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Dyssynchronous ventricular contraction in severe heart failure contributes to low cardiac output, worsening symptoms, and poor prognosis. Recognition of the effect of dyssynchrony in heart failure, and the possibility of manipulating the sequence of electrical cardiac activation to improve the efficiency of mechanical events, led Cazeau et al. to attempt four-chamber pacing in 1994.1 This early system could stimulate both atria and both ventricles extrinsically, and could dictate the temporal relationship between atrial systole and ventricular systole, and the ventriculo-ventricular relationship. Modern cardiac resynchronization therapy (CRT), involving pacing of the right and left ventricles, with right atrial pacing to optimize atrio-ventricular delay, has evolved rapidly from this beginning.Left bundle branch block (LBBB) on the surface electrocardiogram (ECG) has been considered a marker of mechanical dyssynchrony as it represents a delay in conduction of depolarization to the left ventricle, with the greatest delay usually being in the lateral free wall. The presence of LBBB, seen in about one-third of patients with heart failure,2 is associated with a poorer outcome than when LBBB is not present.3,4 Furthermore, the wider the QRS complex in LBBB, the worse the prognosis. The presence of a broad QRS complex with LBBB morphology has been an important entry criterion to most of the trials of CRT, which has now been evaluated in randomized clinical trials involving > 4000 patients.5–9 The European Society of Cardiology has now incorporated recommendations for use in clinical practice guidelines.10 The patients for whom CRT is recommended in these guidelines are those who represent a similar group to those involved in the randomized controlled trials, where in most cases the objective evidence of dyssynchrony was a broad QRS complex on the 12-lead ECG.The pharmacological and device-based therapies recommended in … *Corresponding author. Tel: +64 9 373 7599 ext. 89804; fax: +64 9 367 7146. E-mail address: r.doughty{at}auckland.ac.nz

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