A cardiac MRI with delayed enhancement should be obtained prior to an ablation procedure if at all possible. Post-processing of the images can be used to integrate the endocardial, epicardial and scar contours in 3D reconstruction images. There is a high degree of correlation between electroanatomical voltage mapping and the scar as characterised by MRI (Figure 3).6 Intraprocedural registration of the scar into the electroanatomic map helps to focus on an area of interest and facilitates the ablation procedure.29 The location of scar tissue has been correlated with the location of the arrhythmogenic substrate.5 A recent study demonstrated that preprocedural MRIs in patients with failed VT ablations also helped to localise the arrhythmogenic tissue and thereby indicated the likelihood of whether an epicardial ablation was necessary.30 In this study, patients with cardiac implanted electronic devices (CIEDs) were included. With appropriate precautions and device programming no adverse events were noted in the patients with CIEDs. Artefact production by the ICD generator is a limitation of MRI, as it can obscure parts of the heart, and scarring cannot be excluded with certainty if this is the case. Reduction of artefact, however, has been described using a novel wideband late gadolinium enhancement imaging protocol.31 Although MRIs have been described to be safe in patients with CIEDs32–34 it needs to be pointed out that such an approach is not yet standard of care in most institutions. Most patients with structural heart disease and VT will already have a CIED in place and therefore imaging of these patients might be more difficult depending on institutional practices and protocols. To safely perform imaging in these patients, a protocol detailing specific programming steps of the CIED prior and after the MRI and monitoring of the patient throughout the MRI in addition to specific exclusion criteria are necessary.
A preprocedural echo should rule out an intracavitary thrombus prior to a planned endocardial ablation procedure. ECG criteria have also been used to distinguish endocardial from epicardial origins.35 A delayed precordial maximal deflection index ≥0.55,25 a pseudo ‘delta wave’ at QRS onset ≥34 ms,36 a minimal RS interval ≥121 ms and the presence of a Q wave in lead I35,37 suggest an epicardial VT origin (Figure 4). Those ECG criteria are readily available and should be considered when predicting the VT exit site. However, their accuracy is limited, especially in patients with structural heart disease, and can be improved when using a computerised algorithm focusing on the ECG slope of the initial part of the QRS complex.38 It is essential to note that the QRS morphology solely reflects the VT exit site and cannot exclude the presence of endocardial components of the circuit, which may be eliminated using an endocardial approach.
If endocardial mapping is performed first, in the absence of preprocedural imaging, unipolar mapping can be beneficial to indicate the presence of intramural or epicardial scar. While bipolar mapping is best in assessing local electrograms originating from tissue adjacent to the recording electrodes, unipolar mapping enables recording of deeper tissue layers. Unipolar signals are recorded between the distal tip of the ablation catheter and the Wilson central terminal during baseline rhythm. Unipolar endocardial mapping has been used for identification of nonendocardial scar that is located intramurally or epicardially. Different voltage cut-off values have been reported for identification of intramural scars in patients with nonischaemic cardiomyopathy (Figure 5),39 as well as epicardial scars in patients with ARVC40 and nonischaemic cardiomyopathy.41 In the presence of a normal endocardial bipolar voltage map combined with an abnormal unipolar voltage map an intramural or epicardial substrate needs to be considered and an epicardial approach may be warranted. Identification of intramural septal scarring is of key importance, as an epicardial procedure is unlikely to reach the intramural septal substrate from the epicardial space. Transseptal conduction times have been found to be prolonged, and may indicate intramural septal scarring.42 In patients with prior MIs, but also in patients with other structural heart diseases and transmural scar, it is possible that parts of the reentrant circuit can be reached from sites other than the endocardium. A recent study pointed out that in approximately onethird of patients with prior infarctions, critical VT sites identified in the endocardium were non-exit sites (i.e. the VT exit in these patients was not confined to the endocardium and was located elsewhere, in the epicardium or intramurally), whereas the remaining critical sites had the exit within the endocardial myocardium.43 This further supports our contention that in patients with prior infarctions, an endocardial approach should be the preferred initial method.
In patients with idiopathic outflow tract arrhythmias, late activation times in the absence of matching endocardial pace maps at the sites of earliest activation suggests a deeper or epicardial focus. Mapping within the aortic valve cusps and the coronary venous system is most helpful to identify an epicardial source. The subxiphoid epicardial approach, as mentioned above, is rarely successful in this scenario and should be limited to select cases given the higher risk of complications.26
A failed endocardial ablation procedure can suggest that the critical area might not be reachable from the endocardium. In these patients, preprocedural imaging is particularly helpful, especially if an intramural or epicardial substrate can be demonstrated.
Elimination of all inducible VTs should be the optimal goal for an ablation procedure if this can be safely achieved, as this approach has the best long-term outcome. If VTs after an endocardial approach remain inducible, an epicardial approach should be considered.44 This approach has been beneficial in patients with prior MI, and also in patients with ARVC.
Patients with a mobile intracavitary thrombus, and those with left ventricular scarring who have implanted artificial aortic and mitral valves are not candidates for extensive endocardial mapping procedures. Alternative approaches (i.e. epicardial ablation, transcoronary ethanol ablation45,46) can be considered in such settings
Lesion depth is a major limitation for VTs originating from intramural sites. An epicardial procedure should be considered in the presence of intramural left ventricular free wall scarring. Bipolar ablation procedures from both aspects of the scar,47 ablation with an extendable needle catheter48,49 or simultaneous unipolar ablation49 have been described in this context. For these situations, epicardial access is helpful.