Final answer:
An Implantable Cardioverter Defibrillator (ICD) is the device that monitors heart rhythms and delivers a shock to correct dangerous arrhythmias like ventricular tachycardia or fibrillation.
Step-by-step explanation:
The implanted device that monitors the heart rhythm and delivers a shock to the heart if it detects a dangerous ventricular tachycardia or fibrillation is known as an Implantable Cardioverter Defibrillator (ICD). This device continuously monitors the heart rhythm and can deliver a lifesaving shock during cardiac emergencies, such as ventricular fibrillation or ventricular tachycardia, which are types of arrhythmias where the heart beats irregularly and too fast. Unlike pacemakers which primarily manage slow heartbeats by providing regular electrical pulses to prompt the heart to beat at a normal rate, or Holter monitors which only record heart activity for diagnostic purposes, ICDs can intervene during life-threatening arrhythmias to restore a normal heart rhythm.