Cardiac Resynchronization Therapy

Updated: Dec 17, 2014
  • Author: Adam S Budzikowski, MD, PhD, FHRS; Chief Editor: Richard A Lange, MD, MBA  more...
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Cardiac resynchronization therapy (CRT), also known as biventricular pacing or multisite ventricular pacing, involves simultaneous pacing of the right ventricle (RV) and the left ventricle (LV). To this end, a coronary sinus lead is placed for LV pacing in addition to a conventional RV endocardial lead (with or without a right atrial [RA] lead). (See Permanent Pacemaker Insertion.)

The basic goal of CRT is to restore LV synchrony in patients with dilated cardiomyopathy and a widened QRS, which is predominantly a result of left bundle branch block, in order to improve the mechanical functioning of the LV.

Guidelines from the European Society of Cardiology (ESC), developed in collaboration with the European Heart Rhythm Association (EHRA), indicate that choices about pacing, as well as about the use of implantable cardioverter-defibrillators (ICDs) and CRT, should be influenced by the specific qualities and clinical manifestations of bradyarrhythmias. [2, 3]

The guidelines identify 3 subsets of patients [2, 3] : (1) those with persistent bradycardia (including sinus node disease and atrioventricular [AV] block); (2) those with electrocardiographically documented intermittent bradycardia; and (3) those with suspected (ie, electrocardiographically undocumented) bradycardia.

Appropriate pacing choices are made based on this classification, with a similar scheme used for decisions regarding treatment with CRT. Patients with persistent bradycardia will likely require pacing, while those with atrial defibrillation may, depending on their clinical features, be treated with pacing, CRT, or AV junction ablation. [2, 3]


Practice Essentials

The basic goal of cardiac resynchronization therapy (CRT) is to restore synchrony of the left ventricle in patients with dilated cardiomyopathy and a widened QRS. This involves placement of a coronary sinus (CS) lead for left ventricular (LV) pacing, in addition to placement of a conventional right ventricular (RV) endocardial lead, with or without a (right atrial) lead.


A pacemaker is an electronic device, approximately the size of a pocket watch, that senses intrinsic heart rhythms and provides electrical stimulation when indicated. Cardiac pacing can be either temporary or permanent.

Permanent pacing is most commonly accomplished through transvenous placement of leads to the endocardium (ie, right atrium or ventricle) or epicardium (ie, the LV surface via the CS), which are subsequently connected to a pacing generator placed subcutaneously in the infraclavicular region.

CRT is a specialized type of pacemaker therapy that provides biventricular pacing. This is carried out with or without the use of an implantable cardioverter-defibrillator (ICD), a device employed for treatment and prophylaxis in patients at risk for ventricular tachycardia (VT) or ventricular fibrillation (VF).

See Periprocedural Care and Devices for more detail.


Access to the CS for implantation of the LV lead may be achieved via the axillary, subclavian, or cephalic vein.

To facilitate stable LV lead placement, it is practical first to place the RV pacing lead and then to advance the LV lead into the CS branch, leaving the sheath in place. After the RA lead is positioned, the LV lead guiding sheath is removed, and the LV lead is sutured in place.

RV lead

In most cases, the delivery system sheath is passed over a guidewire into the RA and then advanced slowly into the RV, where 90-180° of counterclockwise rotation is subsequently applied while the sheath is gently withdrawn and then advanced. This maneuver generally brings the sheath to the CS or the vicinity of the CS os, allowing easy cannulation of the CS with a guidewire.

LV lead

Although it may be possible to place the LV lead without knowing the anatomy of the CS and its branches, it is prudent to obtain a CS phlebogram to direct the selection and placement of this lead.

Successful resynchronization can be achieved with placement of the LV lead in almost any CS branch, provided that the site is in the proximal third to the middle third of the LV. [1]

Several techniques have been described for branch cannulation, including the following:

  • Branches with acute-angle origins can often be cannulated with an angioplasty wire without any difficulty

  • If a branch originating at a right or obtuse angle is difficult to cannulate, an inner catheter may be inserted near the preselected branch so that the guidewire can be advanced into the branch; once this is accomplished, the catheter may be exchanged for a packing lead while the wire position is maintained

  • Alternatively, in such problematic cases, a larger-lumen inner catheter may be used to allow delivery of the pacing lead; this technique has been simplified by using a lead with an exaggerated curve, through which a stylet or angioplasty guidewire is advanced to direct the lead tip into the appropriate venous branch

See Technique for more detail.


Indications and Contraindications

Randomized clinical trials have confirmed the effectiveness of CRT for the following purposes:

  • Improving cardiac hemodynamics and symptoms [4, 5]

  • Preventing hospitalization

  • Improving mortality [6] as compared with conventional therapy in patients with advanced heart failure symptoms and severe left ventricular dysfunction

Evidence suggests that CRT may even be beneficial in patients with mildly symptomatic heart failure (New York Heart Association [NYHA] class II). [7, 8, 6] A study of 659 heart failure patients who underwent successful CRT found the procedure safe to use in patients with clinically significant mitral regurgitation (MR). At 12-month followup, patients with more than mild MR had comparable results to patients with mild or no MR. [9] Most of the benefits appear to be associated with the presence of left bundle branch block (LBBB) on electrocardiography (ECG); the longer the QRS duration (particularly >150 msec), the more beneficial CRT is likely to be. [10]

In light of this and other accumulating evidence, the American College of Cardiology (ACC), American Heart Association (AHA), and Heart Rhythm Society (HRS) modified the Class I indication for CRT to include not only patients with NYHA class III and IV symptoms, but also those with NYHA class II symptoms and left-bundle-branch block with a QRS duration that is greater than or equal to 150 ms. [11]

Results from a retrospective study that evaluated long-term outcomes in patients with ambulatory NYHA class III and IV heart failure who underwent CRT over a 4-year period (2003-2007) indicate that, despite inferior outcomes for those with ambulatory NYHA class IV heart relative to those with NYHA class III symptoms, the survival of patients in both groups are parallel over extended follow-up (mean follow-up, 5.0 +/- 2.5 y). [12] Ambulatory NYHA class IV status was an independent factor for poor long-term outcomes, with a 40% survival free of left ventricular assist device (LVAD) or heart transplantation at 5 years.

In patients with other forms of conduction disturbance (eg, right bundle branch block [RBBB] or RV pacing), CRT is of questionable utility and therefore cannot be recommended at this time.