Cardiac resynchronisation therapy

Cardiac resynchronisation therapy with a pacemaker (CRT-P or simply CRT) is used to treat heart failure when there is also a left bundle branch block present. Having a LBBB and heart failure worsens the heart failure because of the desynchronised contraction of the right and the left ventricle. The CRT device is a specialised pacemaker which ensures that the two ventricles contract synchronously, thanks to there being an electrode outside the left ventricle (in addition to the usual electrode in the right ventricle in normal pacemakers).

Another indication for CRT is if a patient with heart failure and another indication for a pacemaker (such as sick sinus syndrome) has poor AV conduction so that it's expected that the pacemaker must pace the ventricles a lot (> 20% of the time). Univentricular pacing may worsen the heart failure, and so adding a left ventricular electrode to the pacemaker and configuring it in CRT mode may alleviate this worsening.

Unfortunately, not all patients fitted with a CRT device achieve synchronicity. Some patients are just CRT non-responders.

CRT-D

There exist CRT devices with ICD functionality as well, called CRT-D devices. These are useful because some patients with an indication for CRT also have indication for ICD, and the same patient can’t have two devices.