Predicting outcome in severe heart failure. Who will benefit from device therapy (CRT)?

Dyssynchronous ventricular contraction in severe heart failurecontributes to low cardiac output, worsening symptoms, and poorprognosis. Recognition of the effect of dyssynchrony in heartfailure, and the possibility of manipulating the sequence ofelectrical cardiac activation to improve the efficiency of mechanicalevents, led Cazeau et al. to attempt four-chamber pacing in1994.¹ This early system could stimulate both atria and bothventricles extrinsically, and could dictate the temporal relationshipbetween atrial systole and ventricular systole, and the ventriculo-ventricularrelationship. Modern cardiac resynchronization therapy (CRT),involving pacing of the right and left ventricles, with rightatrial pacing to optimize atrio-ventricular delay, has evolvedrapidly from this beginning. Left bundle branch block (LBBB) on the surface electrocardiogram(ECG) has been considered a marker of mechanical dyssynchronyas it represents a delay in conduction of depolarization tothe left ventricle, with the greatest delay usually being in