A CRT device sends small electrical impulses through leads to the two lower chambers of your heart. You don't feel them, but they help the chambers beat in synch. This makes it easier for your heart to pump blood and oxygen throughout your body. The "D" stands for defibrillation therapy. (CRT-D) also offers the ability to detect and treat dangerously fast heart rhythms, which some individuals with a damaged heart muscle may be at risk for developing.