG in Figure-2 also simulates LVH (given large amplitude of the R wave in V5,V6) and LBBB (monophasic slurred complex in lead I). However, since the patient has WPW — nothing can be said about the possibility of anterior infarction, LVH, or LBBB. Instead — all we can say is that there is sinus rhythm with WPW.

Figure-2: Arrows highlighting delta waves have been added to the12-lead ECG previously shown in Figure-1. Thus, the patient has WPW. Nothing can be said about the possibility of anterior infarction since the patient has WPW. Where is the likely location of the AP?

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HOW to Determine the Likely Location of the AP?

     Delta wave morphology and orientation on 12-lead ECG may assist in surprisingly accurate localization of the AP (Accessory Pathway) in the patient with WPW. This is of more than academic interest to the EP cardiologist — as it facilitates and expedites localization of the AP during EP (ElectroPhysiology) study. In addition — it helps in planning the procedure as well as in patient discussion, since risks of catheter ablation and likely success rates are based in part on localization of the AP.

• We emphasize that ECG localization of the AP is an advanced topic beyond the clinical needs of the non-cardiologist. Practically speaking — it suffices to recognize WPW and IF there is need for referral. That said — ECG localization of the AP is not necessarily difficult if certain parameters are followed.

My Suggested Approach to ECG AP Localization is based on my synthesis of material primarily derived from the following 2 references:

• Das MK, Zipes DP: Electrocardiography of Arrhythmias — A Comprehensive Review (ePub book). Elsevier-Saunders, Philadelphia, 2012.
• Fitzpatrick AP Gonzales RP Scheinman MM, et al: Algorithm for the Localization of Accessory Atrioventricular Connections Using a Baseline Electrocardiogram. J Am Coll Cardiol 23:107-116, 1994.

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The Basics of AP Localization:

     ECG localization of the AP is not perfect. Accuracy of ECG localization is clearly suboptimal if there is less than maximum preexcitation — as may occur when the QRS is minimally widened because a substantial portion of ventricular activation is occurring by transmission of the impulse over the normal AV nodal pathway.

• Approximate distribution of AP sites is the following: i) Lateral (Left Ventricular) Free Wall — ~50% of APs; ii) Right or Left PosteroSeptal Area — ~20%; iii) RV (Right Ventricular) Free Wall — ~20%; and iv) AnteroSeptal Area — ~10%.
• On occasion — more than a single AP may be present in a given patient. Together with variation in the relative amount of AP vs normal pathway conduction — this may account for changing delta wave morphology that can sometimes be seen from one ECG to the next in a given patient.
• As a general rule — IF the delta wave (first ~40msec of the QRS) is upright (positive) in lead V1 (RBBB pattern) — then there is a LEFT-sided AP (as in Step A-1).
• IF the delta wave is downward (negative) in lead V1 (LBBB pattern) — then as a general rule there is a RIGHT-sided AP (The major exception to this is when transition occurs between leads V1-to-V2 — as described in Step B-1).

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Step A-1: IF the QRS in Lead V1 is all UPRIGHT

     When the QRS complex is all upright in lead V1 — then there is a “RBBB pattern”. In this case — transition (where the R wave becomes taller than the S wave) is said to occur before or by lead V1. This defines a LEFT-sided AP. Proceed as follows:

• Measure the sum of delta wave polarities in the 3 inferior leads (II,III,aVF) — giving a score of +1 if the delta (first 40msec of the QRS) is positive; 0 if the delta is isoelectric; and -1 if the delta is negative.
• IF Sum of inferior lead Polarities is +2 or +3 = AnteroLateral LV Free Wall AP.
• IF Sum of inferior lead Polarities is less than +2 — then the AP is posterior.
• IF Sum of inferior lead Polarities is -2 or -1 — and — the R wave in lead I is at least 0.8mV (8 mm) more than the S wave in lead I = PosteroSeptal AP. Otherwise = there is a PosteroLateral LV Free Wall AP.

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Step A-2: IF Transition Occurs Between Lead V1-to-V2

     IF the R wave in lead V1 is less than the S wave in V1 — but by lead V2 the R wave becomes taller than the S wave in V2 (ie, IF transition occurs between V1-to-V2) — then the AP could be either left or right-sided. Proceed as follows:

• IF the R wave in lead I is less than 1.0mV (10 mm) greater than the S wave in lead I = then there is a LEFT-sided AP. In this case — Proceed as above (Step A-1) for when the QRS is UPRIGHT (beginning by measuring sum of delta polarity in the inferior leads to determine if the AP is anterolateral — posteroseptal — or posterolateral).
• But IF the R wave in lead I is at least 1.0mV (10 mm) more than the S wave in lead I = then there is a RIGHT-sided AP. In this case — Proceed as described in Step B-2.

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Step B-1: How to Tell IF the AP is RIGHT-sided?

     When the QRS complex is predominantly downward (negative) in lead V1 — and — transition (where the R wave becomes taller than the S wave) occurs after lead V2 — then there is a RIGHT-sided AP.

• As stated above there can also be a RIGHT-sided AP — IF transition occurs between V1-to-V2 — and — the R wave in lead I is at least 1.0mV (10 mm) more than the S wave in lead I.

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Step B-2: When the AP is RIGHT-sided

     Localization of a right-sided AP will depend on where transition occurs. There are 3 possibilities. Transition may be: i) before (or by) V2-to-V3 (Step B-3); ii) between V3-to-V4 (Step B-4); or iii) after V4 (Step B-5).

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Step B-3: Right-Sided AP: Transition before (or by) V2-to-V3

     IF the AP is right-sided with transition before (or by) lead V2-to-V3 — then the AP is Septal. To determine which septal area is involved — Proceed as follows:

• Measure the sum of delta wave polarities in the 3 inferior leads (II,III,aVF) — giving a score of +1 if the delta (first 40msec of the QRS) is positive; 0 if the delta is isoelectric; and -1 if the delta is negative.
• IF Sum of inferior lead Polarities is -2 or -3 = PosteroSeptal AP.
• IF Sum of inferior lead Polarities is -1, 0 or +1 = MidSeptal AP.
• IF Sum of inferior lead Polarities is +2 or +3 = AnteroSeptal AP.

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Step B-4: Right-Sided AP: Transition between V3-to-V4

     IF the AP is right-sided with transition between lead V3-to-V4 — then the AP is either septal or right ventricular free wall. To determine which — Proceed as follows:

• Measure delta wave amplitude (first ~40msec of the QRS) in lead II.
• IF the delta wave in lead II is at least 1.0mV (10mm) = Septal AP. To then find out which septal area is involved — Proceed as above for when transition is between V2-to-V3 (Step B-3).
• IF the delta wave in lead II is less than 1.0mV (10mm) = RV Free Wall AP. To determine IF the AP is located on the anterolateral or posterolateral RV Free Wall — Proceed as below for when transition is after V4 (Step B-5).

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Step B-5: Right-Sided AP: Transition after Lead V4

     IF the AP is right-sided with transition after lead V4 — then the AP is located in the RV Free Wall. To determine IF the AP is located on the anterolateral or posterolateral RV Free Wall — Proceed as follows:

• Measure the delta wave frontal axis (looking at delta wave polarity in leads I and aVF).
• IF the delta wave frontal axis is positive (= more than 0 degrees) = Anterolateral RV Free Wall AP.
• But IF the delta wave frontal axis is negative (= less than 0 degrees) — then look at the R wave in lead III.
• IF delta wave frontal axis is negative — and — the R wave in lead III is net positive = AnteroLateral RV Free Wall AP.
• IF delta wave frontal axis is negative — and — the R wave in lead III is net negative = PosteroLateral RV Free Wall AP.

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Figure-3: Where is the AP likely to be?

     Let’s apply the above approach to AP localization to the ECG from Figure-2 that we again show in Figure-3. Where is the AP likely to be?

Figure-3: Sinus rhythm with WPW (ECG repeated from Figure-2). What is the likely location of the AP? (See text). 

ANSWER to Figure-3: Where is the AP?

     We begin by looking to see IF the QRS in lead V1 of Figure-3 is upright or negative:

• Since the QRS complex in V1 is negative — we skip over Step A-1.
• Transition (where the R wave in precordial leads becomes taller than the S wave) is not between V1-to-V2 — therefore we also skip over Step A-2.
• According to Step B-1 — the AP is right-sided (because the QRS is negative in V1 and transition occurs after lead V2).
• Transition occurs between V3-to-V4. Therefore we skip to Step B-4. We are asked to measure delta wave amplitude in lead II. Realizing that it is not always easy to distinguish the precise end of slurring from the delta wave vs fusion with the remaining portion of the QRS — it looks like there is a markedly positive delta wave (of at least 10mm) in lead II. This suggests a Septal location for the AP.
• To determine the likely part of the septum that is involved — We are asked to return to Step B-3. Delta wave polarities are clearly positive in each of the inferior leads — therefore we suspect an AnteroSeptal AP location.

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Figure-4: Where is the AP likely to be?

     Let’s apply the above approach to the example of WPW shown in Figure-4. Where is the AP likely to be?

Figure-4: Sinus rhythm with WPW. What is the likely location of the AP? (See text). 

ANSWER to Figure-4: Where is the AP?

     We begin by looking to see IF the QRS in lead V1 of Figure-4 is upright or negative:

• Since the QRS complex in V1 is upright — we begin with Step A-1. Because the QRS is positive in lead V1 — we already know we are dealing with a LEFT-sided AP.
• The sum of delta wave polarities is at least +2 (decidedly positive delta waves in leads II,aVF — though no more than minimally positive in III, if not isoelectric in this lead). Therefore — we suspect an AnteroLateral LV Free Wall AP.

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Editorial Comment by the Author: Realize that many algorithms have been proposed for ECG localization of the AP. None are perfect — and the “ultimate test” is precise localization by EP study. We hope the above proposed approach simplifies this topic for you and is helpful in allowing a rapid educated guess as to AP localization based on ECG appearance.

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Acknowledgement: My appreciation to Michael Frease for providing me with the ECG in Figure 4, and allowing me to publish this.
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- For more information – GO TO:

• CLICK HERE  — for a pdf of our Section 05.36 on WPW (excerpted from ECG-2014-ePub).
• For an example of very rapid AFib with a wide QRS — Check out our ECG Blog #37.

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