How to use the 12-lead ECG to predict the site of origin of idiopathic ventricular arrhythmias

Lesson Ninety-six

Anatomical considerations

When the heart is viewed in an attitudinal1perspective,the right cardiac chambers are anterior relative to the left chambers.The right ventricle（RV）is positioned anteriorly and to the right of the left ventricle（LV）.The LV lies obliquely in the chest,with the base located posteriorly and the apex positioned to the left.The so-called anterior interventricular sulcus in fact begins superiorly and travels to the left and slightly anteriorly,while the so-called posterior interventricular sulcus is actually positioned inferiorly.

Regarding the outflow tract region,the anatomical relationships are complex and ventricular arrhythmias（VAs）from different structures in this region may have a similar ECG appearance.The RV outflow tract（RVOT）wraps around and crosses the LV outflow tract（LVOT）anteriorly,so that the pulmonary valve lies anterior and to the left of the aortic valve（AoV）.This relation is important,as the anterior aspect of the RVOT is actually the most leftward and highest outflow tract structure.

The LVOT corresponds to the elliptical opening of the LV,also termed the LV ostium by McAlpine.This is composed by the aortic root anteriorly and by the mitral annulus posteriorly and to the left.Both valves are anatomically coupled through a band of fibrous tissue known as the aortomitral continuity（AMC）,which extends between the anterior leaflet of the mitral valve（MV）and the left（LCC）and noncoronary cusp（NCC）of the AoV.The AoV occupies a central position within the heart and is composed of 3 cusps,each one with relevant anatomical relationships.In attitudinal orientation,the right coronary cusp（RCC）is the most anterior cusp relative to the sternum,the NCC is posterior and rightward,and the LCC is posterior and leftward.The NCC is the most inferior and the LCC is the most superior in position.The RCC is in close proximity to the posteroseptal aspect of the RVOT,while the LCC is adjacent to the anterior aspect of the LV ostium,in close proximity to the left anterior descending coronary artery.Conversely,the NCC is in relationship with both the left atrium and the right atrium separated by the interatrial septum.Below the commissure between the RCC and the NCC lies the membranous ventricular septum,where the penetrating bundle of His is located.

ECG features

Several ECG features are relevant for the localization of a particular ventricular arrhythmias（VAs）.The most important are（1）QRS axis,（2）bundle branch block pattern,（3）precordial transition,and（4）QRS width.

The QRS axis has both a vertical（superiorinferior）and a horizontal（right-left）dimension.The vertical dimension is reflected by QRS polarity in bipolar leadsⅡandⅢ.For example,all outflow tract VAs share an inferiorly directed QRS axis,with positive forces in leadsⅡandⅢ.The horizontal dimension is better reflected by leadⅠ.Structures closer to the left arm will produce a deeply negative complex in leadⅠ（rightward axis）;conversely,structures closer to the right arm are strongly positive in leadⅠ（leftward axis）.Additional approximation to the horizontal dimension is given by the relative amplitude between limb leads aVR and aVL：a more positive polarity in lead aVR than in lead aVL suggests a more leftward origin;a more positive polarity in lead aVL than in lead aVR points toward a more rightward origin.

The bundle branch block pattern is related to the sequence of RV and LV activation.VAs with a right bundle branch block（RBBB）appearance typically arise in the LV,while VAs with a left bundle branch block（LBBB）appearance may arise anywhere in the RV,but also in the left side of the interventricular septum.

The precordial transition in RBBB VAs（first lead with a predominant S wave）occurs progressively earlier as the site of origin moves from the base toward the apex of the LV.In LBBB VAs,the precordial transition（first lead with a predominant R wave）occurs progressively later as the site of origin moves from the septum toward the RV free wall.Positive concordance（all positive precordial leads）is seen in VAs arising at the base of the heart,in which case ventricular activation has to move anterior and apical.Conversely,negative concordance（all negative precordial leads）is seen in VAs originating near the apex,such that electrical activity moves away from the chest wall.

Finally,septal VAs have narrower QRS durations than do VAs originating on the free wall of both ventricles because of synchronous rather than sequential ventricular activation.

Comprehensive anatomic approach for the prediction of the site of origin

Step 1

We start looking at the superior/inferior axis,represented by polarity in leadsⅡandⅢ.Inferior-axis VAs（positive QRS complex in leadsⅡandⅢ）arise from basal areas of the heart,including the outflow tracts and the superior aspect of the atrioventricular valves,while superior-axis VAs（negative QRS complex in leadsⅡandⅢ）have their origin at the inferior aspect of both ventricles.

Step 2

Our next step is to separate VAs arising from the right or left side of the chest midline,which does not necessarily mean RV vs LV,especially in the outflow tracts,where there is a significant overlap between the RVOT and the LVOT.

1.For outflow tract VAs,the best single ECG discriminator is the left/right axis reflected by leadⅠ.Rightward structures,such as the posterior aspect of the RVOT,RCC,para-Hisian region,and superior aspect of the tricuspid valve（TV）,are positive in leadⅠ,while leftward structures,such as the anterior aspect of the RVOT,LCC,AMC,anterolateral MV annulus,and LV summit,will produce a negative complex in leadⅠ.The commissure between the RCC and the LCC,a common source of idiopathic VAs,is close to the midline,and,in our experience,arrhythmias from this area may have either a positive,a negative,or a biphasic QRS complex in leadⅠ.

2.For VAs arising from the inferior aspect of the ventricles,the most helpful element is the bundle branch block appearance,as some VAs from the septal portion of the LV may exhibit a left axis.VAs with a superior axis and LBBB appearance may arise from RV structures（inferior aspect of the TV or moderator band[MB]）or the cardiac crux.Conversely,VAs with a superior axis and RBBB pattern arise from LV structures（inferior aspect of the MV,posteromedial papillary muscle[PPM],or left posterior fascicle）.

Step 3

Once we circumscribe the likely site of origin to 1 of these 4 quadrants,a more refined localization relies in other characteristics such as precordial transition,QRS width,or QRS morphology in specific leads.

Right upper quadrant

It includes the posterior aspect of the RVOT,RCC,superior TV and para-Hisian region（Figures 1A-1E）.Outflow tract VAs,in general,can be differentiated from TV and para-Hisian VAs by looking at lead aVL polarity.Lead aVL is a left sided but also a superior lead;thus,the majority of outflow tract VAs show negative deflections in lead aVL（QS waves）as well as in lead aVR.Conversely,TV and para-Hisian Vas are located more inferiorly and rightward in the chest and,therefore,usually exhibit positive deflections in lead aVL（any R or r waves）.In addition,RVOT and RCC VAs show a strong inferior axis,with tall R waves in leadsⅡandⅢ.In VAs from the superior TV and para-Hisian region,positive forces are less pronounced,especially in leadⅢ,which can be even isoelectric or negative.Finally,a narrow QRS duration（usually＜130 ms）is typical of para-Hisian VAs given the early engagement of this His-Purkinje system.

Left upper quadrant

It includes the anterior aspect of the RVOT and most LVOT structures（excluding the RCC）（Figures 1F-1L）.The precordial transition is likely the most helpful characteristic to pay attention in this group.As we move progressively more posterior from the RVOT free wall to the lateral mitral annulus,the precordial transition occurs progressively earlier（lead V4or V5for the RVOT free wall,lead V3or V4for the RVOT septum,lead V1or V2for the LCC）and finally transforms from an LBBB to an RBBB configuration at the AMC or the top of the MV.In addition to the bundle branch block pattern,some specific characteristics of lead V1may orientate to certain locations：RVOT and RCC VAs typically exhibit a QS pattern in lead V1;a QS pattern with notching in downstroke is suggestive of VAs from the RCC/LCC commissure;LCC VAs often have a multiphasic pattern in lead V1（M or W pattern）;a qR pattern in lead V1is often seen in VAs from the AMC;and VAs from the anterolateral MV annulus most often have an R pattern in lead V1with positive precordial concordance.

Right lower quadrant

The most common sources of idiopathic VAs in this quadrant are the inferior TV annulus,the MB,and the cardiac crux（Figures 2A-2C）.The MB is a prominent muscular trabeculation that crosses from the septum to the free wall of the RV and provides support to the anterior papillary muscle of the TV.The crux of the heart is an epicardial region near the junction of the middle cardiac vein and the coronary sinus.MB VAs typically have a left superior axis and late precordial transition（later than lead V4）.Conversely,crux VAs also have a left superior axis,but with early transition（lead V2）and a QS pattern in inferior leads.They may also present features suggesting an epicardial access,such a pseudo delta wave or maximum deflection index＞0.55.TV VAs have a variable precordial transition（leads V2through V5）depending on their septal or lateral origin（lead V2or V3for septal sites and lead V4or V5for free wall sites）.A QS pattern in lead V1is recorded in the majority of VAs arising from the septal portion of the TV annulus,while most VAs from the free wall portion exhibit an rS pattern in lead V1.

Left lower quadrant

Idiopathic VAs with RBBB and superior axis may arise from the inferior MV annulus,the left posterior fascicle,and the PPM（Figures 2D-2F）.These can be differentiated on the basis of 3 main characteristics：precordial transition,QRS duration,and V1morphology.Positive precordial concordance（R＞S in lead V6）is relatively specific of MV VAs,reflecting their more basal location.Conversely,VAs from the left posterior fascicle and PPM usually have R＜S by lead V5.A QRS duration of＜130 ms is highly suggestive of fascicular VAs,reflecting the more rapid ventricular depolarization via the Purkinje system.For the same reason,fascicular VAs typically have an rsR′（r＜R′）pattern in lead V1,mimicking typical RBBB.In comparison,PPM and MV VAs usually have an Rsr′（R＞r′）,R,or qR pattern in lead V1.

Inferior lead discordance

Inferior lead discordance reflects an opposite depolarization vector along bipolar limb leadsⅡ（from the left leg to the right arm）andⅢ（from the left leg to the left arm）.This is most often observed in VAs originating from midcavitary structures（interventricular septum,MB,and APM）and sometimes from the lateral aspect of the atrioventricular valves（Figures 2G-2H）.Positive/negative discordance（positiveⅡ/negativeⅢ）is equivalent to a frontal axis of-30° to+30°,and negative/positive discordance（negativeⅡ/positiveⅢ）is equivalent to a frontal axis of+150°to+210°.In particular,the likely sites of origin are as follows：

Figure 1 Inferior-axis VAs with origin at the（A）posterior RVOT（septal wall）,（B）RCC,（C）para-Hisian region,（D）superior TV,（E）RCC-LCC commissure,（F）anterior RVOT,（G）LCC,（H）AMC,（I）anterolateral MV,（J）LV summit,（K）left anterior fascicle,and（L）anterolateral papillary muscle.

Figure 2 Superior-axis VAs with origin at the（A）inferior TV,（B）moderator band,（C）cardiac crux,（D）inferior MV,（E）left posterior fascicle,and（F）posteromedial papillary muscle.Two examples of VAs with inferior lead discordance ablated from the（G）moderator band and（H）anterolateral papillary muscle.

1.Positive/negative discordance：RV structures,including the lateral TV,MB,and interventricular septum（para-Hisian region）.All these have an LBBB configuration.

2.Negative/positive discordance：LV structures,including the lateral MV and APM.These have an RBBB configuration.

詞 匯

obliquely adv.傾斜地

sulcus n.縱溝;回間溝

elliptical adj.橢圓的，隱晦的

aortomitral adj.主動脈二尖瓣的

commissure n.合縫處，縫口，接合點（緣，處）

approximation n.近似值，粗略估算，類似事物

discriminator n.辨別者，鑒別器

circumscribe v.限制，約束（自由、權利、權力等），畫…的外接圓

quadrant n.四分之一圓，象限;象限儀，四分儀

midcavitary n.中腔，腔中間

注 釋1.Attitudinal一詞通常譯為表示人的主觀意識的表現狀態(tài)，即“態(tài)度的，姿態(tài)的”，醫(yī)學文獻中有時用作表示客觀的身體體態(tài)，特別在描述心臟解剖時強調的是原位。本文中兩處出現該詞，其一為“attitudinal perspective”，譯為“體態(tài)視角”，其二為“attitudinal orientation”，譯為“體態(tài)方向”，強調的是心臟原位的視角和方向。例如“From the nonattitudinal perspective,the anterior pulmonic cusp is the most anterior cusp;however,when viewed from an attitudinal perspective,the right pulmonic cusp is equally anterior relative to the chest wall.”。譯為“從非體態(tài)視角看，前肺竇尖是最靠前的竇尖，然而，從體態(tài)視角看，相對胸壁而言，右肺竇尖一樣靠前”。

參考譯文

第96課 如何應用12導聯心電圖預測室性心律失常起源

解剖基礎

當以體態(tài)視角觀看心臟時，右心腔室較左心腔室靠前。右心室位于左心室的右前方。在胸腔內，左心室傾斜走向，基底部靠后，心尖部朝左。事實上，所謂的前室間溝始于上方，向左走行，略向前，而所謂的后室間溝實際上位于下方。

就流出道區(qū)域而言，解剖復雜，源于該區(qū)域不同結構的室性心律失常（VAs）具有類似的心電圖表現。右心室流出道（RVOT）從前方包繞并跨過左心室流出道（LVOT），因此，肺動脈瓣位于主動脈瓣的前方和左側。這一關系是重要的，因為RVOT的前面部分實際上是最靠左側且最高位的流出道結構。

LVOT相當于左心室的橢圓形出口，McAlpine稱其為左心室口。其組成前方為主動脈根部，后方為二尖瓣環(huán)，朝向左側。解剖上主動脈瓣和二尖瓣通過纖維組織帶相連，即主動脈二尖瓣結合部，從二尖瓣前葉到主動脈瓣的左冠竇和無冠竇。主動脈瓣占據心臟的中心位置，由3個竇組成，各有相應的解剖關系。在體態(tài)方向上，相對于胸骨右冠竇最為靠前，無冠竇靠后偏右，左冠竇靠后偏左。無冠竇位置最低，左冠竇位置最高。右冠竇緊密連接RVOT的后間隔部分，而左冠竇毗連左心室口前部，接近左前降支冠狀動脈。相反，無冠竇與左心房和右心房之間由房間隔分隔。右冠竇和無冠竇結合處下方為室間隔膜部，希氏束穿行于此。

心電圖特征

許多心電圖特征與特定的VAs定位有關。最重要的有（1）QRS波群心電軸，（2）束支傳導阻滯圖形，（3）心前區(qū)移行，和（4）QRS波群寬度。

QRS波群心電軸包含垂直（上-下）與水平（左-右）方向。垂直方向反映在雙極導聯Ⅱ和Ⅲ的QRS極性上。例如，所有流出道VAs的QRS波群心電軸向下，Ⅱ和Ⅲ呈正向波。水平方向主要反映在Ⅰ上。接近左臂的結構將在Ⅰ產生深度負向波（心電軸右偏）；相反，接近右臂的結構將在Ⅰ產生明顯的正向波（心電軸左偏）。另一與水平方向類同的是肢體aVR和aVL間的振幅比：正向極性aVR大于aVL的，提示起源偏左，正向極性aVL大于aVR的起源偏右。

束支傳導阻滯圖形與左右心室的激動順序有關。呈右束支傳導阻滯（RBBB）圖形的VAs來自左心室，而呈左束支傳導阻滯（LBBB）圖形的VAs可來自右心室的任何部位，但也可來自室間隔左側。

當起源部位從間隔部位轉向右心室游離壁時，RBBB圖形VAs的心前區(qū)移行（出現以S波為主的第一個胸導聯）逐漸提前。起源于心臟基底部的VAs心室激動朝向前方和心尖，所有胸導聯一致表現為正向波。相反，起源接近心尖的VAs，電激動背離胸壁，胸導聯均為負向波。

最后，間隔部位起源VAs同步而非先后激動兩心室，因此，QRS波群較心室游離壁起源的窄。

預測起源部位的綜合解剖定位方法

步驟1

我們先看向上/向下心電軸，這反映在Ⅱ和Ⅲ的極性上。心電軸向下的VAs（Ⅱ和Ⅲ正向QRS波群）起源于心臟基底區(qū)域，包括流出道和房室瓣上部，心電軸向上的VAs（Ⅱ和Ⅲ負向QRS波群）起源于兩心室下部。

步驟2

下一步是區(qū)分VAs來自胸部中線的左側或右側，這并不意味著一定是左心室對右心室，尤其在流出道，左右流出道之間存在明顯的重疊。

1.對于流出道起源VAs，最佳單一心電圖鑒別點是Ⅰ左偏/右偏心電軸。偏右結構起源的，如RVOT后部、右冠竇、希氏束旁區(qū)及三尖瓣環(huán)上部，Ⅰ為正向波，而偏左結構起源的，如RVOT的前部、左冠竇、主動脈二尖瓣結合部、二尖瓣環(huán)前外側及左心室頂部，Ⅰ為負向波。右冠竇和左冠竇結合處是特發(fā)性VAs的常見起源部位，接近中線，基于我們的經驗，該區(qū)域起源的VAs在Ⅰ可表現為正向、負向或雙向的QRS波群。

2.對于心室下部起源的VAs，最具有幫助的是束支傳導阻滯圖形，盡管一些左心室間隔部位起源的VAs可呈心電軸左偏。心電軸向上伴LBBB圖形的VAs起源部位可以是右心室結構（三尖瓣環(huán)下部或調節(jié)束）或心臟十字溝。相反，心電軸向上伴RBBB圖形的VAs起源于左心室結構（二尖瓣環(huán)下部，后內側乳頭肌，或左后分支）。

步驟3

當我們將預期的起源部位限定于4個象限之一時，更精細的定位依賴于其他特征，如心前區(qū)移行、QRS波群寬度或特殊導聯的QRS形態(tài)。

右上象限：這包括RVOT的后面部分、右冠竇、三尖瓣環(huán)上部和希氏束旁區(qū)（圖1A-E）?？傮w上，通過觀看aVL的極性即能區(qū)分流出道起源VAs與三尖瓣環(huán)和希氏束旁區(qū)起源VAs。aVL是左側向上導聯，因此，多數流出道VAs在此導聯上與aVR一樣呈現負向波（QS波）。相反，三尖瓣環(huán)和希氏束旁區(qū)位置較低且偏向胸部右側，因此，常在aVL上表現為正向波（R或r波）。另外，RVOT和右冠竇起源VAs顯示明顯的向下心電軸，Ⅱ和Ⅲ表現為高R波。三尖瓣環(huán)和希氏束旁區(qū)起源VAs的正向向量不突出，特別在Ⅲ，可以表現為等電位或負向波。最后，希氏束旁區(qū)起源VAs，因為早期激動希-浦系統(tǒng)，典型表現為窄QRS間期（通常＜130 ms）。

左上象限：這包括RVOT前部和多數LVOT結構（除外右冠竇）（圖1F-L）。在此組中，心前區(qū)移行是值得關注的最有利特征。當從RVOT游離壁逐漸轉向較后面的二尖瓣環(huán)外側時，心前區(qū)移行逐漸提前（RVOT游離壁為V4或V5，RVOT間隔為V3或V4，左冠竇為V1或V2），最后，在主動脈二尖瓣結合部或二尖瓣環(huán)頂部圖形從LBBB轉為RBBB。除了LBBB圖形外，V1的一些特征有助于定位：RVOT和右冠竇VAs典型表現為QS波；QS波伴有切跡時提示來自右冠竇與左冠竇結合處；左冠竇VAs常在V1呈現多相波圖形（M或W形）；V1qR波常見于主動脈二尖瓣結合部起源的VAs；二尖瓣環(huán)前外側起源VAs在V1常呈R波，且心前導聯均為正向波。

右下象限：該象限特發(fā)性VAs最常見起源為三尖瓣環(huán)下部、調節(jié)束和心臟十字溝（圖2A-C）。調節(jié)束是一突出的肌小梁，從右心室間隔部跨越至游離壁，對三尖瓣的前乳頭肌起支撐作用。心臟的十字溝為心外膜區(qū)域，接近心中靜脈與冠狀竇連接處。調節(jié)束起源VAs典型表現為左上心電軸和心前區(qū)移行晚（V4以后）。相反，十字溝起源VAs也呈左上心電軸，但移行早（V2），且下壁導聯呈QS波。它們也可出現提示心外膜起源的特征圖形，如假性delta波或最大反折指數＞0.55。三尖瓣起源VAs心前區(qū)移行范圍大（從V2到V5），這決定于間隔或側壁起源（V2或V3為間隔起源，V4或V5為游離壁起源)。大多數三尖瓣環(huán)間隔部起源VAs V1呈QS波，而多數游離壁起源VAs V1呈rS波。

左下象限：呈RBBB且心電軸向上的特發(fā)性VAs可起源于二尖瓣環(huán)下部、左后分支及左后乳頭?。▓D2D–F）。這可基于三大特征加以鑒別：心前移行、QRS間期和V1圖形。心前導聯一致正向（V6R＞S）對于二尖瓣起源VAs是相對特異的，反映它們較基底的位置。相反，左后分支或左后乳頭肌起源的VAs通常V5R＜S。QRS時間＜130 ms高度提示分支型VAs，反映了經過浦肯氏系統(tǒng)的較快心室除極。基于相同的理由，分支型VAs V1上典型表現為rsR′（r＜R′）波,類似典型的RBBB。相比較，左后乳頭肌和二尖瓣環(huán)起源VAs通常V1呈現Rsr′（R＞r′）、R、或qR波。

下壁導聯不一致性

下壁導聯的不一致性反映在雙極肢體導聯Ⅱ（從左腿到右臂）和Ⅲ（從左腿到左臂）的除極向量對立。這常見于起源于心室腔內結構（室間隔、調節(jié)束和前乳頭肌）的VAs，有時見于房室瓣環(huán)的外側部（圖2G-H）。正/負不一（Ⅱ+/Ⅲ-）相當于額面電軸-30°到+30°，負/正不一（Ⅱ-/Ⅲ+）相當于額面電軸+150°到+210°。特殊情況下，可能的起源部位如下：

1.正/負不一：右心室結構，包括三尖瓣環(huán)外側、調節(jié)束、室間隔（希氏束旁區(qū)）。所有這些均呈LBBB圖形。

2.負/正不一：左心室結構，包括二尖瓣環(huán)外側和前乳頭肌。這些呈RBBB圖形。

圖1心電軸向下的VAs起源（A）RVOT后間隔，（B）右冠竇，（C）希氏束旁區(qū)，（D）三尖瓣環(huán)上部，（E）右冠竇-左冠竇結合處，（F）RVOT前部，（G）左冠竇，（H）主動脈二尖瓣結合部，（I）二尖瓣環(huán)前外側，（J）左心室頂部，（K）左前分支，和（L）前外側乳頭肌

圖2心電軸向上的VAs起源（A）三尖瓣環(huán)下部，（B）調節(jié)束，（C）心臟十字溝，（D）二尖瓣環(huán)下部，（E）左后分支，和（F）后內測乳頭肌。下壁導聯不一致的兩例VAs消融部位（G）調節(jié)束和（H）前外側乳頭肌