Nonspecific intraventricular conduction delay: Definitions, prognosis, and implications for cardiac resynchronization therapy.

Cardiac resynchronization therapy (CRT) is an electrical treatment of heart failure with reduced ejection fraction and wide QRS. It aims to correct the electrical dyssynchrony present in 30% to 50% of patients in this population. Dyssynchrony results in widening of the QRS complex on the electrocardiogram (ECG). CRT was initially developed to treat patients who had left bundle branch block (LBBB) and delayed activation of the lateral left ventricular wall. However, a large proportion of heart failure patients present with a widened QRS that is neither an LBBB nor a right bundle branch block (RBBB): nonspecific intraventricular conduction delay (NICD). Less studied than RBBB or LBBB, its pathophysiology is both complex and varied yet still reflects intramyocardial conduction delay. NICD is most often associated with cardiomyopathy (eg, ischemic or hypertensive). Conduction pathways can be either healthy or affected. Results from CRT are contradictory in this patient group, despite a seemingly neutral trend. Unfortunately, prospective studies are lacking. Guidelines recommending implantation of CRT devices in this group are based solely on analyses of subgroups with small sample sizes. A dedicated prospective study is therefore warranted for this question to be answered properly. A detailed study of the ECG and noninvasive study of ventricular electrical activation may enable clinicians to better identify patients with NICD who will respond to CRT.