7Possible VT mechanisms in ischemic patients Scar related reentrant VTs (most common).Focal VTs (including those originating from the papillary muscles).Fascicular VTs (inter-fascicular)BBR VT
8What do we expect from the ECG ? Localize or at least Regionalize the focus or the exit site.The possible mechanism of the tachycardia.Is it endocardial or Epicardial
9Limitations of the ECG as a mapping tool The presence and the extent of infarction.The degree of intra-myocardial fibrosis.The shape of the heart and its position in the chest .Influence of non-uniform anisotropy in affecting propagation from tachy site.
10Continue…Limitations Effect of acute ischemia,drugs,and metabolic abnormalities on conduction.Integrity of the His-Purkinje system.Presence of increased myocardial mass
11What we are searching for ? QRS initial forcesQRS amplitudeQRS widthQRS frontal plane axisBBB patternConcordanceThe presnece of QR complexes.
12QRS initial forcesRapid initial forces>>> More likely arising from normal myocardiumSlurred initial forces (pseudodelta wave )>>> More likely from a scar or from epicardium
13QRS amplitudeUsually VTs arising from diseased myocardium have lower QRS amplitudes from those arising from normal myocardium
14QRS widthFree wall VTs > Septal VTs ( assuming conduction in all directions is equal )Epicardial VTs > Endocardial VTs
24Basic roles in post MI VTs Almost all VTs arise in the LV or IVSECG looses a lot of its ability to precisely localize VT origin or exit sitesAccuracy of the ECG in anterior MI (greater myocardial damage)patients is much less than in inferior MI.
25Continue…Basic rolesIt is extremely rare for an inferior MI dependent VT to have an exit site at the higher septum close to the aortic valveQS complexes in the lateral leads (V4-V6) reflect origin near the apex ( septal or lateral )Almost impossible to distinguish VTs coming from apical septum and apical free wall based on ECG alone
26Inferior infarction VT Activation goes from back to front>> large R wave in the precordial leads starting from V2LBBB VT in inferior MI >> mainly basal septum (inferobasal septum with left axis and higher septal with normal axis).
28Anterior infarction VT The situation becomes more complicated with less accuracy of the ECG (more myocardial damage).LBBB VT or RBBB VT can occurLBBB VT and LAD is associated usually with inferoapical septal region.It can present with negative concordance and always associated with Q wave in I and aVL
29R wave in V1 and Q in aVL indicates more posterior position on the septum RBBB VT usually shows superior axis. V1 can show monophasic R or qR pattern with QS from V2-V4 or up to V6
33What is epicardial VT ?VTs in which the origin or the critical sites of the reentrant circuit are located in the subepicardial tissue as suggested by entrainment maneuvers and/or termination withen 10 seconds of standard RF pulses.Critical epicardial sites may be entained or interrupted from both the epicardial and endocardial surfaces making it difficult to demonstrate the presence of a truly epicardial circuit in a given case
34LimitationsMost of the adopted ECG criteria to predict Epicardial foci or exit sites have been described in patients with NICM and idiopathic VTs .Even VTs with presumed epicardial exit sites can be still ablated from the endocardial approach (The entrance or the central isthmus).No ECG features distinguished outflow tract epicardial exit sites.Poor sensitivity and specificty.
361-Total QRS durationQRS more than 198 ms has 86% specificity and 69% sensitivity for epicardial origin of VT.
372-Pseudo delta waveEarliest ventricular actiavation to the fastest delection an any precordial leadPdw >34 ms has 80% sensitivity and specificty
383-Intrinscoid deflection time ID from the earlist ventricular activation to the nadir of the first S wave in any precordial lead .ID more than 97 ms has 80% specificity and 50% sensitivity for epicardial VT origin.
394-RS durationRS from the earliest ventricular activation to the peak of R wave in lead V2RS >121 ms is 82% specific and 57% sensitive for epicardial VT
405-Maximum Deflection Index ( MDI) It is defined as the shortest time to maximum positive or negative deflection in any precordial lead divided by the QRS duration.A cut-off value of 0.55 has high sensitivity (100%) and specificity (98%) for epicardial VT.This was mainly adopted for epicardial VTs arising from sinuses of Valsalva.
426-Precordial pattern break (R wave regression progression) This was mainly described by Marchilinski group in Pheladelphia and was in the context of idiopathic VTs (but may still work).There is a brupt loss of R wave in V2 followed by a resumption in R waves from V3 to V6.Unkown predictive value.
45Again….RememberEven with the presence of all of the above mentioned criteria, the ECG is not predictive for epicardial access and mapping .Endocardial mapping should be commenced at first for all casesThe role of the above mentioned criteria in post MI patients has no strong evidence.
46Post MI VTs from papillary muscles When to suspect ?
54Conclusion Different VT mechanisms are involved in patients with IHD ECG, inspite of limitations, is a useful tool in localizing or at least regionalizing the exit sites of VTs in post MI patients.ECG has poorer predictive value in patients with anterior infarction than those with inferior MIDifferent ECG criteria can support epicarial focus or exit site but this does not necessarily indicate the successful ablation site.Finally,it is mapping and not the ECG that determine where you have to ablate