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.

词 汇

regionalisation n.区域化，区域的发展战略，地区化，区划

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

topology n.拓扑结构，构相，地志学

subjacent adj.在下面的，在下方的，表层下的，根本的

corollary n.&adj.必然结果（或结论），推断，推论，系；具有推断性质的

stereotypical n.&adj.铅板印刷，机械重复；铅板的，老一套的，刻板化的，模式化的

predilection n.喜爱，偏爱，钟爱

注 释

1.the crux of the heart可译为房室隔沟，是指心脏靠后间隔的一个四边形锥形空间，由右心房、左心房、右心室和左心室围成，即由房室间沟和心室间隔后沟连接而成。其内包含右冠状动脉的后降支、房室结动脉和心中静脉。

参考译文

第90课 室性心律失常体表心电图-定位教程

室性心动过速心电图总原则

对于室性心动过速（VT）起源的初始定位，确切的说是分类，心电图V1是关键。这是因为它放置的部位与间隔平面近乎正交，与心房中的位置并无二样，最能确定右侧或左侧的初始激动。当V1呈净正向QRS波群（R＞s）时，考虑VT呈右束支传导阻滞图形（RBBB）。相反，V1净负向QRS波群（r＜S）定义为左束支传导阻滞图形（LBBB）。这并非指这些图形看上去像典型的束支传导阻滞图形，但它们却是VT命名中的有助部分。

鉴于人类心室构相的一致性及与胸壁（体表心电图电极安置的标准位置）的关系，一些总原则有助于VT的定位：

1.RBBB图形VT起源于左心室而LBBB图形VT起源于间隔或右心室游离壁，只有极少例外。

2.间隔起源VT同时而非循序激动左心室和右心室，因此，相应的QRS波群较窄。

3.心室基底部位起源激动的净向量指向所有胸部电极，胸导联显示较早移行为QRS正向波，实际上在某些瓣环起源的极端情况下，胸导联呈现一致的全正向波。相反，心尖起源的激动因向量背离胸壁而早移行为QRS负向波。在一些心尖起源VT的极端情况下，胸导联呈现完全一致的负向波。

4.尽管QRS额面电轴通常反映VT起源沿头足方向的范围，鉴于左右心室间相互螺旋状环绕，它也反映波峰起源的左-右偏转。

流出道特发性VT

必须适度掌握流出道解剖去理解流出道VT的心电图图形。流出道位于基底部，心脏的中心结构即主动脉根部成为考虑该部位解剖关系的有用参考点。由于胚胎心球的折叠形式，RVOT（漏斗部）从前面包绕主动脉根部和左心室顶部，指向下面心室间隔（IVS）的左侧。因为肺动脉瓣位于主动脉瓣头侧约2cm处，RVOT的所谓“间隔”部分的大部分位于主动脉瓣上方，与其相关的是主动脉而非IVS。新月形RVOT的前间隔部分是其最左侧部分，骑跨在左主动脉乏氏窦（ASOV）和左心室顶部上。后间隔部分紧邻右ASOV。远端肌性RVOT支撑着位于心室动脉连接处的肺动脉瓣窦，进一步延伸成为窦管肺动脉。此处走行接近左心耳。

对多数人而言，包括那些无临床VT者，RVOT不同程度的心肌延伸超出漏斗圆锥和窦管连接而达肺动脉壁（肺窦上方），这些具有致心律失常特性。从肺窦上方消融VA很多是靶向这些延伸部分。在左侧肺窦必须特别谨慎，要考虑该处接近左冠状动脉。也常见心肌延伸到ASOV上方，右ASOV多于左ASOV，偶见于无冠ASOV。

如上所述，流出道VT的基本心电图是电轴向下的LBBB图形。虽然这是见于多数RVOT和肺动脉VT的经典图形，各种左侧起源的也可呈现类似图形。因此，特别从射频消融角度看，解读OT VT图形时最为重要的是首先区分右侧与左侧起源。鉴于上述的解剖关系重叠，虽然有一些总原则可应用，但并非总能分辨。通常情况下最有用的是胸导联QRS移行，左侧起源的从r＜S到r＞S的逆转发生较早。窦性心律时的胸导联移行是判断VT或异位搏动移行的最佳参考（图1）。这已量化成一度量，称为V2移行比率，尽管比率≥0.6高度预测左侧起源，但在许多情况下，目测比较VT和窦性心律移行足也。

在确立心律失常起源于一侧后，进一步的心电图线束确定各侧起源的特定部位。这些线束中的每一种均基于特定的解剖考量。在右侧，RVOT间隔、RVOT游离壁、希氏束旁区、肺动脉左右和前窦均有其特征，见表1总结。在左侧，左ASOV、右ASOV、左/右ASOV连接处、主动脉二尖瓣连接处、前外侧二尖瓣环和左心室顶部起源的各种特征见表2。

非流出道特发性VT

作为上述描写的流出道部位的推演，非流出道特发性VT通常表现为Ⅱ、Ⅲ和aVF（或不一致）为负向波的总体向上电轴（表3）。以触发激动为主的类似电生理机制引发这些心律失常。然而，有一重要的例外是一种少见形式的VT，被定为分支型VT、左间隔VT或异博定敏感VT。虽然是一种发生于结构正常心脏的特发性VT，分支型VT的发作机制是涉及左后分支的折返。表3为达完整而列出另外两种形式的浦肯氏系统折返，即束支折返VT和分支间折返，尽管这些不属于特发性VT，而是通常发生于心脏结构明显异常的患者。

在各不相同的部位，起源的模式化定位再次成为灶性非流出道特发性VT的规则，其中最常见的及其相关的心电图特征列于表3。解剖上重要的结构包括后内侧乳头肌、前外侧乳头肌、后外侧二尖瓣环、三尖瓣环、右心室乳头肌、调节束和房室冠状沟交叉区。

心外膜特发性VT

虽然多数特发性VT源于充分描述的心内膜病灶，两个特别区域促成心外膜VT发作。其中更为常见的区域称作左心室顶部，是左心室外膜的最高点。其头端以左冠状动脉主干分叉为界，而心大静脉横跨其尾端部分。此处特发性VT病灶呈现LBBB图形（尽管V1、V2上呈宽R波）和右下额面电轴的流出道特征。由于远离位于心内膜的浦肯氏系统，外膜病灶倾向于呈现延迟的类本位曲折，特别在胸导联，这已量化成一种度量，称作最大反折指数。另外，鉴于这些顶部病灶最接近V2，可观察到特征性的胸导联移行图形的断裂现象，即相对于V1和V3，V2出现净负向电势。

第2个典型的特发性心外膜VT发作来自心脏的房室隔沟，即锥形空间的顶端。此处基本的心电图是电轴朝向左上而下壁导联呈现明显QS波的LBBB图形。这些病灶通常也显示延长的最大反折指数＞0.55以及胸前V2图形的断裂。

图112导联心电图显示源于流出道的室性异位搏动在下壁Ⅱ/Ⅲ/aVF为正向电势。不支持右心室流出道起源的特征包括异位波动的胸前移行早于窦律，QS振幅aVR小于aVL，以及V1的宽大R波。该异位搏动点在左冠窦得到消融。