A pacemaker isn’t the only device that stimulates heartbeat. People with more serious problems — especially those whose heartbeat has become so weak or irregular that they are at risk of sudden death — may need an implantable cardioverter defibrillator (ICD).

An ICD monitors the heart’s rhythm; when it senses an arrhythmia, it delivers a shock that reverts the heart to normal rhythm.

These devices are reserved for people with serious heart failure, whose ejection fraction — a measure of the output of blood — has fallen below 35 percent, putting them at risk of life-threatening arrhythmia.

Studies have also shown ICDs help prevent cardiac arrest in high-risk patients who haven’t had, but are at risk for, life-threatening ventricular arrhythmias.