Cardiac Resynchronization

Cardiac resynchronization (cardiac resynchronization therapy, CRT) is a new pacemaker procedure to resynchronize cardiac contraction for patients with heart failure (heart failure; NYHA stages III and IV) when drug therapy has been exhausted. Cardiac resynchronization can be used to increase the efficiency of the heart. Furthermore, cardiac resynchronization can be combined with an implantable cardioverter defibrillator ICD (electric shock device for immediate therapy of dangerous cardiac arrhythmias). The efficacy of classical cardiac resynchronization as well as of the combination procedure with an ICD has been demonstrated, among others, in two large studies published in 2004 and 2005: the COMPANION study (“Comparison of Medical Therapy, Pacing and Defibrillation in Heart Failure”) and the CARE-HF study (“Cardiac Resynchronization in Heart Failure“). Current guidelines for the treatment of heart failure list the procedures. In patients with symptomatic mild heart failure (heart failure; NYHA stage II) and wide QRS complex (ventricular complex; wide QRS complex ≥ 120 ms) on ECG, cardiac resynchronization therapy (CRT) achieved a significant long-term survival benefit. Patients with heart failure and narrow QRS complex did not benefit from cardiac resynchronization therapy (CRT). Cardiac resynchronization therapy (CRT) reduces mortality (death rate) in progressive heart failure in patients with and without diabetes mellitus. However, mortality is higher in patients with diabetes.

Indications (areas of application)

  • Mild heart failure (heart failure; NYHA stage II) – The use of resynchronization even in mild heart failure is intended to slow or stop structural change (remodeling processes). Three studies were conducted to demonstrate the efficacy of the procedure in mild heart failure. In summary, both functional and structural improvements of the left ventricle were observed after only one year of therapy. Significant reductions in end-diastolic and end-systolic ventricular volumes (filling of the ventricles at different times) and increases in LVEF (left ventricular blood ejection fraction) were noted, among others. However, not all patients benefit equally from the procedure. In the presence of left bundle branch block (disruption of conduction in the left part of the heart), cardiac resynchronization pacing is most effective, whereas patients with right bundle branch block do not benefit significantly from CRT.
  • Severe heart failure (NYHA stages III and IV) – Even with optimal medical therapy, some patients with heart failure remain (severely) symptomatic. If these patients reach the criteria for cardiac resynchronization, it should be performed in the absence of possible contraindications (contraindications). Criteria include the following:
    • Reduced ejection fraction/percentage of blood volume ejected from a ventricle during a cardiac action (left ventricular erection fraction (LVEF) ≤ 35%).
    • Asynchronous ventricular contraction (contraction of the ventricle) due to complete left bundle branch block (QRS complex ≥ 130 ms)
    • End-diastolic left ventricular dilatation (“dilation”) > 55 mm.
    • Pacing fraction > 40% with pacemaker supply.

    In the heart failure guidelines no longer the strict restriction to patients with sinus rhythm (regular heart excitation).

  • Symptomatic patients who meet the following requirements:
    • Reduced ejection fraction ≤ 35% despite optimal drug therapy.
    • Sinus rhythm
    • QRS complex ≥ 150 ms
    • Non-left bundle branch block
  • Atrial fibrillation – In exceptional cases, cardiac resynchronization may be considered in patients with atrial fibrillation if the following conditions are met:
    • LVEF ≤ 35%
    • NYHA class III-IV despite optimal drug therapy.
    • QRS complex ≥ 130 ms
    • Nearly complete biventricular pacing.

Cardiac resynchronization therapy is recommended by the guidelines, particularly for patients with the following constellations:

  • Left bundle branch block with QRS width > 120 ms, sinus rhythm, LVEF ≤ 35%, and NYHA II-IV despite optimal drug therapy.
  • With pacemaker already in place, stage NYHA III-IV, LVEF < 35% and ventricular fibrillation despite optimal drug therapy.

Indications for cardiac resynchronization therapy in patients with sinus rhythm and LVEF ≤ 35%.

QRS (ms) Left bundle branch block Non-left bundle branch block
< 130 ↓↓ ↓↓
130-149 ↑↑ ↔
≥ 150 ↑↑ ↑

Cardiac resynchronization should be recommended to symptomatic patients who meet the following requirements:

  • Reduced ejection fraction ≤ 35% despite optimal drug therapy.
  • Sinus rhythm
  • QRS complex ≥ 130 ms

Contraindications

  • QRS duration < 130 ms

Reversible causes associated with ventricular tachyarrhythmias are a contraindication:

  • Digitalis intoxication
  • Electrolyte changes
  • Sepsis

Before therapy

Before implantation of the CRT pacemaker, it is necessary to verify the extent to which the particular patient would benefit from cardiac resynchronization and whether he or she is suitable for the treatment. Furthermore, it must be ensured that all non-invasive methods (adequate drug therapy, etc.) have been fully exhausted. When establishing the indication for CRT, the guidelines state that the indication for a pacemaker should also be considered. Patients in whom CRT but not implantation of a cardioverter-defibrillator is indicated should be treated with a combination device with pacing (CRT-P).

The procedure

Cardiac resynchronization is based on the elimination of a condition called dyssynchrony (noncoordinated cardiac action). As systolic heart failure progresses, increased dilatation (widening) of the left or both ventricles (heart chambers) can be seen. These structural changes also lead to molecular changes as the disease progresses, as gene expression and ion channel function may be affected. The consequences of these multiple changes are delays in electrical excitation propagation (left bundle branch block) and the aforementioned dyssynchrony of the ventricular contraction pattern. Thus, in addition to electrical dyssynchrony, which is manifested, among other things, by a delay in ventricular excitation, there is also mechanical dyssynchrony, which is recognizable as a delay in contraction. Increasing ventricular excitation delay (ventricular excitation) is associated with an increase in cardiac pump dysfunction and concomitant worsening of prognosis. Like a conventional pacemaker (HSM; pacemaker), the CRT pacemaker is implanted under the skin slightly below the left or right clavicle. The device is connected to the heart by means of three electrodes (probes; cables). With conventional pacemakers, only a maximum of two electrodes are used. The third electrode is inserted into the left side of the heart via the coronary veins. The CRT pacemaker can thus deliver weak electrical impulses to both ventricles (= biventricular pacing; biventricular pacemaker). As a result, both ventricles can beat simultaneously again, increasing the pumping force of the heart and improving the previously reduced ejection fraction (percentage of blood volume ejected by a ventricle during a cardiac action). There are now also CRT pacemakers with four electrodes. These allow better alignment of left and right ventricular contraction (muscle contraction of the heart chamber). This is said to be especially true for patients with scarred myocardium (heart muscle). A quadripolar electrode is intended to produce a larger wave of depolarization along the target vein. In the presence of a sleep-related breathing disorder, a combination with a respiratory rate sensor is possible with some implantable therapy systems. The sensor is used to measure respiratory movements so that nocturnal pauses in breathing during sleep can be detected.

After therapy

Following implantation, the function must be checked. In approximately 5% of all implantations, the probe cannot be clearly placed correctly.

Possible complications

Because cardiac resynchronization therapy is an invasive procedure, complications are possible during implantation, among other procedures:

  • Implantation problems and probe placement difficulties.
  • Technical complications of the pacemakers
  • Dislocations of the probes (with possible unpleasant diaphragm stimulation).
  • Coronary sinus injuries with the risk of pericardial tamponade (vascular injury with the risk of bleeding into the pericardium and obstruction of cardiac action)
  • Hematoma (bruise)
  • Infections