Mechanical dyssynchrony from the perspective of a cardiac electrophysiologist

Purpose of review Cardiac resynchronization therapy has been shown to improve survival, quality of life, exercise capacity, left ventricular ejection fraction and New York Heart Association functional class in patients with a prolonged QRS and moderate-to-severe heart failure resistant to optimal medical treatment. This patient population has an approximately 30-40% nonresponder rate to cardiac resynchronization therapy. It has been proposed that the use of echocardiographic imaging could be a more specific marker of intraventricular conduction delay rather than the surrogate marker of electrical delay demonstrated by prolonged QRS duration. Recent findings Recent clinical trials have evaluated not only how well imaging predicts dyssynchrony (Predictors of Response to Cardiac Resynchronization Therapy - PROSPECT) but also clinical outcomes in patients with dyssynchrony and narrow QRS duration (resynchronization therapy in narrow QRS - RETHINQ, evaluation of CRT in narrow QRS patients with mechanical dyssynchrony from a multicenter study - ESTEEM- CRT). The former trial failed to demonstrate a single reliable parameter for predicting dyssynchrony, whereas the latter trials did not demonstrate a clinical benefit from cardiac resynchronization therapy in patients with narrow QRS and evidence of dyssynchrony. Summary Results of recent clinical trials have challenged the applicability of contemporary echocardiographic techniques in evaluating dyssynchrony to clinical practice at the present time. Currently the optimal lead position is a lateral or posterolateral position with reasonable capture threshold and lack of diaphragm stimulation. With the refining of echocardiographic techniques and technology, perhaps areas of greatest delay may be targeted and used for guiding lead placement.