Wednesday, June 11, 2014

ECG Blog #91 (Basic Concepts-4) – Lead Groupings


     This is my 4th installment of Basic ECG Concepts. Rather than specific cases — the goal of these Basic ECG Concepts is concise review of some less advanced topics that comprise the fundamentals of ECG interpretation. This material is excerpted with modification from my new introductory book to ECG Interpretation = A 1st Book on ECGs-2014 and/or the expanded 1st-ECG-Book-ePub version (which is out in kindle-nook-kobo-ibooks). Your feedback on this series is WELCOME!

  • NOTE: To enhance relevance — some advanced points have been added, illustrating selected concepts about Anatomic and ECG Lead Groupings (with this more advanced material excerpted from our ECG-2014-ePub). In this way — We hope this post is of interest and value to ECG interpreters of any level.
LINKS to Previous Basic ECG Concepts:
     When interpreting ECGs — It is important to know which leads view which areas of the heart. The focus of this Basic ECG Concepts #4 — is to present a concise pictorial review of the various ECG Lead Groupings designed to accomplish this goal.
  • Realize that the ECG terminology used for anatomic localization is subject to variation even among expert electrocardiographers. Variation depends on regional differences — as well as preference of the experts residing in your particular geographic area or on staff at the institution in which you practice. While such differences in ECG terminology may be confusing to the less experienced interpreter — We feel it offers a broader perspective that ultimately leads to enhanced understanding. Our purpose in this Basic Concepts #4 — is to offer the perspective we have found most helpful over time for classifying the location of the ECG findings you detect.
Basic Lead Groups
     The “trained eye” keys into the specific Lead Groups that are shown in Figure-1.
  • NOTE-1: There is overlap in the precordial lead areas (ie, lead V2 is both septal and anterior and lead V4 is both anterior and lateral).
  • NOTE-2: We do not assign a specific number of degrees to the vantage point for lead aVR. It suffices to think of lead aVR as a remote right-sided lead.
Figure-1: Basic Lead Groups — and, the leads they encompass. Note overlap in the area of the heart visualized by lead V2 and lead V4. (Figure reproduced from A 1st Book on ECGs-2014-ePub). NOTE — Enlarge by clicking on Figures — Right-Click to open in a separate window.
Looking Closer at the View from the Chest Leads
     The view of the heart that each of the 6 Precordial (Chest) Leads receives is schematically shown in Figure-2 (red arrows).
  • As already suggested in Figure-1 — there is overlap in the anatomic area viewed by lead V2 and lead V4 (red and green brackets in Figure-2).
  • Note that there are 2 areas of the heart that are not directly visualized by any of the 6 chest leads (labeled X and Y in Figure-2). Special leads can be used to visualize the RV (Right Ventricle = X) and the posterior wall of the LV (Left Ventricle), which is the point labeled “Y” in Figure-2. For simplicity — We do not explore the topic of right-sided and/or posterior leads at this time. Just Be Aware that the ECG is not perfect and does not fully visualize all areas of the heart.
Figure-2: The view of the heart that each of the 6 Precordial (Chest) Leads receives (red arrows). Note that certain areas of the heart (ie, “X” and “Y” = the right ventricle and posterior wall of the left ventricle) — are not well visualized by any of the standard 12 leads. Additional Points: i) Leads V2 and V4 manifest “overlap” with regard to their anatomic perspective; and ii) Leads V1 and V2 are also often referred to as either “antero-septal” leads and/or “right-sided” leads. (Figure reproduced from A 1st Book on ECGs-2014-ePub).
Advanced Concept: Leads V1 and V2 are labeled as “septal leads” in Figure-1 and Figure-2. That said — two other designations are commonly used to describe the anatomic perspective of these 2 leads:
  • Leads V1 and V2 — are often referred to as “antero-septal” leads (since lead V2 overlaps in its view of the septum and anterior wall of the left ventricle). Clinically — ischemia or infarction of the septum is almost always associated with at least some involvement of the anterior wall.
  • Leads V1 and V2 — are also often referred to as “right-sided” leads — since anatomic placement of the electrodes to record leads V1 and V2 is clearly to-the-right with respect to placement of the other 4 chest lead electrodes (See Figure-2). Appreciation that leads V1 and V2 are “right-sided” in their anatomic placement — will help in understanding the rationale for the ECG signs of LVH (Left Ventricular Hypertrophy) and bundle branch block that we will present in future Basic Concept installments.
Distinction between Anterior vs Antero-Septal MI
     Confusion often arises as to distinction between infarctions that are “anterior” in location vs “antero-septal” in location. Clinically — specific septal involvement is most likely when there is proximal occlusion of the LAD (Left Anterior Descending coronary artery) in which the occlusion occurs prior to takeoff of the first septal perforator branch.
  • Proximal LAD occlusion is more likely than mid- or distal LAD occlusion to result in conduction system damage with need for a pacemaker.
  • Similar profound conduction problems may occur with acute left main occlusion — but such patients usually die of cardiogenic shock before they reach the hospital.
     ECG identification of acute proximal LAD occlusion is not discussed here. What we do cover in this Basic ECG Concepts #4 post — is semantics of the terminology we favor for identifying anterior infarcts that are more likely to have septal involvement.
  • The correlation between ECG identification of “septal involvement” and risk of developing complete AV block is far from perfect. That said — appreciation that leads V1 and V2 are “septal” leads facilitates application of our system for localizing ECG changes.
     Figure 3 illustrates why leads V1 and V2 are considered “septal” leads. With normal sinus rhythm — the electrical impulse starts in the SA node — travels through the atria — and then arrives at the AV Node. After passage through the AV Node and the Bundle of His — the very first part of the ventricles to depolarize is the left side of the ventricular septum. As a result — septal depolarization normally moves from left-to-right (red arrow pointing left-to-right in Figure-3).
  • As a result of this initial left-to-right direction of septal depolarization — left-sided leads (I,aVL; V4,V5,V6) normally see initial ventricular activation as moving away — which is why small septal q waves are often normally seen in one or more of the lateral leads (Figure-3).
  • In contrast — right-sided leads (V1,V2) normally view initial ventricular activation as coming toward the site on the chest where these leads lie. This explains why a small initial r wave is commonly seen in lead V1 and/or lead V2 (Figure-3). This initial small r wave represents septal depolarization. It is lost when there is septal infarction (Figure 4).
Figure-3: Normal septal activation moves from left-to-right (red arrow). This accounts for the small initial r wave that may normally be seen in right-sided leads V1 and/or V2 — and for the normal small and narrow septal q waves that may be seen in one or more of the lateral leads. (Figure adapted from ECG-2014-ePub).
     Normal R Wave Progression is shown in the precordial lead sequence depicted in Panel A of Figure-4. Note that there is progressive increase in R wave amplitude as one moves across precordial leads until the point of “Transition” is reached (where the R wave becomes taller than the S wave is deep). Transition normally occurs between leads V2-to-V4. It occurs between leads V3-to-V4 in Panel A. Other normal findings in Panel A include:
  • An isolated Q wave (or QS complex) that is localized to lead V1 (with a small positive r wave developing by lead V2 and with normal R wave progression after lead V2).
  • Small and narrow septal q waves in one or more of the lateral leads (seen here in leads V5,V6 of Panel A).
BOTTOM Line — There is no ECG evidence of infarction in the precordial lead sequence that is seen in Panel A of Figure 4. While not impossible — septal infarction is unlikely to have occurred when a QS complex is only seen in lead V1.
Figure-4: Three precordial lead sequences. Panel A — shows normal RWP (R Wave Progression) without sign of infarction. Panel B — suggests isolated anterior infarction has occurred. Panel C — suggests antero-septal infarction has occurred. (See text; Figure adapted from ECG-2014-ePub).
     Panel B in Figure-4 — suggests anterior MI (Myocardial Infarction) has occurred at some point. Note the following:
  • Lead V1 manifests an initial positive deflection (r wave). This suggests that the septum is likely to be intact.
  • There is then “loss of r wave” between lead V1 and leads V2,V3. Instead of progressive increase in r wave amplitude — a deep QS complex is seen in leads V2,V3. A small q wave is present in lead V4 — but there is no q wave in either V5 or V6. This picture suggests an isolated anterior infarction (ie, without septal involvement).
  • Advanced Point: The ECG picture in Panel B may also be produced by lead placement error. If clinically indicated — repeating the ECG after verifying lead placement can resolve this issue.
BOTTOM Line — While accuracy of the above terminology for identifying "septal involvement" is admittedly not perfect — thinking of V1,V2 as “septal leads” allows for an easy and functional system of lead localization.

     Panel C in Figure-4 — suggests antero-septal MI has occurred.
  • In contrast to Panel B — the presence of no r wave at all in leads V1, V2 and V3 suggests septal involvement in addition to infarction of the anterior wall.
Where to Look on a 12-Lead ECG?
Returning to pictorial review of which leads view which area(s) of the heart — Figure 5 schematically illustrates where to look for the 3 Lead Groups on a 12-lead ECG:
  • Group 1 consists of the 3 Limb Leads = leads I, II, III.
  • Group 2 consists of the 3 Augmented Leads = leads aVR, aVL, aVF.
  • Group 3 consists of the 6 Chest Leads = leads V1-thru-V6.
Figure-5: Where to look on a 12-lead for the Limb Leads — Augmented Leads — and Precordial Chest Leads. (Figure reproduced from A 1st Book on ECGs-2014-ePub).
     In Figure-6 — We color code the area of the heart viewed by each of the 12 leads on a standard ECG. The “trained eye” automatically takes in leads of the same color in the same glance:
  • High-Lateral Leads Leads I, aVL (purple).
  • Inferior Leads Leads II, III, aVF (pink).
  • Septal Chest Leads Leads V1,V2 (green).
  • Anterior Chest Leads Leads V2,V3,V4 (green).
  • Lateral Chest Leads Leads V4,V5,V6 (green).
Figure-6: The area of the heart viewed by each of the 12 leads is color-coded. (Figure adapted from A 1st Book on ECGs-2014-ePub).
NOTE: Similar information is conveyed in different formats in the various Figures from this post in our hope of clarifying the topic of Lead Groups and the anatomic areas of the heart that each lead views.
  • Remember when viewing Figure-6 that there is overlap in the precordial chest lead areas (ie, lead V2 is both septal and anterior and lead V4 is both anterior and lateral).
Advanced Point (Beyond-the-Core): Recently — MRI correlations with cardiac anatomy, coronary artery distribution, and ECG findings suggest that traditional ECG terminology is not as accurate as previously thought (Bayes de Luna et al: Circulation 114:1755,2006). Thus, the anatomic relationship of the “posterior” wall — is in reality not as directly posterior as depicted by point “Y” in Figure-2. Instead — what traditionally has been thought of as “posterior” wall involvement is more accurately referred to as involvement of part of the lateral LV wall.
  • A new, more anatomically accurate terminology has been proposed. This would change reference to the posterior LV wall to the lateral wall instead.
  • MRI correlations have also suggest that classification of lead aVL as a “high lateral lead” is anatomically inaccurate. Instead, the ECG finding of infarction Q waves in lead aVL and/or lead I — but without a Q wave in lead V6 indicates a mid-anterior wall MI rather than “high lateral involvement”.
  • MY BIAS: Realizing the potential tremendous benefit MRI correlations may provide toward more accurate anatomic localization — traditional ECG terminology appears entrenched at the current time. Rather than confusing the issue with a novel ECG terminology that is not yet in general use — We favor continued use of the term posterior infarction (with continued distinction between lateral vs posterior walls of the heart) — and continued description of lead aVL as a “high lateral" lead. We fully acknowledge that at some point in the future — more widespread acceptance of MRI-correlated terminology may change the way we localize anatomic areas on ECG.
— For more information — GO TO 
  • Most material and Figures #1,2 and 5,6 have been excerpted from our newest publication = A 1st Book on ECGs-2014and/or from the expanded 1st Book ePub version (available in kindle-kobo-nook-ibooks).
  • More advanced material and Figures #3,4 have been excerpted from ECG-2014-Pocket Brain and/or from the expanded ECG-2014 ePub version (available in kindle-kobo-nook-ibooks).
  • Please check out Free Download of our expanded GLOSSARY of ECG-related terms -

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