Cardiac Resynchronization Therapy (CRT)

The decision to treat an arrhythmia depends on its symptoms and its potential severity. Treatment is directed at causes. If necessary, direct antiarrhythmic therapy, including antiarrhythmic drugs, cardioversion-defibrillation, implantable cardioverter-defibrillators (ICDs), pacemakers (and a special form of pacing, cardiac resynchronization therapy), catheter ablation, surgery, or a combination, is used.

In some patients, the normal, orderly, sequential relationship between contraction of the cardiac chambers is disrupted (becomes dyssynchronous). Dyssynchrony may be

• Atrioventricular: Between atrial and ventricular contraction

• Interventricular: Between left and right ventricular contraction

• Intraventricular: Between different segments of left ventricular contraction

Patients at risk for dyssynchrony include those with the following:

• Ischemic or nonischemic dilated cardiomyopathy

• Prolonged QRS interval (≥ 130 millisecond) particularly in the form of typical left bundle branch block

• Left ventricular end-diastolic dimension ≥ 55 mm

• Left ventricular ejection fraction ≤ 35% in sinus rhythm

Cardiac dyssynchrony can be suspected based on electrocardiogram (ECG) parameters (eg, left bundle branch block) and advanced echocardiography techniques (eg, tissue Doppler index, strain rate).

Cardiac resynchronization therapy (CRT) involves use of a cardiac pacing system to resynchronize cardiac contraction by restoring more physiologic timing to the electrical activation of different chambers. Such systems usually include a right atrial lead, right ventricular lead, and left ventricular lead. Biventricular cardiac resynchronization therapy (BiV-CRT) is a form of CRT that typically involves pacing of the right and left ventricles simultaneously. Leads may be placed transvenously or surgically via thoracotomy (1). Cardiac resynchronization may also be accomplished by conduction system pacing (His bundle or left bundle branch area) (2).

In patients with heart failure who have New York Heart Association (see table NYHA classification) class II, III, or IV symptoms, CRT can reduce hospitalization for heart failure and reduce all-cause mortality (1). However, there is little to no benefit in patients with permanent atrial fibrillation, right bundle branch block, nonspecific intraventricular conduction delay, or only mild prolongation of QRS duration (< 150 millisecond).