The surface electrocardiograph in ventricular arrhythmias:lessons in localisation

General Principles of Ventricular Tachycardia Electrocardiography

The key electrocardiograph（ECG）lead in the initial regionalisation,and indeed classification,of ventricular tachycardia（VT）origin is V1.This is because it is located nearly orthogonal to the septal plane and is,not dissimilarly to the situation in the atrium,best able to resolve initial right verses left sided activation.When V1has a net positive QRS（R＞s）,the VT is considered to have right bundle branch block（RBBB）configuration.Conversely,net negative QRS （r＜S）defines a left bundle branch block（LBBB）configuration.This is not to imply that these configurations look anything like typical bundle branch block patterns,but they are a helpful part of VT nomenclature.

Given the overall uniformity of human ventricular topology and its relationship to the chest wall（where the surface ECG electrodes are applied in the standard positions）,a few general principles are useful in VT localisation:

1.With some rare exceptions only,VTs of RBBB configuration arise from the left ventricle（LV）while VTs of LBBB configuration arise from the septum or from the free wall of the right ventricle（RV）.

2.Septal origins of VT give rise to simultaneous rather than sequential activation of the LV and RV,and hence are associated with narrower QRS widths.

3.Basal sites of origin in the ventricle have a net activation vector towards the overlying chest wall electrodes and hence show earlier transitions to QRS positivity in the precordial leads,or indeed in the extreme case of some annular origins,full positive precordial concordance.Conversely,apical sites have early transitions to QRS negativity given that their activation vector points away from the chest wall.In the extreme case of some apical VTs,this may result in complete negative precordial concordance.

4.Whilst the QRS axis in the frontal plane generally reflects VT origin along a craniocaudal extent,given the way the ventricles spiral around each other,it may also reflect right-left shifts in wavefront origin.

Outflow Tract Idiopathic VA

The anatomy of the outflow tracts must be properly understood to appreciate the ECG patterns seen in OTVT.The outflow tracts are basally located,and the central structure of the heart,namely the aortic root,forms a useful reference point to consider the anatomic relations here.Due to the pattern of folding of the embryonic bulbus cordis,the RVOT（infundibulum）wraps anteriorly around the aortic root and LV summit,leftward of the subjacent interventricular septum（IVS）.Since the pulmonary valve is located around 2cm cranial of the aortic valve,much of the so-called'septal'aspect of the RVOT is supravalvular and is applied to the aortic root,not the IVS.The anteroseptal aspect of the crescent-shaped RVOT is the most leftward part of it and sits overlying the left aortic sinus of Valsalva（ASOV）and the LV summit.The posteroseptal aspect is in close relationship to the right ASOV.The distal muscular RV infundibulum supports the pulmonary valve cusps at the ventriculoarterial junction and extends further to become the sinotubular pulmonary artery.Here it comes into close proximity to the left atrial appendage.

In the majority of people,including those with no clinical VT,there are variable myocardial extensions from the RVOT beyond the infundibular conus and sinotubular junction and into the wall of the pulmonary artery（above the pulmonary sinus cusps）and these may be arrhythmogenic.Ablation of VA from the pulmonary sinus cusp（PSC）probably targets these extensions,and must be undertaken with particular care in the left PSC where proximity to the left coronary artery is a consideration.Myocardial extensions are also commonly noted above the right ASOV more than the left ASOV,and only rarely in the non-coronary ASOV.

As described above,the basic ECG pattern of outflow tract VT is that of a LBBB configuration with inferior axis.Whilst this is the classical pattern seen in most forms of RVOT and pulmonary artery VT,various left-sided sites of origin may also exhibit a similar pattern.Thus,particularly from an ablation perspective,the most important initial consideration in interpreting OT VT morphologies is distinguishing right-sided from left-sided sitesoforigin.Given the overlapping anatomic relations described above,this is not always straight-forward although some generalprinciples apply.Most helpful is generally the precordial QRS transition,with r＜S to r＞S reversal occurring earlier with left-sided foci.The best reference for this transition during VT or ectopy is the sinus rhythm precordial transition[Figure 1] .This was quantified in a metric called the V2transition ratio and although a measured ratio of≥0.6 is highly predictive of a left-sided origin,in many cases,a visual comparison of the VT and sinus rhythm transitions suffices.

After establishing the likely side of the arrhythmogenicfocus,furtherECG clueslocalise particular sites of origin on each side.Each of these clues is based on specific anatomic considerations.On the right side,the septal aspect of the RVOT,the free wall of the RVOT,the parahisian region,and the right,leftand anteriorpulmonary sinuscusps of the pulmonary artery all have specific features that are summarised in Table 1.The specific features of the various left-sided sites of origin,namely the left ASOV,the right ASOV,the left/right commissural junction of the ASOV,the aortomitral continuity,the anterolateral mitralannulus and the LVsummitareoutlined in Table2.

Figure 1 A 12-lead ECG showing ventricular ectopy arising from the outflow tract with positive forces in the inferior leads II,III and aVF.Features against an origin in the right ventricular outflow tract include the early precordial transition in the ectopic relative to the sinus transition,the negative ratio of QS amplitude in aVR to aVL,and the broad prominent R in V1.This ectopic focus was ablated in the left aortic sinus of Valsalva.

Table1 Specific ECGfeatures of right-sided idiopathic outflow tract VAfoci.

Table2 Specific ECGfeatures of left-sided idiopathic outflow tract VAfoci.

Non-Outflow Tract Idiopathic VT

As a corollary to the outflow sites described above,non-outflow tract idiopathic VAs generally have an overall superior axis with negative（or discordant）forces in leads Ⅱ ,Ⅱ and aVF （Table 3）.Similar electrophysiologic mechanisms drive these arrhythmias with triggered activity being prevalent.However,there is one important exception,a rare form of VT designated variably fascicular VT, left septal VT or verapamil-sensitive VT.Although an idiopathic VA occurring in structurally normal hearts,the operative mechanism in fascicular VT is re-entry involving the posterior fascicle.Also listed for completeness in Table 3 are two other forms of Purkinje-system re-entry,namely bundle branch re-entry VT and interfascicular re-entry,although these are not idiopathic VTs and instead usually occur in patients with significant structural heart disease.

Table 3 Specific ECG features of non-outflow tract VA.

Stereotypicalsites oforigin in regions of heterogeneity are again the rule in focal non-outflow tract idiopathic VA,and the commonest of these with their associated ECG features are listed in Table 3.Anatomically important structures here include the posteromedial papillary muscle, the anterolateral papillary muscle,posterolateral mitral annular sites,tricuspid annularsites,rightventricularpapillary muscles,the moderator band and the crux.

Epicardial Idiopathic VT

While most idiopathic VT arises from well described endocardial foci,two regions in particular have a predilection for epicardial VT breakouts.The more common of these is the region known as the LV summit,the highest point on left ventricular epicardium.It is bounded cranially by the bifurcation of the left main coronary artery,and the great cardiac vein traverses across its caudal aspect.Idiopathic VT foci here display outflow type characteristics with LBBB-type morphology（albeit often with a broad R in V1and V2）and right inferior frontal plane axis.Due to their remoteness from the endocardially located Purkinje system,epicardial foci tend to have delayed intrinsicoid deflections particularly in the precordial leads and this was quantified into a metric called the maximum deflection index.Additionally,since these summit foci lie closest to lead V2,a characteristic precordial transition pattern break may be observed with net negative forces observed in V2relative to V1and V3.

The second typically epicardial breakout of idiopathic VT is from the crux of the heart1,apical of the pyramidal space.The basic ECG pattern here is a LBBB configuration with left superior axis but the inferior leads here display markedly negative QS complexes.These focialsotypically exhibita prolonged maximum deflection index （MDI）＞0.55 and may display precordial V2pattern break also.

詞 匯

regionalisation n.區域化，區域的發展戰略，地區化，區劃

orthogonal n.&adj.正交直線；正交的，直角（交）的，（相互）垂直的，矩形的

topology n.拓撲結構，構相，地志學

subjacent adj.在下面的，在下方的，表層下的，根本的

corollary n.&adj.必然結果（或結論），推斷，推論，系；具有推斷性質的

stereotypical n.&adj.鉛板印刷，機械重復；鉛板的，老一套的，刻板化的，模式化的

predilection n.喜愛，偏愛，鐘愛

注 釋

1.the crux of the heart可譯為房室隔溝，是指心臟靠后間隔的一個四邊形錐形空間，由右心房、左心房、右心室和左心室圍成，即由房室間溝和心室間隔后溝連接而成。其內包含右冠狀動脈的后降支、房室結動脈和心中靜脈。

參考譯文

第90課 室性心律失常體表心電圖-定位教程

室性心動過速心電圖總原則

對于室性心動過速（VT）起源的初始定位，確切的說是分類，心電圖V1是關鍵。這是因為它放置的部位與間隔平面近乎正交，與心房中的位置并無二樣，最能確定右側或左側的初始激動。當V1呈凈正向QRS波群（R＞s）時，考慮VT呈右束支傳導阻滯圖形（RBBB）。相反，V1凈負向QRS波群（r＜S）定義為左束支傳導阻滯圖形（LBBB）。這并非指這些圖形看上去像典型的束支傳導阻滯圖形，但它們卻是VT命名中的有助部分。

鑒于人類心室構相的一致性及與胸壁（體表心電圖電極安置的標準位置）的關系，一些總原則有助于VT的定位：

1.RBBB圖形VT起源于左心室而LBBB圖形VT起源于間隔或右心室游離壁，只有極少例外。

2.間隔起源VT同時而非循序激動左心室和右心室，因此，相應的QRS波群較窄。

3.心室基底部位起源激動的凈向量指向所有胸部電極，胸導聯顯示較早移行為QRS正向波，實際上在某些瓣環起源的極端情況下，胸導聯呈現一致的全正向波。相反，心尖起源的激動因向量背離胸壁而早移行為QRS負向波。在一些心尖起源VT的極端情況下，胸導聯呈現完全一致的負向波。

4.盡管QRS額面電軸通常反映VT起源沿頭足方向的范圍，鑒于左右心室間相互螺旋狀環繞，它也反映波峰起源的左-右偏轉。

流出道特發性VT

必須適度掌握流出道解剖去理解流出道VT的心電圖圖形。流出道位于基底部，心臟的中心結構即主動脈根部成為考慮該部位解剖關系的有用參考點。由于胚胎心球的折疊形式，RVOT（漏斗部）從前面包繞主動脈根部和左心室頂部，指向下面心室間隔（IVS）的左側。因為肺動脈瓣位于主動脈瓣頭側約2cm處，RVOT的所謂“間隔”部分的大部分位于主動脈瓣上方，與其相關的是主動脈而非IVS。新月形RVOT的前間隔部分是其最左側部分，騎跨在左主動脈乏氏竇（ASOV）和左心室頂部上。后間隔部分緊鄰右ASOV。遠端肌性RVOT支撐著位于心室動脈連接處的肺動脈瓣竇，進一步延伸成為竇管肺動脈。此處走行接近左心耳。

對多數人而言，包括那些無臨床VT者，RVOT不同程度的心肌延伸超出漏斗圓錐和竇管連接而達肺動脈壁（肺竇上方），這些具有致心律失常特性。從肺竇上方消融VA很多是靶向這些延伸部分。在左側肺竇必須特別謹慎，要考慮該處接近左冠狀動脈。也常見心肌延伸到ASOV上方，右ASOV多于左ASOV，偶見于無冠ASOV。

如上所述，流出道VT的基本心電圖是電軸向下的LBBB圖形。雖然這是見于多數RVOT和肺動脈VT的經典圖形，各種左側起源的也可呈現類似圖形。因此，特別從射頻消融角度看，解讀OT VT圖形時最為重要的是首先區分右側與左側起源。鑒于上述的解剖關系重疊，雖然有一些總原則可應用，但并非總能分辨。通常情況下最有用的是胸導聯QRS移行，左側起源的從r＜S到r＞S的逆轉發生較早。竇性心律時的胸導聯移行是判斷VT或異位搏動移行的最佳參考（圖1）。這已量化成一度量，稱為V2移行比率，盡管比率≥0.6高度預測左側起源，但在許多情況下，目測比較VT和竇性心律移行足也。

在確立心律失常起源于一側后，進一步的心電圖線束確定各側起源的特定部位。這些線束中的每一種均基于特定的解剖考量。在右側，RVOT間隔、RVOT游離壁、希氏束旁區、肺動脈左右和前竇均有其特征，見表1總結。在左側，左ASOV、右ASOV、左/右ASOV連接處、主動脈二尖瓣連接處、前外側二尖瓣環和左心室頂部起源的各種特征見表2。

非流出道特發性VT

作為上述描寫的流出道部位的推演，非流出道特發性VT通常表現為Ⅱ、Ⅲ和aVF（或不一致）為負向波的總體向上電軸（表3）。以觸發激動為主的類似電生理機制引發這些心律失常。然而，有一重要的例外是一種少見形式的VT，被定為分支型VT、左間隔VT或異博定敏感VT。雖然是一種發生于結構正常心臟的特發性VT，分支型VT的發作機制是涉及左后分支的折返。表3為達完整而列出另外兩種形式的浦肯氏系統折返，即束支折返VT和分支間折返，盡管這些不屬于特發性VT，而是通常發生于心臟結構明顯異常的患者。

在各不相同的部位，起源的模式化定位再次成為灶性非流出道特發性VT的規則，其中最常見的及其相關的心電圖特征列于表3。解剖上重要的結構包括后內側乳頭肌、前外側乳頭肌、后外側二尖瓣環、三尖瓣環、右心室乳頭肌、調節束和房室冠狀溝交叉區。

心外膜特發性VT

雖然多數特發性VT源于充分描述的心內膜病灶，兩個特別區域促成心外膜VT發作。其中更為常見的區域稱作左心室頂部，是左心室外膜的最高點。其頭端以左冠狀動脈主干分叉為界，而心大靜脈橫跨其尾端部分。此處特發性VT病灶呈現LBBB圖形（盡管V1、V2上呈寬R波）和右下額面電軸的流出道特征。由于遠離位于心內膜的浦肯氏系統，外膜病灶傾向于呈現延遲的類本位曲折，特別在胸導聯，這已量化成一種度量，稱作最大反折指數。另外，鑒于這些頂部病灶最接近V2，可觀察到特征性的胸導聯移行圖形的斷裂現象，即相對于V1和V3，V2出現凈負向電勢。

第2個典型的特發性心外膜VT發作來自心臟的房室隔溝，即錐形空間的頂端。此處基本的心電圖是電軸朝向左上而下壁導聯呈現明顯QS波的LBBB圖形。這些病灶通常也顯示延長的最大反折指數＞0.55以及胸前V2圖形的斷裂。

圖112導聯心電圖顯示源于流出道的室性異位搏動在下壁Ⅱ/Ⅲ/aVF為正向電勢。不支持右心室流出道起源的特征包括異位波動的胸前移行早于竇律，QS振幅aVR小于aVL，以及V1的寬大R波。該異位搏動點在左冠竇得到消融。