There are two types of CRT-

1) Cardiac Resynchronization Therapy Pacemaker (CRT-P)

• This device functions only as a pacemaker.
• It sends a mild electrical pulse to regulate the rate of ventricular contractions and restores a coordinated pumping action.
• It is an option for people who are not at high risk of developing life-threatening arrhythmias such as ventricular tachycardia.

2) Cardiac Resynchronization Therapy Defibrillator (CRT-D)

• This device combines a pacemaker and defibrillator in one device.
• It synchronizes the contractions of the ventricles like a CRT-P but it also senses and corrects dangerous arrhythmias that can develop in the ventricles.
• CRT-D may be right for people who are at a high risk of developing ventricular arrhythmias that may sometimes lead to sudden cardiac arrest or sudden cardiac death.  

Reference : Cardiac Resynchronization Therapy. (n.d.). Retrieved from https://www.azheartarrhythmia.com/contents/ep-procedures/cardiac-resynchronization