A pacemaker is a small, implantable electronic device that helps the heart maintain a normal rhythm and rate. It functions by delivering electrical impulses to the heart muscle, taking over the role of the heart’s natural electrical system when it malfunctions. This intervention is necessary when the heart beats too slowly or irregularly, preventing the body from receiving the blood flow it needs. The device relieves symptoms and restores a functional heartbeat, improving the patient’s quality of life.
Understanding the Heart’s Electrical System
A healthy heartbeat is initiated and coordinated by a specialized electrical conduction system within the heart muscle. This process begins with the sinoatrial (SA) node, a cluster of cells in the upper right chamber, which acts as the heart’s natural pacemaker. The SA node generates an electrical impulse that causes the upper chambers (atria) to contract, pushing blood into the lower chambers.
The electrical signal then travels to the atrioventricular (AV) node, situated near the center of the heart, where the impulse is momentarily delayed. This pause allows the atria to fully empty their blood into the ventricles before they contract. From the AV node, the signal moves rapidly into the muscular walls of the ventricles, causing them to contract and pump blood out to the rest of the body.
This organized sequence ensures the four chambers contract in a coordinated manner, establishing a steady rhythm, typically between 60 to 100 beats per minute at rest. Problems requiring a pacemaker arise when this natural system of signal generation or transmission becomes too slow or blocked.
Primary Medical Indications for Pacing
Pacemakers are indicated for conditions causing symptomatic bradycardia—a heart rate too slow to meet the body’s metabolic needs. A common reason is Sinus Node Dysfunction (SND), where the SA node fails to generate electrical impulses reliably. This leads to an abnormally slow heart rate, frequent pauses, or an alternation between slow and fast rhythms.
Another indication is Atrioventricular (AV) Block, where electrical signal transmission between the atria and the ventricles is impaired. In a second-degree block, some impulses are blocked from reaching the ventricles, causing dropped beats. A third-degree, or complete, heart block means no impulses pass, forcing the lower chambers to rely on a slower, less reliable backup rhythm insufficient for proper blood circulation.
Symptoms leading to pacemaker implantation include fatigue, dizziness, lightheadedness, and fainting (syncope). These symptoms occur because the slow heart rate results in an inadequate supply of oxygen-rich blood to the brain and vital organs. Pacemakers may also be necessary for certain types of chronic bifascicular block or following a heart attack if the conduction system is significantly damaged.
The Mechanism of Pacemaker Function
The artificial pacemaker corrects rhythm problems using two components: the pulse generator and the leads (wires). The pulse generator is a small, sealed titanium case containing the battery and the computer circuitry. This circuitry monitors the heart and delivers electrical energy, powered by a battery that often lasts several years before replacement is needed.
The leads are insulated wires threaded through a vein directly into the heart chambers from the pulse generator. Electrodes at the tips of the leads serve a dual purpose: they sense the heart’s intrinsic electrical activity and deliver the corrective electrical pulse when necessary. This monitoring process, called sensing, allows the device to remain dormant if the heart is beating normally.
If the heart rate drops below a pre-set minimum or an expected beat is missed, the device initiates pacing. The pulse generator sends a low-energy electrical impulse through the lead to the heart muscle, forcing the chamber to contract and restore a functional rhythm. Pacemakers are categorized as single-chamber (pacing one chamber) or dual-chamber (using two leads to pace and sense both upper and lower chambers, maintaining coordination).