Cardiac Resynchronization Therapy (CRT)

What is Cardiac Resynchronization Therapy (CRT)?

Some women with heart failure lose coordination between the two sides of their heart, preventing the heart from pumping efficiently. This causes fluid to build up in the lungs and legs, worsening heart failure symptoms such as shortness of breath and swelling. Cardiac Resynchronization Therapy (CRT) uses an implanted pacemaker to send electrical signals to the main pumping chambers on both sides of the heart, making sure they contract at the same time. This enables the heart to pump more efficiently, reducing strain on the heart and relieving heart failure symptoms. CRT can help these women live longer, require fewer hospitalizations, and have a better quality of life. Because CRT uses a pacemaker device to send signals to the two main pumping chambers of the heart (ventricles), it is sometimes called biventricular pacing.

Some women who need CRT also need an Implantable Cardioverter Defibrillator (ICD) to prevent and correct more serious heart rhythm problems that can result in sudden cardiac death. For these patients, a CRT and ICD can be combined into one device and implanted in the same procedure.

Why do women with heart failure need CRT?

In women with heart failure the heart does not pump as efficiently as it should, either because of damage to the heart muscle from a heart attack or toxins, or because gradual changes over time have caused the heart’s walls to thicken and become stiff. This damage impairs the heart’s mechanical ability to pump blood, but it can also damage the electrical pathways along which the heartbeat travels.

The yellow line is the path the electrical heartbeat signal takes through the heart.

To pump blood effectively, the heart muscle must contract in a coordinated fashion with a regular speed and rhythm. The heart has an electrical system that carries signals telling different areas of the heart muscle when to contract. In some patients with heart failure, damage to the heart’s electrical system has caused the two main pumping chambers of the heart (the left and right ventricles) to fall out of sync and not contract together at the same time called cardiac dyssynchrony. This happens in about one-third of women and men with a low ejection fraction (systolic heart failure) and moderate to severe heart failure symptoms.

When the two sides of the heart are not synchronized, the heart cannot pump as efficiently and fluid builds up in the lungs and legs, worsening heart failure symptoms such as shortness of breath and swelling. A pacemaker corrects this problem by sending small timed electrical impulses to the left and right main pumping chambers (ventricles), ensuring that the two sides of the heart are synchronized and working together.

How does CRT work?

Cardiac Resynchronization Therapy uses a permanently implanted pacemaker to coordinate the heart rhythm. There are two main parts to a pacemaker: the generator and the leads.

Example of CRT generators

The generator contains a battery and a small computer that constantly monitors and analyzes the heart’s rhythm. It is programmed by your doctor to send out small electrical pulses to coordinate both sides of your heart. The generator is implanted underneath the skin, usually on the left side of the chest. It may also be placed elsewhere in the abdomen.

Wires called leads are connected to the generator and run through large veins that lead to the heart, ending in the heart muscle. These leads carry information from the heart muscle to the generator, and electrical pulses from the generator to the heart muscle. Most pacemakers have one or two leads that go to the same side of the heart. A CRT pacemaker has an extra lead so that it can send signals to both sides of the heart. One lead ends in the main pumping chamber on the left side of the heart (the left ventricle) another in the main chamber on the right side (the right ventricle). A third lead ends in the upper right chamber of the heart (the right atrium) to help ensure the heart beats in a coordinated way.

Who should receive CRT?

A woman with heart failure may be a candidate for CRT if she:

  • Has systolic heart failure (blood pumping problems) and an ejection fraction of less than 35%. (What is ejection fraction?)
  • Moderate to severe heart failure symptoms (symptoms at rest or with minimal physical activity) even while taking all appropriate heart failure medications
  • The two sides of her heart are not in electrically in step with each other. Your doctor can tell this from the pattern on an electrocardiogram (ECG).

Who should NOT receive CRT?

CRT should not be used in a woman with only mild heart failure symptoms, or who has not tried all the medication options to get her heart failure under control. CRT does not help heart failure patients who do not have blood pumping problems (it is not used to treat diastolic heart failure) or who do not have problems with coordination between the two sides of the heart.

Does CRT work as well in women as in men?

Although women only account for 1 in 3 participants in clinical trials of CRT, most studies agree that women benefit just as much from the devices as men do.

In one study of 173 patients (21% were women), both women and men who had CRT pacemakers implanted had less severe heart failure symptoms, a better quality of life, and were more able to perform physical tasks. Women benefited just as much from the treatment as men did. There were no significant difference in survival between women and men after two years (84% of women compared with 80% of men).

Compared with medical therapy alone, CRT reduces the chances of hospitalization for heart failure by 30% and lowers the chances of dying by 20%.

Are women missing out on the benefits of CRT?

It is unclear if women are less likely than men to receive CRT when they are eligible for the device. While only about 25% of CRT implantation performed each year are done in women, it is not clear if this is because women are under treated. It is possible that women are more likely to have clinical factors that make CRT unsuitable for them: for example, women with heart failure are more likely than men to have a normal ejection fraction, and these patients are not known to benefit from CRT.

Are there any alternatives to CRT?

CRT pacemaker implantation is currently the only way to re-synchronize the two sides of the heart in patients with heart failure. This can be done in a small procedure in which the device is inserted through a vein, or through surgical implantation.

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