Ventricular tachycardia substrate mapping: What's been done and what needs to be done.

Substrate mapping is an important component of electrophysiological (EP) study for the treatment of reentrant ventricular tachycardia (VT). It is used to detect characteristics of the electrical circuit and, in particular, the location and properties of the central common pathway, aka the isthmus, where multiple circuit loops can coincide. Typically, reentrant circuits are single or double loop, but as the common pathway size increases, 4-loop patterns may emerge, consisting of 2 parallel isthmuses or a single isthmus with 4 loops. Arrhythmogenic substrate contains a mixture of scar, calcification, and fibrofatty regions blended with viable ventricular myocytes, which can slow conduction. It is identified in the EP laboratory in part by the presence of low-amplitude electrograms and a zone of uniform slow conduction resulting from a sparsity of remaining viable myocytes and molecular-level remodeling. The electrograms recorded near isthmus boundaries frequently exhibit an abnormal morphology, such as fractionation and late or split deflections, due to the separation of muscle fiber bundles by fibroadipose tissue or calcification, and due to other conduction impediments such as source-sink mismatch, wherein topographic changes to the viable myocardial structure occur. Substrate mapping facilitates the identification of arrhythmogenic regions during sinus rhythm, whereas inducible VT with periods of ongoing reentry, when recordable, can be used for further assessment. Substrate modeling augments substrate mapping by seeking to predict electrogram morphology and mapped features and properties to be encountered during EP study based on an accurate depiction of arrhythmogenic tissue. Herein, we elaborate on the details of VT substrate mapping and modeling to the present time.