Final answer:
A defibrillator, especially the Automated External Defibrillator (AED), is used to deliver a strong electrical current to a patient's heart to correct life-threatening arrhythmias. It operates by delivering a calculated shock from a stored charge in a capacitor directly through electrodes on the patient's chest.
Step-by-step explanation:
The device used to pass a strong electrical current into a patient's heart in case of cardiac emergencies is known as a defibrillator. Specifically, the variant often used in such scenarios by both medical personnel and trained laypersons is the Automated External Defibrillator (AED). AEDs deliver a large electrical charge from a capacitor to the heart muscle via electrode paddles directly applied to the bare chest wall. The purpose is to correct arrhythmias, such as ventricular fibrillation, which can be a cause of cardiac arrest.
The shock delivered by the AED allows the heart to potentially reset to a normal rhythm. Modern AEDs are also equipped with an electrocardiogram (ECG or EKG) function to assess the patient's heart rhythm and provide the correct level of energy and shock waveform as needed. It's recommended that CPR is administered before the use of an AED if the situation allows.