A CRT-D device delivers cardiac resynchronization therapy, which helps coordinate your heart rhythm so that your body gets the blood and oxygen it needs. When a heart is in good working order, the upper chambers–the atria–contract first, followed by the lower chambers–the ventricles. That's the thump-thump you hear in a heartbeat. If those contractions fall out of synch, the heart can't work efficiently. The CRT device works by delivering small electrical impulses through leads to both of the ventricles. You don't feel them, but it helps the chambers beat in synch. The "D" stands for defibrillation therapy. This means the device can detect and treat dangerously fast heart rhythms, which some individuals with a damaged heart muscle may be at risk for developing. Also known as biventricular ICD. Your heart rhythm is unique, so your doctor will personalize and program what the device does for you.