A standard pacemaker does not deliver a high-energy electrical shock that a person would feel. The device is designed to treat heart rhythms that are too slow, a condition known as bradycardia. The confusion about a “pacemaker shock” arises because a different, but related, implantable device exists that does deliver a strong electrical discharge.
How a Standard Pacemaker Regulates Heart Rhythm
A cardiac pacemaker regulates heart rhythm by delivering small, low-energy electrical impulses, not a powerful shock. The system consists of a pulse generator, which contains the battery and circuitry, and leads, which are thin, insulated wires extending from the generator to the heart muscle. These leads have electrodes at the tip that make contact with the heart tissue, typically in the right atrium or ventricle.
The pacemaker operates on a demand basis, meaning it constantly monitors the heart’s intrinsic electrical activity, a process called sensing. If the device senses that the heart’s natural rate has fallen below a pre-set minimum, it generates a tiny electrical signal. This signal, measured in millivolts or microjoules, travels through the lead to the heart muscle, causing it to contract and initiate a heartbeat.
This therapeutic action, known as pacing, is typically imperceptible to the patient. The electrical output is just enough to stimulate the nearby cells of the myocardium and trigger a contraction. The goal is to ensure the heart beats at a steady, reliable pace. Modern pacemakers can also adjust the pacing rate to match the body’s physical activity levels.
The Difference Between Pacing and Defibrillation
The term “pacemaker shock” is often a misnomer, confusing the function of a standard pacemaker with that of an Implantable Cardioverter-Defibrillator (ICD). While both devices are implanted and use electricity, they serve distinct purposes and deliver vastly different levels of energy. The pacemaker focuses on correcting rhythms that are too slow, whereas the ICD is designed to treat rhythms that are dangerously fast and chaotic.
Pacing uses low-voltage, low-energy pulses to prompt a heart contraction, similar to the heart’s natural electrical signal. These impulses are measured in fractions of a joule, or microjoules. Defibrillation, conversely, is a high-energy intervention delivered to stop life-threatening arrhythmias, such as ventricular fibrillation or ventricular tachycardia.
The energy delivered by an ICD for defibrillation is measured in joules and can be up to 40 joules, an energy level thousands of times greater than a pacing impulse. This strong current is meant to instantaneously depolarize all the heart muscle cells simultaneously, effectively “resetting” the electrical system to allow a normal rhythm to resume. Some ICDs also have an anti-tachycardia pacing function, which delivers a rapid sequence of low-energy impulses to try and interrupt a fast rhythm before a high-energy shock is required.
Many modern ICDs are combination devices that include full pacemaker functionality. This means a single implanted device can perform low-energy pacing for slow rhythms and high-energy shock for fast, lethal rhythms. Standard pacemakers, however, lack the large capacitor needed to store and deliver the high energy required for defibrillation.
What Receiving a Shock Feels Like
The sensation of receiving a high-energy shock comes from the ICD, not a standard pacemaker, and is a significant physical event. Patients who have experienced a shock frequently describe it as a sudden, intense jolt or a sharp punch to the chest. The pain is brief, lasting less than a second, but it is startling and can cause the muscles in the chest and arms to contract abruptly.
On a pain scale, the sensation is often rated around a six out of ten, reflecting its intensity. The shock’s purpose is therapeutic, indicating the device has successfully corrected a potentially fatal heart rhythm. However, the experience can cause significant anxiety. If the ICD delivers a low-energy shock, such as for anti-tachycardia pacing, the sensation may be much milder, feeling like a flutter or a rapid thumping in the chest, or it may be imperceptible.
If a person receives a high-energy shock, medical guidance generally advises contacting the cardiology team immediately for a device check, even if they feel fine afterward. Experiencing multiple shocks in a short period suggests a serious underlying issue or a problem with the device’s programming and requires immediate emergency medical attention. Having an action plan in place for a shock is important for managing the physical and emotional aftereffects of the event.