The primary function of an ICD is to treat hearts that are beating too fast. It does this by first delivering small pulses to the heart to disrupt the fast heart rate. If this attempt does not work, the ICD will deliver a shock to reset the heart to its normal rhythm. Meanwhile, the primary function of a CRT-D is to get the chambers of the heart to beat in synchrony. This allows the heart to effectively pump blood to the body. The added benefit of a CRT-D is that it also contains a defibrillator, which can treat the heart just like regular ICD.