Electrocardiographic and echocardiographic dyssynchrony parameters that might better predict the response to cardiac resynchronization therapy than QRS morphology and duration

C ardiac resynchronization therapy (CRT) is an evidence-based effective therapy of symptomatic heart failure with reduced ejection fraction refractory to optimal medical therapy and associated with intraventricular conduction disturbance, that results in a significant electrical ventricular dyssynchrony. However, the nonresponse rate to CRT is still 20%−40%. The potential underlying causes of CRT non-response are: (1) suboptimal localization of the left ventricular (LV) electrode, far away from the latest activated LV region; (2) placement of the LV electrode over a transmural scar; (3) absence of significant dyssynchrony; (4) suboptimal A-V, V-V delay settings; and (5) too severe heart disease. The most important way to further reduce the number of CRT non-responders is the optimal patient selection for CRT. The current guidelines recommend the determination of QRS morphology, QRS duration and the measurement of LV ejection fraction for patient selection for CRT. The main determinant of CRT outcome is the presence or absence of significant ventricular dyssynchrony and the ability of the applied CRT technique to eliminate it. Moreover, CRT can only eliminate mechanical dyssynchrony due to primary electrical dyssynchrony, it is ineffective in mechanical dyssynchrony without primary electrical dyssynchrony, such as inflammation, myocardial ischemia and myocardial scar. The explanation of the still non-negligible non-response rate to CRT is that QRS morphology and QRS duration are not perfect indicators of electrical ventricular dyssynchrony. Therefore, finding better markers of electrical ventricular dyssynchrony may significantly improve the outcome of CRT. There are some promising methods, which can assess the presence of electrical dyssynchrony before CRT implantation in patients who are already on the operating table for CRT implantation, such as the determination of the QLV interval [the interval from the onset of the QRS from the surface electrocardiogram (ECG) to the first large peak of the LV electrogram], or the determination of the right ventricular electrogram-LV electrogram interval in sinus rhythm reflecting interventricular delay. However, these methods can only be applied in the patient who is already on the operating table with invasive procedures. The greatest challenge is to find methods that can assess electrical dyssynchrony before the patient is on the operating table, which can determine, which patient should be selected for CRT implantation. Thus, the main purpose of this Special Issue is to discuss non-invasive (electrocardiographic and echocardiographic) methods that may improve patient selection for CRT, may better indicate the presence of electrical ventricular dyssynchrony than QRS morphology and duration, and can be applied before starting the CRT implantation. Since CRT is only effective in mechanical dyssynchrony due to primary electrical dyssynchrony, this Special Issue is mainly focusing on electrocardiographic methods, but also discusses echocardiographic methods, that can detect dyssynchrony. In this Special Issue, we discuss non-invasive methods that can improve patient selection for the conventional biventricular pacing CRT technique, and we are not dealing with newer CRT techniques, such as endocardial LV pacing, multisite pacing, and conduction system pacing (His-bundle pacing and left bundle branch pacing). In this Special Issue Ghossein MA, et al. sumJournal of Geriatric Cardiology