How does a ICD work?

Most modern ICDs have three main functions;

• If your heart rhythm is too slow, the device can give your heart extra beats by working as a normal pacemaker. This is called anti-bradycardia pacing

• If your heart beats too fast, the ICD can give you a burst of extra beats at an even faster rate which will normally return your heart back to a normal rhythm. This is called anti-tachycardia pacing (or ATP)

• If the anti-tachycardia pacing doesn’t bring your heart back to a normal rhythm, or if the ICD senses a faster rhythm called ventricular fibrillation, the ICD can then give a higher energy shock. This is called defibrillation

What can CRT do?

There are 2 different types of CRT devices:

  • CRT Pacemaker
  • CRT ICDs

CRT Pacemakers

These devices work in the same way as traditional pacemakers. They help the heart to beat in a regular rhythm and are especially useful when the heart’s natural pacemaker stops working properly. An implanted pacemaker sends out electrical signals which make the heart beat in a normal way. A
traditional pacemaker has either one or two leads which are put into the right side of your heart. One lead will be put into the lower right chamber (right ventricle) and a second lead may be put in the right upper chamber (right atrium). However, if you have heart failure you may need a more advanced type of pacemaker.

The main pumping chamber of the heart is on the left side, called the left ventricle. In a normal heart, both the right and left ventricles pump out blood at the same time in a coordinated way. In heart failure, the left side may not pump at the same time as the right, meaning the two sides of the heart lose their coordination. This leads to the heart being unable to pump effectively and you may become breathless, dizzy and have no energy. Over time, the heart may get even weaker and you may feel more unwell.

In this case, your doctor may feel that you could benefit from having a CRT pacemaker implanted. The main difference is that an additional lead is placed on the left side of the heart and electrical signals can make the left ventricle beat at the same time as the right. This can restore the heart’s coordination and make the heart pump more efficiently. In two out of three people implanted with a CRT pacemaker, symptoms of breathlessness improve and they have more energy and generally feel healthier.


You may have had a dangerous rhythm problem in the past known as ventricular tachycardia (VT) or ventricular fibrillation (VF). These happen when an electrical problem causes the heart to beat too fast and may lead to the heart being unable to pump blood around the body, causing a cardiac arrest. If not treated with an electric shock, death can occur in a few minutes.

You may not have had a dangerous rhythm before, but you may be at risk of having one in the future because of your underlying heart condition. In this case your doctor may recommend that you are fitted with an ICD instead of a pacemaker.

Having an ICD means that if you ever need an electric shock because your heart rhythm has become dangerously fast, it will be delivered within seconds. However, you may not always need a shock to return your heart rhythm to normal. Quite often extra signals from the pacemaker part of the ICD can make the heart rhythm normal again. This is known as anti-tachycardia pacing (ATP) and works about 85% of the time to treat ventricular tachycardia.

A standard ICD is fitted like a normal pacemaker, with either one or two leads in the right side of the heart. A CRT ICD has the extra lead on the left ventricle which makes the heart pump in a coordinated way, improving symptoms of heart failure.


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