**8. Future directions to optimize cardiac resynchronization therapy**

CRT is well known since its inception to be a promising electrical therapeutic device to treat CHF. After more than two decades in clinical use, we know that around 30–40% of CRT subjects do not exhibit any detectable clinical or echocardiographic benefit. As a matter of fact, some of them are deteriorating after resynchronization. For this reason, most of the discussion in this chapter and selected recent literature is devoted to non-responders toward optimizing resynchronization therapy [80, 81]. The special diagnostic tools mentioned earlier in this chapter are to refine our CRT subjects selection especially the subgroup without conspicuous LBBB criteria. Those special diagnostic tools can be still considered as future direction that has been started and in the way for mature applicable understandings in the field of CRT science. *Promising new directions can be classified as new diagnostic tools and new basic knowledge with deeper investigation in the biomechanics of cardiac electromechanical coupling and spatial orientation of the ventricular muscles, as well as ,new advances in implant and resynchronization site.*

#### **8.1 Vectorcardiography guided cardiac resynchronization therapy**

Vectorcardiography (VCG) was developed by E. Frank in the mid-1950s. The magnitude and direction of the electrical forces that are generated by the heart are recorded in 3-dimensional information format by means of a continuous series of vectors that form curving lines around a central point. The area under the 3-dimensional QRS complex (QRS area) is reflecting the electrical forces during depolarization and the area under the 3-dimensional T-wave (T area) is reflecting the electrical forces during repolarization. Volumetric response and survival after CRT were thought to be predicted strongly by the QRS area, but also T area and the sum of QRS and T areas (QRST area) [82, 83]. QRS area was also found repeatedly to be superior to QRS duration and morphology as a predictor of CRT response [28, 82, 84, 85]. One retrospective multicenter study displayed that this was true for a cohort of patients that received CRT and also for patients without a Class I indication for CRT according to American guideline recommendations [56] (QRS duration 120–149 ms or non-LBBB) [28]. Only the QRS area in these patients, was significantly associated with all-cause mortality. Reviewing volumetric CRT response, demonstrated that both QRS area and LBBB morphology were associated with an LV end-systolic volume reduction of = or >15 [28]. The advantage of the QRS area is that it is an objective measure and observer-independent parameter, whereas the definition for LBBB is subjective measure and operator-dependent. Variability in the QRS area is less than QRS duration as it is determined by QRS complex amplitude, not the beginning and end of the QRS complex [28]. VCG is not yet commercially available in clinical practice, but the QRS area is a promising non-invasive diagnostic evaluation tool for identifying possible CRT responders.

*CRT Past, Present, and Future Directions: Toward Intelligent Responders Selection… DOI: http://dx.doi.org/10.5772/intechopen.101608*
