Pacemakers are used to help treat a slow heart rate, a condition known as bradycardia. An ICD is implanted to help treat a fast heart rate (called tachycardia), or when there is risk for sudden cardiac arrest. Most ICDs now have both therapies available in the same device. There's also a heart device called a CRT-D. This device is needed when the lower chambers of the heart do not beat in synchrony, which may reduce the amount of oxygen and blood pumped to the body.